CT600 Box-by-Box Guide: Every Core Box Explained (2025/26)

CT600 Box-by-Box Guide: Every Core Box Explained (2025/26)

The CT600 form has hundreds of boxes. Most guides skip straight to "the important ones" and leave you wondering what all the others are for. This guide covers every core box on the main CT600 — boxes 1 through 235 and 430 through 620 — so you know exactly what each one does and whether you need to fill it in.

If you're filing for a straightforward small company, you'll probably use about 15-20 of these boxes. Our DIY CT600 filing guide walks through the full process step by step. But knowing what the rest are for means you won't accidentally skip one that applies to you.

Before You Start: What You Need

Have these ready:

• Your company's accounts for the period (profit & loss and balance sheet)
• UTR (Unique Taxpayer Reference) and Company Registration Number
• Your accounting period start and end dates (check your CT600 filing deadline too)
• Details of any capital allowances, losses, or special claims

Company Information (Page 1)

These boxes identify your company and the return period. Most are pre-populated if you're filing via software.

Boxes 1–30: Turnover, Income, and Profits

This is the core income section. For most small companies, boxes 1, 3, and 21 are the only ones that need figures.

Box 1 — Turnover from the accounts

Your company's total turnover (revenue/sales) as shown in the accounts. This is gross income before deducting expenses. If your company is purely a trading company, this matches the top line of your profit & loss account.

Box 2 — Turnover from trade(s) included in box 1

If Box 1 includes turnover from more than one source (e.g., trading income plus other income), this box captures just the trading turnover. For most small companies with a single trade, Box 2 equals Box 1.

Box 3 — Trading profits

Your adjusted trading profit after deducting allowable expenses and capital allowances. This is not the same as your accounting profit — you need to add back disallowable expenses (entertaining, depreciation) and deduct capital allowances.

Box 4 — Trading losses brought forward against trading profits

Losses from previous periods used specifically against this period's trading profits (pre-April 2017 losses only). Most companies use Box 285 instead for post-April 2017 losses. For a full explanation of how to carry forward corporation tax losses and which boxes to use, see our losses guide.

Box 5 — Net trading profits

Box 3 minus Box 4. The trading profit after deducting any pre-2017 brought-forward losses.

Box 6 — Bank, building society, or other interest and profits from non-trading loan relationships

Interest your company earned on bank accounts, savings, or loans it has made. Even small amounts of bank interest go here.

Box 7 — Annual payments not otherwise charged to Corporation Tax

Rare for small companies. This covers specific types of annual payments received (not trading income, not interest).

Box 8 — Non-exempt dividends or distributions from non-UK companies

Dividends received from overseas companies. UK dividends are exempt and don't go here.

Box 9 — Income from non-trading loan relationships (Box 6 + Box 7)

The total of boxes 6 and 7. Calculated automatically by most software.

Box 10 — Non-trading gains on intangible fixed assets

Gains from selling or disposing of intangible assets (goodwill, patents, trademarks) where the disposal isn't part of a trade.

Box 11 — Tonnage tax profits

Only relevant for shipping companies in the tonnage tax regime. Leave blank.

Box 14 — Income from property business

Rental income from property owned by the company, after deducting allowable property expenses.

Box 16 — Gross chargeable gains

Capital gains from selling company assets (shares, property, equipment) before deducting capital losses.

Box 17 — Allowable losses (including losses brought forward)

Capital losses to set against Box 16. This includes current-year capital losses and any brought forward from earlier periods.

Box 18 — Net chargeable gains (Box 16 minus Box 17)

The taxable capital gain after deducting allowable losses.

Box 21 — Profits before deductions and reliefs

The total of all income and gains: trading profits + non-trading income + property income + chargeable gains. This is your total profits before any further deductions.

This is a key box. Corporation tax is ultimately calculated on the figure that flows from this total after applying reliefs and deductions.

Box 24 — Non-trade deficits on loan relationships

Losses on non-trading loans or interest arrangements. Rarely used by small companies.

Box 25 — Losses on unquoted shares

Specific relief for losses on shares in unquoted trading companies. Very niche.

Box 26 — Management expenses

Costs of managing investments (for investment companies, not trading companies). Most trading companies leave this blank.

Box 27 — UK property business losses for this or previous periods

Losses from UK rental property that can be set against total profits.

Box 28 — Capital allowances for management of investment business

Capital allowances specifically for companies managing investments.

Box 29 — Total deductions from profits

The sum of boxes 24 through 28.

Box 30 — Profits chargeable to Corporation Tax

Box 21 minus Box 29. This is the figure corporation tax is calculated on (before reliefs in boxes 31–35).

Boxes 31–40: Tax Calculation

Box 31 — Profits matched with non-corporate distributions (FrankedInvestmentIncome)

Technical matching of profits against distributions for determining tax rates. Automatically calculated.

Box 32 — Profits matched against non-corporate distributions (ring-fence)

Ring-fence profits matched against distributions. Oil and gas companies only.

Box 35 — Charity donations/qualifying charitable donations

Total qualifying charitable donations made by the company. These reduce your taxable profits.

Box 37 — Corporation Tax chargeable

The corporation tax calculated on your profits. For 2025/26:

• 19% on profits up to £50,000
• 25% on profits over £250,000
• Marginal relief for profits between £50,000 and £250,000

Box 38 — Marginal relief for ring-fence trades

Ring-fence marginal relief (oil and gas only). Not relevant for most companies.

Box 39 — Corporation Tax chargeable less marginal/ring-fence relief

Box 37 minus any ring-fence relief. For most companies, this equals Box 37.

Box 40 — Net Corporation Tax

Corporation tax after all reliefs. For most small companies, this is the bottom-line tax figure.

Boxes 45–65: Reliefs and Deductions from Tax

These boxes reduce the amount of tax you pay (not profits — actual tax).

Box 45 — Community investment tax relief

Relief for investments in accredited Community Development Finance Institutions.

Box 50 — Double taxation relief

Relief to avoid being taxed twice on overseas income. Claim here if you've paid foreign tax on income also taxable in the UK.

Box 55 — Advance Corporation Tax (historical)

ACT was abolished in 1999. Shadow ACT claims may still appear for very old companies.

Box 60 — Total reliefs and deductions

Sum of boxes 45 through 55.

Box 65 — Net Corporation Tax liability

Box 40 minus Box 60. This is what you owe HMRC.

Boxes 70–90: Tax Payable and Repayable

Box 70 — Tax payable

The amount of corporation tax you need to pay. Usually equals Box 65 unless there are instalments or earlier payments.

Box 75 — Self-assessment of tax payable

Your self-assessed tax liability. Should match Box 70 in most cases.

Box 80 — Tax repayable

If you've overpaid (e.g., from carry-back claims or excessive payments on account), the refund amount goes here.

Box 85 — Tax already paid (includes R&D credit)

Corporation tax already paid for this period, including any R&D tax credits.

Box 90 — Tax outstanding

Box 70 minus Box 85. The remaining amount still to pay.

Boxes 95–145: Supplementary Information

Box 95 — Associated companies for this period

The number of associated companies (companies under common control). This affects the thresholds for the small profits rate and marginal relief. If you have no associated companies, enter 0 or leave blank.

Why this matters: The £50,000 small profits threshold is divided by (1 + number of associated companies). Two associated companies means the threshold drops to £16,667 each.

Box 100 — Associated companies for the previous period

Same as Box 95 but for the prior accounting period.

Box 110 — Quarterly instalment payments

Whether the company pays tax in quarterly instalments (for companies with profits over £1.5 million). Most small companies pay in one lump sum after the period end.

Box 145 — Tax chargeable

The total corporation tax charged for the period. A key summary box.

Box 155 — Tax outstanding

The final amount of tax still owed.

Boxes 160–235: Reliefs, Allowances, and Adjustments

Box 160 — R&D enhanced expenditure

The enhanced amount of qualifying R&D expenditure. Only for companies claiming R&D relief.

Box 165 — R&D SME payable credit/RDEC credit

R&D tax credits payable to the company. Enter the credit amount.

Box 170 — Creative industry enhanced expenditure

Enhanced expenditure for creative industry tax reliefs (film, TV, video games, etc.).

Box 175 — Creative industry payable credit

Tax credits for creative industries.

Box 190 — Group relief claimed

Losses surrendered from other group companies. Only for company groups.

Box 195 — Net chargeable gains

Cross-reference with Box 18. The net capital gains figure.

Box 200 — Qualifying donations

Total qualifying charitable donations — feeds into Box 35.

Box 205 — Community investment scheme

Details of community investment scheme participation.

Box 210 — CIR — Corporate Interest Restriction

Restriction on interest deductions for large companies. Rarely affects small companies.

Box 215 — Annual investment allowance

Whether you're claiming the AIA for capital expenditure. Most small companies with equipment purchases tick yes.

Box 220 — Losses, deficits, and excess amounts

Summary of losses across all categories.

Box 225 — Carried forward non-trading losses on intangible fixed assets

Non-trading intangible asset losses brought forward.

Box 230 — Carried-forward non-trading loan relationship deficits

Brought-forward losses on non-trading loans.

Box 235 — Carried-forward property losses

Property business losses carried forward from previous periods.

Boxes 430–620: Tax Computation

This section is effectively your tax computation — the detailed breakdown that supports the figures in boxes 1–40.

Box 430 — Turnover/sales

Total turnover from trading. Should match Box 1 for a single-trade company.

Box 435 — Trading profits/losses from this period

The detailed trading profit or loss. If a loss, enter a negative figure (or the loss amount, depending on the software). This feeds into Box 3 (profit) or Box 275 (loss).

Box 440 — Expenses of management of investment business

Management expenses for investment companies. Leave blank for trading companies.

Box 445 — Income from property

Gross rental income before property expenses.

Box 450 — Non-trading gains on intangible fixed assets

Matches Box 10 — gains from selling intangible assets outside a trade.

Box 455 — Tonnage tax profits

Shipping companies only.

Box 460–490 — Loan relationship and non-trading income

Detailed breakdown of interest income, loan relationship profits, and other non-trading income. Box 460 covers non-trading loan relationship profits. Box 470 relates to non-trading loan relationship deficits.

Box 495 — Qualifying donations

Amount of charitable donations being claimed as a deduction.

Box 500–530 — Capital gains details

Detailed capital gains information including disposal proceeds, costs, indexation, and losses.

Box 535 — Group relief claimed

Amount of group relief being claimed.

Box 540 — Group relief for carried-forward losses

Brought-forward losses surrendered via group relief.

Box 545 — Losses brought forward against total profits

Post-April 2017 losses brought forward and used against total profits this period.

Box 550 — Non-trading loan relationship deficits from previous periods

Historical non-trading loan relationship deficits claimed this period.

Box 555 — Management expenses brought forward

Management expenses from previous periods claimed now.

Box 560–580 — Further adjustments

Various adjustments for specific situations — ring-fence trades, tonnage tax adjustments, property business surplus, and other technical adjustments.

Box 585 — Total profits

The total of all taxable profits after all adjustments and reliefs. This should equal Box 30.

Box 590 — Corporation tax rate

The applicable tax rate. For 2025/26, this is 19% or 25% depending on profit level (with marginal relief between £50,000 and £250,000).

Box 600 — Corporation tax

The tax calculated at the applicable rate(s).

Box 605 — Marginal relief

The amount of marginal relief, if applicable. This reduces the tax for companies with profits between £50,000 and £250,000.

Box 610 — Net Corporation Tax

Box 600 minus Box 605. Your net corporation tax liability.

Box 615 — Tax reliefs

Total reliefs deducted from the tax amount (double taxation relief, community investment relief, etc.).

Box 620 — Corporation Tax chargeable

The final corporation tax payable after all reliefs. This is the headline number — what you owe HMRC.

Which Boxes Do Most Small Companies Actually Use?

For a typical small limited company (single trade, no property, no investments, no group), here's the realistic list:

That's about 16 boxes. Everything else stays blank.

Filing Tips

Don't panic about blank boxes

The CT600 is designed for every type of company — from single-director micro-entities to multinational corporations. Most boxes don't apply to you. Blank boxes are normal and expected.

Cross-check your figures

Several boxes should contain the same (or closely related) figures:

• Box 1 should match Box 430
• Box 3 should match Box 435 (if profitable)
• Box 21 should relate to Box 585
• Box 37/40/65/70 should all be close to each other for simple returns

If these figures don't reconcile, something's wrong.

Use software

Filing software like Taxpipe calculates the box cross-references automatically. Enter your income and expenses in the guided wizard, and the boxes are populated correctly — including the ones you didn't know existed. It costs £59 per filing with direct HMRC submission.

Frequently Asked Questions

Do I need to fill in every box on the CT600?

No. Most small companies use fewer than 20 boxes. Leave irrelevant boxes blank — HMRC expects this for simple returns.

What's the difference between Box 3 and Box 435?

Box 3 is in the summary section (page 1–2 of the CT600) and Box 435 is in the computation section (the detailed tax calculation). They should show the same trading profit figure.

What goes in Box 1 — gross turnover or net?

Gross turnover (total sales/revenue before deducting any expenses). If you're VAT-registered, enter the VAT-exclusive figure.

Why do boxes 37, 40, 65, and 70 all seem to show the same number?

For a simple return with no special reliefs, they do. Each box applies a different layer of relief or adjustment. When none of those apply, the same figure flows through all of them.

What if I have bank interest — which box?

Bank interest goes in Box 6 (interest and non-trading loan relationship profits). It then feeds into Box 9 and ultimately into Box 21 (total profits). See our full guide: Corporation Tax on Bank Interest: How It Works.

Don't want to figure out 600+ boxes manually? File with Taxpipe — our guided wizard asks simple questions and fills in every box for you. Direct HMRC submission, just £59.

Related Articles

• CT600 Box by Box Guide: Every Section Explained in Plain English
• CT600 Supplementary Pages Explained: Which Ones Does Your Company Need?
• Filing a CT600 for a Dormant Company — Complete Guide
• Directors' Loan Accounts and Your CT600 — What Every Director Needs to Know