Why 200+ Boxes?
The CT600 form covers every possible type of UK company — from dormant shell companies to multinational insurers. Most of those 200+ boxes don't apply to you.
A typical small company or contractor fills in about 20-30 boxes. This guide covers the ones that matter.
Company Information (Boxes 1-50)
| Box | Name | What Goes Here |
|---|---|---|
| 1 | Company name | Exactly as registered at Companies House |
| 2 | Company Registration Number | 8-digit CRN |
| 3 | Tax reference (UTR) | 10-digit Unique Taxpayer Reference |
| 4 | Company type | 0 for a normal trading company. Use other codes only for special types: 2 = close investment-holding company, 6 = members’ club or voluntary association, 8 = charity. Most small limited companies enter 0. |
| 30 | Period start | First day of accounting period (e.g., 2024-04-01) |
| 35 | Period end | Last day of accounting period (e.g., 2025-03-31) |
Common mistake: Getting the accounting period dates wrong. HMRC counts both start and end dates as inclusive. A period from 1 April to 31 March is exactly 365 (or 366) days. Don't overlap with previous periods.
Another common mistake: Entering the wrong company type code. A normal UK limited company enters 0 — meaning none of the special categories apply. Entering 6, for instance, would mis-declare your company as a members’ club or voluntary association, not a normal trading company.
Turnover (Boxes 145-155)
| Box | Name | What Goes Here |
|---|---|---|
| 145 | Total turnover from trade | All income from trading activities |
| 150 | Banks/financial concerns (tick-box) | Tick only if your company is a bank, building society, insurance company or other financial concern that has no recognised turnover and has not made an entry in box 145. Normal trading companies leave this blank. |
| 155 | Trading profits | Tax-adjusted profit after allowable expenses and capital allowances |
| 160 | Trading losses brought forward | Unused trading losses from earlier periods set against this year’s trading profits |
Box 145 is your top-line revenue — everything your company invoiced or earned from its trade.
Box 155 is your tax-adjusted trading profit. This is NOT the same as your accounting profit. It starts with accounting profit, then adds back disallowable expenses (depreciation, client entertaining) and deducts capital allowances. The result is your taxable trading profit. Example: accounting profit £50,000 + depreciation £3,000 − capital allowances £4,000 = taxable trading profit £49,000.
Box 160 is for trading losses from previous accounting periods that you want to set against this year’s trading profits from the same trade. If your company made a loss in the current period, that goes in box 275 (to claim it against total profits this year) or box 780 (to carry it forward to future years).
Tax Calculation (Boxes 235-440)
This is where corporation tax is actually calculated.
| Box | Name | What Goes Here |
|---|---|---|
| 235 | Profits before deductions and reliefs | Auto-calculated: net sum of trading profits, interest, property income, and gains |
| 275 | Trading losses (current period) | Current period trading losses claimed against total profits |
| 285 | Losses brought forward | Post-April 2017 losses carried forward, claimed against total profits |
| 305 | Qualifying donations | Cash gifts to registered charities or Community Amateur Sports Clubs (CASCs) |
| 315 | Total profits chargeable to CT | Final taxable profit — this drives the rate band |
| 326 | Associated companies (this period) | Number of associated companies for the accounting period |
| 330 | FY1 year | First financial year (e.g., 2024) |
| 335 | FY1 profit | Profit apportioned to first financial year |
| 340 | FY1 rate | Corporation tax rate for FY1 |
| 345 | FY1 tax | Tax calculated for FY1 |
| 380 | FY2 year | Second financial year (if period spans two FYs) |
| 385 | FY2 profit | Profit apportioned to second financial year |
| 390 | FY2 rate | Corporation tax rate for FY2 |
| 395 | FY2 tax | Tax calculated for FY2 |
| 430 | Corporation tax (gross) | Total CT before marginal relief |
| 435 | Marginal relief | Relief for profits between £50k–£250k |
| 440 | Corporation tax (net) | CT after marginal relief |
Box 315 is your total taxable profits — this determines which rate band you fall into.
Box 305 (qualifying donations) covers cash gifts to registered charities or CASCs. These reduce your taxable profits pound-for-pound. The amount cannot exceed box 300 (profits before donations and group relief).
Box 326 is the number of associated companies for the accounting period. Two companies are associated if the same person (or group of people) controls them both. This matters because the £50,000 and £250,000 rate thresholds are each divided by (associated companies + 1). If you only have one company, enter 0.
Box 435 (marginal relief) only applies if your profits are between £50,000 and £250,000 (adjusted for associated companies). Below £50,000, you pay 19%. Above £250,000, you pay 25%. In between, marginal relief reduces your effective rate.
Tax Payable (Boxes 510-600)
| Box | Name | What Goes Here |
|---|---|---|
| 510 | Tax chargeable | Total CT plus any additional charges (e.g. Section 455 tax on director loans) |
| 515 | Income Tax deducted from gross income | Income Tax already withheld at source from income received — acts as a credit |
| 525 | Self-assessment of tax payable | Box 510 minus box 515 — your CT self-assessment |
| 600 | Tax outstanding | Balance to pay after any tax already paid |
Box 510 is the total of all tax charges — for most small companies, this equals your basic Corporation Tax (box 475) plus any Section 455 tax on outstanding director loans (box 480).
Box 515 is not a tax charge — it is a credit for Income Tax already deducted at source from income your company received (for example, patent royalties received net of tax). This reduces your CT liability.
Box 525 is your self-assessed CT liability — box 510 minus box 515. This is the figure HMRC uses to determine what is owed.
Box 600 is the final amount to pay, after deducting any tax already paid in advance (box 595). This is the bottom line: what you owe HMRC for the period, due 9 months and 1 day after the period end.
Declaration
| Box | Name | What Goes Here |
|---|---|---|
| — | Name | Director’s full name |
| — | Status | “Director” |
| — | Declaration | Confirmation that the return is correct |
The declaration is signed by a company director (or authorised person). You’re confirming under penalty that the information is correct and complete.
Split Financial Years
If your accounting period spans two financial years (e.g., 1 January 2024 to 31 December 2024), profits are split between FY2023 (1 Jan – 31 Mar) and FY2024 (1 Apr – 31 Dec).
This matters because tax rates can differ between financial years. The split is done by number of days in each FY.
Your software handles this automatically — Taxpipe calculates the day-based apportionment and fills both FY1 and FY2 boxes.
Boxes You Can Usually Ignore
Unless they specifically apply to your company:
- Boxes 95-144: Supplementary pages (CT600A through CT600N)
- Boxes 480-509: CFC charges, bank levies, supplementary charges
- Boxes 530-615: R&D credits, creative industry credits
- Boxes 790-965: Overseas losses, NI information, nominee details
Let Software Handle the Boxes
The beauty of filing software is that you don’t need to know which box is which. Enter your financial data, and the software maps it to the correct boxes, calculates your tax, generates the iXBRL, and submits to HMRC.
Try Taxpipe’s free calculator to see your estimated tax, then file your CT600 for £59.