CT600 Box by Box Guide 2025/26: Every Box Explained

The CT600 has over 200 boxes. Most companies only need to fill in about 30. Here's what goes in each section, so you know exactly what to enter.

Company Information (Boxes 1-80)

These identify your company and the accounting period.

Box	What It Is	Example
1	Company name	ABC Trading Ltd
2	Company registration number	12345678
3	Tax reference (UTR)	1234567890
30	Period start date	01/04/2024
35	Period end date	31/03/2025
40	Repayment due indicator	Tick if HMRC owes you money for this period
55	Estimated figures indicator	Tick if any figures in this return are estimates
80	Accounts attached indicator	Tick to confirm accounts and computations are attached

Box 40 — tick this if you expect a repayment for this period. This happens when your company overpaid Corporation Tax, has R&D credits, or had Income Tax deducted at source from income it received. If you are simply paying the normal CT bill, leave it blank.

Box 55 — tick if you have used any estimated figures because your accounts are not yet finalised. HMRC may ask you to amend the return later with final numbers. Most small companies have final figures and leave this blank.

Box 80 — tick to confirm you are submitting accounts and tax computations for the same period as this return. This is the normal case — almost every company ticks this box.

Common mistake: Getting the UTR wrong. Box 3 is your 10-digit Unique Taxpayer Reference, not your company number. Find it on any HMRC letter or your Government Gateway account.

Income (Boxes 145-220)

The core of your return — how much did the company make?

Box	What It Is	Notes
145	Total turnover from trade	Your company's total sales revenue before any expenses
155	Trading profits	Tax-adjusted profit after capital allowances and adjustments
160	Trading losses brought forward	Old trading losses used against this year's trading profit (don't enter more than box 155)
165	Net trading profits (155 minus 160)	Calculated automatically
170	Bank/loan interest and non-trading loan relationship profits	Bank interest earned and other non-trading financial income, net of non-trading interest paid
172	Deficit carried-back indicator	Tick if box 170 is net of a non-trading deficit carried back from a later period
175	Annual payments not otherwise charged to CT	Rare — patent royalties to individuals and similar; most companies leave blank
190	Income from a property business	Net rental income after allowable expenses
210	Gross chargeable gains	Total profit from selling capital assets (property, shares, equipment above tax written-down value)
215	Allowable losses	Capital losses deducted from box 210 — cannot exceed box 210
220	Net chargeable gains (210 minus 215)	Calculated automatically

Box 155 is your main trading profit. Start with your accounting profit and adjust: add back depreciation and any disallowable expenses (client entertaining, private expenses), then deduct capital allowances. The result is your taxable trading profit — different from your accounting profit. Example: accounting profit £50,000 + depreciation £3,000 − capital allowances £4,000 = taxable trading profit £49,000.

Box 170 is for bank interest and other non-trading loan relationship income. For most small companies, this is just the interest your business account earned. Don't include trading interest (e.g. interest charged to clients on overdue invoices) — that goes in box 155.

Box 190 is for net rental income. If your company owns rental property: rental income minus allowable expenses (mortgage interest, repairs, insurance, letting agent fees). If the property business made a loss, report it in box 805 instead.

Boxes 210–220 are for capital gains. Capital assets include property, shares in other companies, and equipment sold for more than its tax written-down value. Do not use these boxes for selling stock or inventory — that is trading income in box 155.

Capital Allowances (Boxes 688–730)

Tax relief for buying equipment, vehicles, and other assets. Capital allowances reduce your taxable trading profit (box 155) and are reported in the capital allowances section of the CT600.

Box	What It Is	Notes
688	Full expensing — allowances	100% write-off on new plant and machinery (permanent since April 2023)
690	Annual Investment Allowance (AIA)	100% deduction up to £1,000,000 per year on most qualifying equipment
705	Main pool — writing-down allowance	18% per year on plant and machinery in the main pool
695	Special rate pool — writing-down allowance	6% per year on integral building features, long-life assets, and high-emission cars

For most small companies, the Annual Investment Allowance (AIA) in box 690 covers all capital purchases up to £1 million per year at 100%. Qualifying items include computers, office furniture, tools, machinery, and commercial vehicles (not cars). If you bought brand-new equipment or machinery after April 2023, you can also claim full expensing (box 688) for 100% relief with no annual limit — this is now permanent.

Capital allowances feed directly into your taxable profit calculation: the allowances you claim are deducted in arriving at the box 155 figure you enter.

Tax Calculation (Boxes 300-440)

HMRC calculates your Corporation Tax here. If your period spans two financial years (e.g., 1 January to 31 December), the profit is split across both years based on the number of days in each.

Box	What It Is
300	Total profits before qualifying donations and group relief
315	Total profits chargeable to CT — your final taxable profit
330/380	Financial year (e.g. 2024 means 1 April 2024 to 31 March 2025)
335/385	Amount of profit allocated to that financial year
340/390	Rate of tax for that financial year (e.g. 19% or 25%)
345/395	Tax calculated (profit × rate)
430	Total Corporation Tax before marginal relief
435	Marginal relief (if applicable)
440	Corporation Tax net of marginal relief

Financial year apportionment: CT financial years run from 1 April to 31 March and are named by the year they start (FY2024 = April 2024 to March 2025). If your accounting period sits within a single financial year, all your profit goes in boxes 335 and 345. If it straddles two financial years — for example a January to December year — your profit is apportioned by the number of days in each: a period of 1 January to 31 December 2025 puts 90 days (January to March) in FY2024 at boxes 335/340/345, and 275 days (April to December) in FY2025 at boxes 385/390/395.

Tax rates for FY2023 onwards: 19% if profits (adjusted for associated companies) are £50,000 or less; 25% if above £250,000. Between those thresholds, you pay 25% but claim marginal relief (box 435), which reduces your effective rate towards 19%.

Marginal relief (box 435): applies when your taxable profits fall between £50,000 and £250,000 (divided by associated companies + 1). The relief is calculated using the standard fraction 3/200. Taxpipe calculates this automatically.

Tax Payable (Boxes 510-595)

Box	What It Is
510	Tax chargeable — total tax your company owes (CT plus any additional charges)
515	Income Tax deducted from gross income received — a credit against your CT bill
520	Income Tax repayable — calculated automatically if box 515 exceeds box 510
525	Self-assessment of tax payable — box 510 minus box 515 (enter 0 if 515 exceeds 510)
528	Final self-assessment total (boxes 525 plus any COVID-scheme overpayments and restitution tax)
595	Tax already paid — enter any Corporation Tax already paid for this period

Box 515 is a credit, not a payment. If your company received income where the payer deducted Income Tax before paying you (e.g. certain patent royalties), the tax withheld acts as a credit against your CT bill. You enter the gross amount in the relevant income box and the tax deducted here. Most small companies leave this blank.

Box 595 is for tax already paid for the accounting period. Most small companies pay their Corporation Tax in a single payment 9 months and 1 day after the period ends, so this box is blank when filing. It would only contain a figure if your company made advance voluntary payments or is a large company paying quarterly instalments.

Repayment (Boxes 600-940)

If your company has overpaid tax or has payable credits:

Box	What It Is
600	Tax outstanding — the amount your company still owes
605	Tax overpaid — the amount HMRC will repay to you
610	Group tax refunds surrendered to this company (only relevant if part of a group)

Boxes 600 and 605 are calculated automatically. Only one will have a figure — either you owe money (box 600) or HMRC owes you (box 605).

Bank details for any repayment go in boxes 920–940: bank or building society name (box 920), sort code (box 925), account number (box 930), and account name (box 935). Fill these in whenever box 605 has a figure to avoid delays in receiving your repayment.

Declaration (Boxes 975-985)

Box	What It Is
975	Signatory name
980	Date of declaration
985	Status (e.g. Director, Company Secretary)

The person signing declares the return is correct and complete. This is usually a company director. Box 980 records the date you are making the declaration, and box 985 confirms your role — for most small companies this will be "Director".

Filing with Taxpipe

Taxpipe fills in all these boxes automatically based on your answers to simple questions. You don't need to know which box is which — we handle the mapping.

Related: Corporation Tax rates for 2025/26

Ready to file? Start your CT600 with Taxpipe — £59, every box calculated and validated for you.

CT600 Box by Box Guide 2025/26: Every Box Explained