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 | "0" 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 | Tax-adjusted trading profit — accounting profit with depreciation added back and capital allowances deducted |
| 160 | Trading losses brought forward | Losses from earlier years used against this period's trading profit |
| 165 | Net trading profits | Box 155 minus Box 160 |
| 170 | Non-trading interest income | Bank interest and non-trading loan relationship profits |
| 190 | Property business income | Net rental income (if the company owns property) |
| 235 | Profits before deductions | Sum of all income types |
| 275 | Total trading losses of this period | Amount of trading loss made this period, claimed against total profits |
| 285 | Carried-forward losses against total profits | Post-2017 losses from earlier years set against total profits (not just trading profits) |
| 305 | Qualifying donations | Gift Aid and other qualifying charitable payments |
| 315 | Profits chargeable to Corporation Tax | The final taxable profit |
For most small companies: Box 145 = turnover, Box 155 = taxable trading profit, Box 165 = Box 155 (no losses brought forward), Box 315 = Box 165 (no other income or deductions).
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 profits rate) or 25% (main rate) |
| 345 | FY1 tax | Box 335 × Box 340 |
| 380 | Financial year 2 | Only if period spans two financial years |
| 430 | Corporation Tax (before marginal relief) | Sum of tax from all financial year rows |
| 435 | Marginal relief | Deduction if profits are between £50k–£250k |
| 440 | Corporation Tax chargeable | 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 Amounts Due (Boxes 475–605)
| Box | What It Is |
|---|---|
| 475 | Net Corporation Tax liability — CT after any double taxation or other tax reliefs |
| 510 | Total tax chargeable — includes CT plus any S455 tax on loans to directors |
| 525 | Self-assessment of tax payable — the key "amount owed" figure for this return |
| 595 | Tax already paid — any payments on account made during the year |
| 600 | Tax outstanding — still owed after deducting payments already made |
| 605 | Tax overpaid — refund due if you have paid too much |
Declaration (Boxes 975–985)
The director (or authorised signatory) signs the declaration that the return is correct and complete. You enter the declarant's name (box 975), date of declaration (box 980), and their status — for example, "Director" (box 985). This is a legal declaration — false statements can lead to prosecution.
Note: Box 80 is separate — it is the checkbox confirming that you are attaching accounts and computations for the same period as the return.
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.