CT600 Box-by-Box Guide: Which Boxes Do You Actually Need to Fill In?

CT600 Box-by-Box Guide: Which Boxes Do You Actually Need to Fill In?

The CT600 form has over 200 boxes. Most of them, you'll never touch.

If you're a small limited company director looking at the CT600 for the first time, the sheer number of boxes can be paralysing. Boxes about group relief, controlled foreign companies, tonnage tax, derivative contracts... none of that applies to 95% of small companies.

This guide cuts through the noise. We'll walk you through the boxes that actually matter for a typical small limited company — the ones you need to fill in, what each one means in plain English, and which ones you can safely ignore.

By the end, you'll know exactly what goes where — or you'll realise that Taxpipe can fill them all in for you for £59 in about 15 minutes.

How the CT600 Is Structured

The CT600 form is divided into sections, each covering a different aspect of your company's tax affairs. Here's the high-level structure:

For a small company with straightforward affairs, you'll mostly be working with the company information, income, and tax calculation sections. Let's go through each critical box.

Section 1: Company Information (Boxes 1–4)

This is the admin section. Get this right and everything else flows from it.

Box 1: Company Name

Your company's full legal name exactly as registered at Companies House. Not your trading name — your registered name. If your company is "ABC Solutions Limited" but you trade as "ABC Solutions," use the full registered name.

Box 2: Company Registration Number (CRN)

Your 8-digit Companies House registration number. You can find this on any Companies House document or by searching at find-and-update.company-information.service.gov.uk.

Box 3: Tax Reference (UTR)

Your 10-digit Unique Taxpayer Reference for corporation tax. HMRC sent this to you when your company was registered for corporation tax. It's different from your personal UTR (if you have one). If you can't find it, check your HMRC Government Gateway account or any previous correspondence from HMRC about corporation tax.

Lost your UTR? See our guide on HMRC corporation tax online account setup.

Box 4: Type of Company

A code for special company types — unit trusts, close investment-holding companies, members' clubs, charities, real estate investment trusts, and similar. Most small trading limited companies leave this blank (or enter 0 where the software requires it). Only enter a code if one of HMRC's special categories genuinely applies to your company.

Section 2: About This Return (Boxes 30–90)

Boxes 30 and 35: Accounting Period Start and End Dates

Box 30 is the start date and box 35 is the end date of the accounting period this return covers. For most companies, this is a 12-month period (e.g., 1 April 2024 to 31 March 2025). Your first accounting period might be shorter — but a CT600 can never cover more than 12 months. If your accounting period is longer, you'll need to file two CT600s.

More on accounting periods: Accounting periods explained

Box 40: A Repayment Is Due

Tick this if you expect HMRC to repay you for this period — for example, because you've overpaid corporation tax or have a payable tax credit.

Box 80: Accounts and Computations Attached

Tick this to confirm you're attaching accounts and a tax computation for the same period as the return. This is the normal situation for almost every company.

Section 3: Turnover and Income (Boxes 145–235)

This is where you declare what your company earned.

Box 145: Total Turnover From Trade

Your company's total sales/revenue for the accounting period. This is the top line of your profit and loss account — all the money your company earned from its business activities, before deducting any costs.

What to include: Sales of goods or services, fees earned, commissions received.

What NOT to include: Bank interest (that goes in box 170), money you personally loaned to the company, VAT (if you're VAT registered — use the net figure).

Box 150 is a tick box for banks, building societies, and financial institutions that don't have a recognised turnover figure. Standard trading companies leave it blank.

Box 155: Trading Profits

This is a key box. It's your taxable trading profit — revenue minus allowable expenses and capital allowances. It is not the same as your accounting profit because certain expenses are not deductible for tax:

• Add back: Client entertaining, accounting depreciation, personal expenses put through the company
• Deduct: Capital allowances (the tax version of depreciation — claimed in your tax computation and broken down in boxes 688–755)

A detailed tax computation showing how you arrived at this figure must be filed alongside your CT600.

Deep dive on this box: CT600 Box 155: trading profits explained

Box 160: Trading Losses Brought Forward Set Against Trading Profits

If your company made trading losses in previous accounting periods that you haven't yet used, you can set them against this year's trading profit by entering them here. This is for brought-forward losses from earlier years only — not a loss your company made in the current period.

If your company made a trading loss in the current period, that goes in box 780 (in the losses section) and can be carried forward to future years or, if you prefer, set against total profits via box 275.

Box 165: Net Trading Profits

Box 155 minus box 160. If you have no brought-forward losses to use, this equals box 155.

Section 4: Income From Other Sources (Boxes 170–235)

Box 170: Bank Interest and Non-Trading Loan Relationship Profits

This is where bank interest goes. If your company's bank account earned interest during the period, declare the net amount here — even if it's only a few pounds. Box 170 covers all non-trading profits from loan relationships and derivative contracts; for most small companies, that's just bank interest.

Note: Box 172 is a tick box used only if the figure in box 170 has been reduced by a non-trading loan deficit carried back from a later period. Almost all small companies leave box 172 blank.

Box 190: Income From a Property Business

Net rental income from property your company owns, after deducting allowable property expenses (repairs, insurance, agent fees, mortgage interest). If your company doesn't own rental property, leave this blank. A property business loss goes in box 805.

Property companies: see our corporation tax for property companies guide.

Box 205: Income Not Falling Under Any Other Heading

A catch-all for any taxable income that doesn't fit into boxes 145–200. For most small trading companies, this is zero.

Box 235: Profits Before Other Deductions and Reliefs

The total of all your income streams — net trading profits, bank interest, property income, net chargeable gains, and any other income — before further deductions. This is the starting figure for the deductions section and the tax calculation.

Section 5: Deductions and Reliefs (Boxes 240–315)

Box 245: Management Expenses

Costs of managing an investment portfolio. This only applies to investment companies — companies whose primary activity is holding investments rather than trading. Standard trading companies leave this blank.

Box 275: Current-Period Trading Losses Against Total Profits

If your company made a trading loss this year (box 780) and you want to claim it against your total profits for the same period — including interest, property income, and capital gains — enter the amount here.

Box 285: Carried-Forward Trading Losses Against Total Profits

For losses arising after 1 April 2017, you can carry them forward and set them against total profits (not just future trading profits). That election is made here. Most small companies using brought-forward losses against trading profits use box 160 instead.

Box 305: Qualifying Charitable Donations

Donations your company made to registered charities or Community Amateur Sports Clubs during the period. These reduce taxable profits pound-for-pound. The donations must be from the company, not from you personally.

More details: Corporation tax relief on charity donations

Box 315: Profits Chargeable to Corporation Tax

Your total taxable profits after all deductions, losses, and reliefs. This is the number that gets taxed — the most important figure on your return.

Section 6: Capital Allowances (Boxes 688–755)

Instead of deducting accounting depreciation (which isn't tax-allowable), you claim capital allowances. These are already reflected in the trading profit in box 155. Boxes 688–755 are where you report the breakdown to HMRC.

Box 690: Annual Investment Allowance

The most commonly used allowance. The AIA gives 100% tax relief on qualifying plant and machinery in the year of purchase, up to £1 million per year (pro-rated for short periods). Covers computers, equipment, tools, machinery, and most commercial vehicles. Cars don't qualify for AIA — different rules apply depending on CO2 emissions.

This is one of the most valuable tax reliefs available: instead of writing off a piece of equipment over three or five years in the accounts, you get the full deduction this year.

Boxes 688 and 733: Full Expensing

A permanent 100% first-year deduction on qualifying new main-pool plant and machinery, introduced in 2023. Available to companies only (not sole traders or partnerships). Similar to AIA but applies to main-pool assets where AIA has already been used or where you're above the annual limit.

Boxes 705 and 710: Main Pool — Allowances and Charges

Plant and machinery not covered by AIA or full expensing goes into the main pool, which attracts a writing-down allowance of 18% per year. Box 710 is for balancing charges — a clawback when an asset is sold for more than its remaining tax value.

Boxes 695 and 700: Special Rate Pool

Long-life assets, integral features (heating, electrical systems, lifts), and similar items attract a lower 6% writing-down allowance.

Detailed guide: Capital allowances for small companies

Section 7: Tax Calculation (Boxes 326–528)

This is where it all comes together.

Box 326: Number of Associated Companies

The number of companies associated with yours (under common control, broadly). If you only have one company, enter 0. This matters because both the £50,000 small profits threshold and the £250,000 marginal relief upper limit are divided by (associated companies + 1).

Box 329: Small Profits Rate / Marginal Relief Indicator

Tick this if the company's profits qualify for the 19% small profits rate or for marginal relief — i.e., if profits are £250,000 or below.

Boxes 330–425: Financial Year Rows

The tax calculation is split across financial year rows because a company's accounting period can straddle two of HMRC's financial years (each running 1 April to 31 March). For each year, you enter the amount of profit chargeable at that year's rate and the resulting tax.

Most companies whose period falls entirely within one financial year only need to complete the FY1 row:

• Box 330: The financial year (e.g., enter 2024 for FY 2024, which runs 1 April 2024 to 31 March 2025)
• Box 335: Your profit chargeable at this rate
• Box 340: The tax rate (19% or 25%)
• Box 345: The tax calculated (box 335 × box 340)

If your period straddles two financial years, you'll need the FY2 rows as well (boxes 380–425), with profits split proportionally by days.

Tax rates summary:

• 19% if augmented profits are £50,000 or below (threshold divided by associated companies + 1)
• 25% if augmented profits exceed £250,000 (threshold divided by associated companies + 1)
• Between 19% and 25% (marginal relief) if profits fall between those thresholds

Calculate your rate: Use our corporation tax calculator or read about marginal relief explained.

Box 430: Corporation Tax

The total tax from all the financial year rows — sum of boxes 345, 360, 375, 395, 410, and 425. This is before any marginal relief.

Box 435: Marginal Relief

If your profits fall between £50,000 and £250,000, you're entitled to marginal relief, which reduces your tax bill. Claim it here on the main form by entering the relief amount — no supplementary page is needed for marginal relief. The formula is:

Marginal relief = 3/200 × (£250,000 – Profits) × (Profits ÷ Augmented Profits)

Augmented profits are taxable profits plus dividends received from other UK companies. For a company with no such dividends, augmented profits equal taxable profits and the fraction equals 1.

Taxpipe calculates this automatically. But it's important to know it exists, because many directors on profits of £60,000–£200,000 overpay by not claiming it.

Box 440: Corporation Tax Chargeable

Box 430 minus box 435 — your CT after marginal relief.

Box 475: Net Corporation Tax Liability

Box 440 minus double taxation relief and any other credits at boxes 445–465. For most small companies this equals box 440.

Box 480: Tax on Director Loans (Section 455)

If your company has an outstanding loan to a director or shareholder at the nine-months-and-one-day payment date, a 33.75% tax charge applies. The amount comes from the CT600A supplementary page. The charge is repaid by HMRC when the loan is eventually repaid. If you have no overdrawn director loans, leave this blank.

Important topic: Director's loan accounts and your CT600

Box 595: Tax Already Paid

Any corporation tax already paid for this period (e.g., quarterly instalment payments). Box 600 shows the outstanding balance after this.

Section 8: Supplementary Pages (Indicated by Boxes 95–144)

Supplementary pages handle special situations. The tick boxes at 95–144 on the main form flag which supplementary pages you're attaching. Most small companies won't need any:

CT600A: Loans to Participators (Box 95)

You need this if: A director or shareholder owes the company money at the year-end — an overdrawn director's loan account. The Section 455 tax charge is 33.75% of the outstanding amount, due nine months and one day after the period end. The charge is refunded once the loan is repaid. Anti-avoidance rules prevent repaying and re-borrowing: if a loan of £5,000 or more is repaid and a new loan is taken within 30 days, the S455 charge continues as if the original loan were still outstanding.

CT600B: Controlled Foreign Companies (Box 100)

Skip this unless your UK company controls a foreign subsidiary. Virtually no small companies need this.

CT600C: Group Relief (Box 105)

Skip this unless your company is part of a corporate group claiming losses from other group companies.

If applicable: Corporation tax group relief explained

CT600E: Charities and CASCs (Box 115)

You need this if your company is a registered charity or Community Amateur Sports Club claiming corporation tax exemptions. Taxpipe supports CT600E filings.

CT600J: Disclosure of Tax Avoidance Schemes (Box 140)

Skip this for almost all small companies. CT600J is for disclosing notifiable tax avoidance schemes under the DOTAS rules — specific marketed schemes with a Scheme Reference Number issued by HMRC. Normal tax planning (claiming capital allowances, paying dividends, making pension contributions) is not a disclosable avoidance scheme.

CT600L: Research and Development (Box 142)

You need this if you're claiming R&D tax relief under the merged RDEC scheme or the enhanced R&D intensive support (ERIS) scheme. If your company spends money solving scientific or technological uncertainties — developing new software, engineering a new product, or creating new processes — this relief can be significant.

Worth investigating: R&D tax relief for SMEs

Marginal relief does not require a supplementary page. It is claimed in box 435 on the main CT600 form.

Full guide: CT600 supplementary pages explained

The Boxes Most Small Companies Can Ignore

Here's a quick list of sections you can safely skip if you're a standard small limited company:

That eliminates the majority of the form.

Example: A Typical Small Company CT600 Walkthrough

Let's say you run a small consulting company. Here's what your CT600 might look like:

Company: Smith Consulting Ltd
Accounting period: 1 April 2024 – 31 March 2025
Income: £85,000 in consulting fees
Expenses: £15,000 (software, travel, home office, insurance, accountancy)
Equipment purchased: £2,000 laptop
Bank interest: £120
Director's salary: £12,570 (already in expenses)

Trading profit before capital allowances: £85,000 − £15,000 = £70,000. Deduct AIA on laptop (£2,000) → taxable trading profit £68,000 (box 155).

* Capital allowances are deducted when arriving at box 155. The £2,000 AIA is also separately reported in box 690 as a breakdown item in the capital allowances section.

** Marginal relief: 3/200 × (£250,000 – £68,120) ≈ £2,728. The company has no dividends from other UK companies, so augmented profits equal taxable profits and the augmented-profits fraction is 1.

Without claiming marginal relief, this company would pay £17,030 instead of £14,302 — a difference of £2,728. That's real money left on the table if you don't claim it.

Taxpipe's calculator works this out automatically. Try it now →

The iXBRL Accounts Requirement

Filling in the CT600 boxes is only half the job. You also need to submit iXBRL-tagged accounts alongside your return. These are your company's accounts (profit and loss, balance sheet) in a machine-readable format that HMRC's systems can automatically process.

You cannot submit:

• ❌ PDF accounts
• ❌ Scanned documents
• ❌ Excel spreadsheets
• ❌ Plain text

The accounts must be in iXBRL format, with each number properly tagged to the HMRC taxonomy. This is a legal requirement for all online CT600 filings.

Creating iXBRL accounts requires specialist software. This is often the biggest barrier for directors trying to file themselves — you might be able to work out the tax calculation, but generating compliant iXBRL accounts is a different challenge entirely.

What is iXBRL? iXBRL accounts explained

Should You Fill In the CT600 Yourself?

Honestly? It depends on your situation.

You Might Be Fine DIY If:

• Your company is very simple (one trade, no property, no R&D, no loans to directors)
• You're comfortable with basic tax concepts
• You have software that handles iXBRL generation
• You understand marginal relief and capital allowances

You'll Struggle DIY If:

• This is your first time
• You have director's loans, property income, or R&D claims
• You don't have iXBRL software
• You're not sure about capital allowances or marginal relief
• You need to deal with losses or prior period adjustments

The Middle Ground: Use Taxpipe

Taxpipe gives you the best of both worlds. You keep control — entering your own figures and making your own decisions — but we handle the complexity:

• We calculate marginal relief — so you never overpay
• We generate iXBRL accounts — micro-entity (FRS 105) accounts included, no extra software needed
• We file with HMRC — through your own Government Gateway account
• We explain each question — in plain English, no jargon

All for £59. That's cheaper than most accountants charge just to review your figures.

Get started with Taxpipe →

Common CT600 Box Mistakes

Mistake 1: Using Gross Figures Instead of Net (VAT-Registered Companies)

If your company is VAT-registered, use net figures (excluding VAT) for turnover and expenses. The VAT you collect and pay over belongs to HMRC, not your company — so it shouldn't appear in your corporation tax calculations.

Mistake 2: Not Claiming Capital Allowances

Many directors forget to claim capital allowances on equipment, computers, and vehicles. Instead, they deduct accounting depreciation — which isn't allowable for corporation tax. You need to add back depreciation in your tax computation and claim capital allowances instead. The difference can be significant: AIA gives you 100% relief in year one, while accounting depreciation might spread the cost over 3–5 years.

Mistake 3: Missing Marginal Relief

Companies with profits between £50,000 and £250,000 need to claim marginal relief in box 435 on the main CT600 form — no supplementary page required. If you don't claim it, you'll pay the full 25% rate instead of the reduced effective rate. On £100,000 of profit, the difference is around £2,250.

Mistake 4: Not Declaring Bank Interest

Even tiny amounts of bank interest need to be declared in box 170. Leaving it out makes your return inaccurate and could cause problems if HMRC cross-references your information with banks.

Mistake 5: Confusing Accounting Profit With Taxable Profit

Your accounting profit (from your company accounts) won't match your taxable profit (box 155) because certain expenses are disallowed for tax purposes. Common disallowable expenses include client entertainment, some car costs, and depreciation.

Full list: Disallowable expenses for corporation tax

Frequently Asked Questions

Which boxes on the CT600 do I actually need to fill in?

For a standard small limited company, you typically need to complete: boxes 1–4 (company information), boxes 30 and 35 (period dates), box 80 (accounts attached), box 145 (turnover), box 155 (trading profits), box 170 (bank interest, if any), box 235 (total profits), box 315 (profits chargeable), box 326 (associated companies), boxes 330–345 (FY1 profit, rate, and tax), boxes 430, 435, 440 (CT, marginal relief, CT chargeable), box 475 (net CT liability), box 690 (AIA), and the declaration. That's roughly 20 boxes out of 200+.

What is Box 155 on the CT600?

Box 155 is your trading profits — the taxable profit from your company's trade after deducting allowable expenses and capital allowances. It's not the same as your accounting profit because some expenses (like client entertaining and depreciation) aren't deductible for tax, while capital allowances are claimed instead of accounting depreciation. See our detailed Box 155 guide.

Do I need to fill in supplementary pages?

Most small companies don't need any. The most common exception is CT600A if a director or shareholder has an outstanding loan from the company, and CT600E for registered charities. You do not need a supplementary page for marginal relief — it's claimed in box 435 on the main form. CT600L is only needed if you're claiming R&D relief.

How do I calculate marginal relief on the CT600?

Marginal relief applies to profits between £50,000 and £250,000 (adjusted for associated companies). The formula is: 3/200 × (£250,000 – Profits) × (Profits ÷ Augmented Profits). Claim it in box 435 on the main CT600 form — no supplementary page is needed. Or use Taxpipe, which calculates it automatically. See our marginal relief guide with calculation examples.

What's the difference between Box 145 and Box 155?

Box 145 is your total turnover (revenue/sales) — the total money your company earned from its trade before any costs. Box 155 is your taxable trading profit — turnover minus allowable expenses and capital allowances. Box 145 is the top line, box 155 is the tax-adjusted bottom line.

Where do I put bank interest on the CT600?

Bank interest goes in box 170 (bank, building society or other interest, and profits from non-trading loan relationships). It does NOT go in box 145 (turnover) or box 155 (trading profits) because bank interest is non-trading income. It is taxed at the same corporation tax rate, but it needs to be reported separately. Box 172 is a tick box for a specific deficit carry-back situation — leave it blank in normal circumstances.

Do I need to include a tax computation with my CT600?

Yes. Alongside the CT600 form, you must submit a tax computation (showing how you calculated your taxable profit) and iXBRL-tagged accounts (your profit and loss account and balance sheet). Taxpipe generates both automatically for micro-entity (FRS 105) companies.

Can I submit my CT600 without iXBRL accounts?

No. HMRC requires iXBRL-tagged accounts for all CT600 submissions filed online (which is now mandatory — paper filing was discontinued). Without iXBRL accounts, your submission will be rejected. Taxpipe generates micro-entity (FRS 105) iXBRL accounts automatically.

What accounting standard should I use for my CT600 accounts?

Most small companies use FRS 105 (the micro-entity standard) if they qualify, or FRS 102 Section 1A (the small company standard). FRS 105 is simpler and has fewer disclosure requirements. Taxpipe generates accounts under FRS 105; for FRS 102 Section 1A accounts, you can prepare them in other software and upload the iXBRL file. See our FRS 105 vs FRS 102 comparison.

How long does it take to fill in a CT600?

For a simple company doing it manually with proper software, expect 2–4 hours including preparing the accounts and tax computation. With Taxpipe, the same process takes about 15 minutes because we handle the calculations, iXBRL conversion, and submission automatically. Most of your time is spent entering your income and expense figures.

Let Taxpipe Fill In the Boxes for You

You now know which boxes matter and what goes in them. But knowing and doing are two different things — especially when iXBRL accounts are involved.

Taxpipe does the heavy lifting:

• ✅ Asks plain English questions — we translate your answers into the right CT600 boxes
• ✅ Calculates automatically — marginal relief, capital allowances, losses carried forward
• ✅ Generates iXBRL accounts — compliant with HMRC requirements, no extra software needed
• ✅ Files directly with HMRC — through your own Government Gateway account
• ✅ £59 flat fee — one price, everything included

You don't need to memorise box numbers. You don't need to understand iXBRL. You just need to know your company's income and expenses — and Taxpipe handles the rest.

File your CT600 now → | See pricing → | Calculate your tax first →

Related Reading

• Accounting Periods Explained — When Does Your Corporation Tax Year Start and End?
• HMRC Free Filing Ended March 2026 — What You Need to Know
• What Is a CT600? The Plain English Guide for Company Directors