CT600 Box-by-Box Guide for Small Companies
The CT600 has over 200 boxes. For a typical small company, you only need about 20. Here's what each one means in plain English.
Company Information (Boxes 1-40)
| Box | What It Is | What to Enter |
|---|---|---|
| 1 | Company name | As registered at Companies House |
| 2 | Company registration number | 8-digit CRN (or SC/NI prefix + 6 digits) |
| 3 | Tax reference (UTR) | 10-digit number from HMRC |
| 4 | Type of company | "00" for most private limited companies |
| 30 | Start of accounting period | First day of the period |
| 35 | End of accounting period | Last day of the period |
Turnover and Profits (Boxes 145-315)
This is where the actual numbers go.
| Box | What It Is | Example |
|---|---|---|
| 145 | Total turnover | Your total sales/revenue |
| 155 | Trading profits | Profits from trading (after allowable expenses) |
| 160 | Losses brought forward | Trading losses from previous years used this year |
| 165 | Net trading profits | Box 155 minus Box 160 |
| 170 | Non-trading loan profits | Interest income, loan relationship profits |
| 190 | Property business income | Rental income (if company owns property) |
| 235 | Profits before deductions | Sum of all income types |
| 275 | Qualifying donations | Gift Aid payments |
| 285 | Trading losses of this period | If you made a loss (not a profit) |
| 315 | Total profits chargeable | The final taxable profit |
For most small companies: Box 145 = turnover, Box 155 = profit, Box 165 = Box 155, Box 315 = Box 165.
Tax Calculation (Boxes 330-440)
| Box | What It Is | What to Enter |
|---|---|---|
| 330 | Financial year 1 | e.g., "2024" for FY starting April 2024 |
| 335 | FY1 profit | Profit apportioned to FY1 |
| 340 | FY1 Corporation Tax rate | 19% (small), 25% (main), or marginal |
| 345 | FY1 tax | Box 335 × Box 340 |
| 380 | Financial year 2 | Only if period spans two financial years |
| 430 | Corporation Tax chargeable | Total CT before reliefs |
| 435 | Marginal relief | If profits between £50k-£250k |
| 440 | Corporation Tax net of relief | Box 430 minus Box 435 |
Marginal relief applies if your profits are between £50,000 and £250,000 (divided by associated companies + 1). Taxpipe calculates this automatically.
Payments and Returns (Boxes 470-525)
| Box | What It Is |
|---|---|
| 470 | Tax already paid (payments on account) |
| 475 | Tax outstanding (or repayable) |
| 510 | Tax repayable |
| 515 | Payable with this return |
| 525 | Tax you owe (or are owed) |
Declaration (Box 80)
The director signs a declaration that the return is correct and complete. This is a legal declaration — false statements can lead to prosecution.
Why You Should Use Software
You can see why filling this in manually would be error-prone. The calculations for marginal relief alone involve apportioning profits by days, prorating thresholds, and applying HMRC's specific rounding rules.
Taxpipe fills all these boxes automatically from the information you provide about your company. The guided wizard asks plain English questions — you never have to think about box numbers.
£59 per filing. No subscription. HMRC-validated.