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 | Usually "6" for a non-close investment holding company, or another code |
| 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.
Turnover (Boxes 145-155)
| Box | Name | What Goes Here |
|---|---|---|
| 145 | Total turnover | All income from trading activities |
| 150 | Other financial concerns | Income from non-trading activities |
| 155 | Trading profits | Turnover minus allowable expenses |
| 160 | Trading losses | If expenses exceed income |
Box 145 is your top-line revenue — everything your company invoiced or earned.
Box 155 is your profit after deducting all allowable business expenses (salaries, rent, supplies, travel, etc.). This is the number that drives your tax calculation.
Tax Calculation (Boxes 235-440)
This is where corporation tax is actually calculated.
| Box | Name | What Goes Here |
|---|---|---|
| 235 | Charges paid | Qualifying charitable donations |
| 275 | Trading losses | Current period trading losses |
| 285 | Losses brought forward | Losses carried forward from previous periods |
| 315 | Total profits | Sum of all income minus deductions |
| 326 | Associated companies (FY1) | Number of associated companies in first financial year |
| 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 435 (marginal relief) only applies if your profits are between £50,000 and £250,000. Below £50,000, you pay 19%. Above £250,000, you pay 25%. In between, marginal relief reduces your effective rate.
Tax Payable (Boxes 510-515)
| Box | Name | What Goes Here |
|---|---|---|
| 510 | Tax chargeable | Corporation tax after all reliefs |
| 515 | Tax payable | Final amount you owe HMRC |
Box 515 is the bottom line — this is what you pay.
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 95-146: Supplementary pages (CT600A through CT600N)
- Boxes 480-527: 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.