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 section and the tax calculation section. Let's go through each critical box.

Section 1: Company Information (Boxes 1-40)

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.

Boxes 4-5: Accounting Period Start and End Dates

The start and end dates 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 or longer — 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 6: Notice to Deliver Reference

This is the reference number from HMRC's notice telling you to file. It's on the letter they sent you. If you're filing proactively (before receiving a notice), you may not have this — and that's fine, it's not mandatory.

Boxes 8-11: Type of Company

Checkboxes indicating what type of company you are:

• Box 8: Starting/ceasing — tick if this is your first or last return
• Box 9: Type — small, medium, large, or very large (most readers: small)
• Box 10: Close company — most small companies are "close companies" (5 or fewer shareholders, or directors who control it). If it's just you and perhaps a spouse as shareholders, you're a close company. Tick yes.
• Box 11: Charities/community amateur sports clubs — ignore unless applicable

What's a close company? Read close company rules and corporation tax explained.

Section 2: Turnover and Income (Boxes 145-165)

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 elsewhere), money you personally loaned to the company, VAT (if you're VAT registered — use the net figure).

Box 150: Other Income From Trade

Income that isn't from your main trade but is still trading income. For most small companies, this is £0.

Box 155: Trading Profits

This is a key box. It's your taxable trading profit — turnover minus allowable expenses. It's not the same as your accounting profit because certain expenses aren't allowable for tax purposes.

Common adjustments:

• Add back: Client entertaining, some depreciation, personal expenses put through the company
• Deduct: Capital allowances (which replace accounting depreciation for tax purposes)

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

Box 160: Trading Losses

If your expenses exceeded your income, enter the loss here. This is the flip side of Box 155 — you'll have either a trading profit or a trading loss, not both.

Box 165: Net Trading Profits

Your trading profit after deducting any losses brought forward from previous years. If this is your first year or you haven't had losses before, this will be the same as Box 155.

Section 3: Income From Other Sources (Boxes 170-205)

Box 170: Trading Profits From Non-Trading Loan Relationships

Income from loans your company has made — most small companies don't have this.

Box 172: Income From Non-Trading Loan Relationships (Including Bank Interest)

This is where bank interest goes. If your company's bank account earned interest during the period, declare it here. Even if it's only a few pounds.

Box 190: Income From Property

Rental income from property your company owns. If your company is a property investment company, this is where your main income goes. Note: this is rental income, not property trading profits (which would be in Box 155).

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

Box 205: Total Profits Before Deductions and Reliefs

This is the sum of all your income streams — trading profits, bank interest, property income, and anything else. It's the figure HMRC uses to determine your tax rate and calculate your bill.

Section 4: Capital Allowances (Boxes 490-500)

Instead of deducting accounting depreciation (which isn't tax-allowable), you claim capital allowances — HMRC's version of depreciation.

Box 490: Capital Allowances

The total capital allowances you're claiming for the period. This reduces your taxable profits. Common claims include:

• Annual Investment Allowance (AIA): 100% deduction on most plant and machinery up to £1 million per year
• Full Expensing: 100% deduction on qualifying plant and machinery (permanent from 2023)
• Writing Down Allowance: 18% or 6% per year for items not qualifying for AIA or full expensing
• Motor cars: Different rates depending on CO2 emissions (100% for electric, lower rates for others)

Detailed guide: Capital allowances for small companies

Most small companies can claim the AIA and get 100% relief on equipment, computers, machinery, and tools in the year of purchase. This is one of the most valuable tax reliefs available.

Box 495: Balancing Charges

If you sold an asset for more than its tax written-down value, you may have a balancing charge — effectively clawing back some of the capital allowances you previously claimed. Most small companies won't have this.

Section 5: Deductions and Reliefs (Boxes 530-600)

Box 530: Losses Brought Forward

Trading losses from previous accounting periods that you're using to reduce this year's profit. You can carry forward trading losses indefinitely and offset them against future trading profits.

Box 535: Non-Trade Loan Relationship Deficit

Losses from non-trading loan relationships. Most small companies: skip this.

Box 540: Management Expenses

Costs of managing investments (relevant for investment companies). If your company is a straightforward trading company, this doesn't apply.

Box 565: Qualifying Charitable Donations

Donations your company made to registered charities during the period. These are deducted from profits before calculating corporation tax. Note: the donations must be from the company, not from you personally.

More details: Corporation tax relief on charity donations

Section 6: Tax Calculation (Boxes 600-625)

This is where it all comes together.

Box 600: Profits Chargeable to Corporation Tax

Your total taxable profits after all deductions, losses, and reliefs. This is the number that gets taxed.

Box 605: Corporation Tax

The actual tax calculated on your profits. The rate depends on your profit level:

• Small profits rate (19%): Applies if profits are £50,000 or below
• Main rate (25%): Applies if profits are £250,000 or above
• Marginal relief: If profits are between £50,000 and £250,000, you pay an effective rate between 19% and 25%

The thresholds are divided by the number of associated companies plus one — so if you have associated companies, the bands are smaller.

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

Box 610: Marginal Relief

If your profits fall between £50,000 and £250,000, you're entitled to marginal relief — which reduces your tax from the 25% rate towards the 19% rate. The calculation is:

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

Don't worry about the formula — 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 620: Net Corporation Tax

Your corporation tax liability after all deductions — this is what you actually owe HMRC.

Box 625: Tax Already Paid

Any corporation tax you've already paid for this period (e.g., quarterly instalment payments for larger companies, or tax credits).

Section 7: Supplementary Pages (Boxes 700+)

Supplementary pages handle special situations. Most small companies only need one or two (if any):

CT600A: Loans to Participators (Box 700+)

You need this if: A director or shareholder owes the company money at the year-end (an overdrawn director's loan account). There's a special tax charge (Section 455 tax) of 33.75% on outstanding loans.

Important topic: Director's loan accounts and your CT600

CT600B: Controlled Foreign Companies

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

CT600C: Group Relief

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

Skip this unless your company is a registered charity.

CT600J: Research and Development

You need this if: You're claiming R&D tax credits. This is a valuable relief that many tech companies and innovative businesses should claim.

Worth investigating: R&D tax relief for SMEs

CT600L: Marginal Relief

You need this if: Your taxable profits are between £50,000 and £250,000. It's where you claim the marginal relief calculation.

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 about 60% of the form right away.

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)

Here's how the key boxes get filled:

* £85,000 – £15,000 expenses = £70,000. Note: the director's salary is part of the £15,000 expenses.

** Marginal relief: 3/200 × (£250,000 – £68,120) × (£68,120 / £68,240) ≈ £2,728

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 — so you meet HMRC's requirements
• We file with HMRC — so there's no fumbling with Government Gateway
• 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 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 on a CT600L supplementary page. If you don't, you'll pay the full 25% rate instead of the reduced effective rate. On £100,000 profit, the difference is about £2,250.

Mistake 4: Not Declaring Bank Interest

Even tiny amounts of bank interest need to be declared in Box 172. 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 1-11 (company information), Box 145 (turnover), Box 155 (trading profits), Box 172 (bank interest, if any), Box 205 (total profits), Box 490 (capital allowances), Box 600 (profits chargeable), Boxes 605-625 (tax calculation), and the declaration. That's roughly 15-20 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 but before capital allowances and other reliefs. It's not the same as your accounting profit because some expenses (like client entertaining and depreciation) aren't allowed for tax purposes, while others (like capital allowances) are added. See our detailed Box 155 guide.

Do I need to fill in supplementary pages?

Most small companies only need CT600L (for marginal relief, if profits are between £50,000 and £250,000) and possibly CT600A (if there's a director's loan). You don't need supplementary pages for group relief, controlled foreign companies, oil and gas, or creative industries unless those situations apply to you.

How do I calculate marginal relief on the CT600?

Marginal relief applies to profits between £50,000 and £250,000. The formula is: 3/200 × (£250,000 – Profits) × (Profits ÷ Augmented Profits). You claim it on supplementary page CT600L. Or use Taxpipe, which calculates it automatically and ensures you don't overpay. 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. Box 155 is your trading profit — turnover minus allowable expenses. Box 145 is the top line, Box 155 is the bottom line (adjusted for tax purposes).

Where do I put bank interest on the CT600?

Bank interest goes in Box 172 (income 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's taxed at the same corporation tax rate, but it needs to be reported separately.

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). These must be in the specific iXBRL format HMRC requires. Taxpipe generates all of these automatically.

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 handles iXBRL generation automatically — you don't need separate software.

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 the appropriate standard. 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 — instant electronic submission
• ✅ £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 →

Over 10,000 directors have used Taxpipe to file their CT600. Join them today →

Related Reading

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