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 | Return type (1=new, 2=amended) | 1 |
| 55 | Franked investment income | Usually 0 |
| 80 | Declaration (1=yes) | 1 |
Common mistake: Getting the UTR wrong. This is your 10-digit Unique Taxpayer Reference, not your company number. Find it on any HMRC letter or your Government Gateway account.
Trading Profits (Boxes 145-175)
The core of your return — how much did the company make?
| Box | What It Is | Notes |
|---|---|---|
| 145 | Net trading profits | Turnover minus allowable expenses |
| 150 | Net trading losses | If you made a loss (don't enter negative in box 145) |
| 155 | Net chargeable gains | Profits from selling assets |
| 160 | Net chargeable losses | Losses from selling assets |
| 165 | Non-trading loan relationship profits | Bank interest, loan interest received |
| 170 | Non-trading loan relationship deficits | Interest paid on non-trading loans |
| 172 | Income stated net of income tax | UK income taxed at source |
| 175 | Property business income | Rental income from properties |
Key point: Box 145 is your main trading profit. For most small companies, this is turnover minus expenses from your accounts. Don't include VAT, dividends received, or bank interest here.
Capital Allowances (Boxes 230-245)
Tax relief for buying equipment, vehicles, and other assets.
| Box | What It Is | Notes |
|---|---|---|
| 230 | Capital allowances claimed | Annual Investment Allowance, WDA, FYA |
| 235 | Total income | Sum of trading + other income |
For most small companies, the Annual Investment Allowance (AIA) covers all capital purchases up to £1 million per year at 100%.
Tax Calculation (Boxes 300-440)
HMRC calculates your Corporation Tax here. If your period spans two financial years (e.g., 1 April to 31 March), the profit is split across both years.
| Box | What It Is |
|---|---|
| 300 | Total profits before deductions |
| 315 | Total profits chargeable to CT |
| 330/380 | Financial year |
| 335/385 | Number of days in FY |
| 340/390 | FY profit (apportioned) |
| 345/395 | Tax rate for that FY |
| 430 | Corporation Tax (before reliefs) |
| 435 | Marginal relief (if applicable) |
| 440 | Corporation Tax net of marginal relief |
Marginal relief: If your profits are between £50,000 and £250,000, marginal relief reduces your effective tax rate from 25% towards 19%. This is calculated automatically by Taxpipe.
Tax Payable (Boxes 510-530)
| Box | What It Is |
|---|---|
| 510 | Tax chargeable |
| 515 | Tax payable (after deductions) |
| 520 | Tax outstanding |
| 525 | Tax payable |
| 528 | Tax already paid |
Repayment (Boxes 600-615)
If you've overpaid tax (e.g., via CIS deductions or R&D credits):
| Box | What It Is |
|---|---|
| 600 | Tax overpaid |
| 605 | Repayment due |
| 610-615 | Bank details for repayment |
Declaration (Boxes 975-990)
| Box | What It Is |
|---|---|
| 975 | Signatory name |
| 980 | Signatory status |
| 985 | Date |
| 990 | Declaration tick |
The person signing declares the return is correct and complete. This is usually a company 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: company capital gains and Corporation Tax
Related: Corporation Tax rates for 2025/26
Ready to file? Start your CT600 with Taxpipe — £59, every box calculated and validated for you.