CT600 Box by Box Guide: Every Section Explained in Plain English
The CT600 form has over 200 boxes. Most companies only need to fill in about 20-30 of them. But which ones?
This guide walks through every section of the CT600 in plain English — no jargon, no accounting degree required.
Before You Start
You'll need:
- Your company's accounts for the period (profit & loss, balance sheet)
- Your UTR (Unique Taxpayer Reference) — 10-digit number from HMRC
- Your Company Registration Number (CRN) from Companies House
- Your accounting period start and end dates
Use our filing checklist to make sure you have everything →
Section 1: Company Information (Boxes 1-35)
This section identifies your company.
| Box | What It Means | What to Enter |
|---|---|---|
| 1 | Company name | Exactly as registered at Companies House |
| 2 | Company registration number | 8-character CRN (e.g., 12345678) |
| 3 | Tax reference (UTR) | 10-digit number from HMRC |
| 30 | Start of accounting period | First day of the period this return covers |
| 35 | End of accounting period | Last day of the period |
Common mistake: Your accounting period for CT600 purposes might not match your financial year at Companies House. The CT600 period can't exceed 12 months. If your accounts cover more than 12 months, you'll need to file two CT600 returns.
Section 2: Type of Return (Boxes 40-55)
These boxes tell HMRC what kind of company you are and what's included.
| Box | What It Means |
|---|---|
| 40 | Amended return (tick if correcting a previous filing) |
| 45 | Accounts and computations attached (always yes) |
| 50 | iXBRL accounts attached (always yes if filing electronically) |
| 55 | iXBRL computations attached (always yes if filing electronically) |
For a standard first-time filing, you'll tick boxes 45, 50, and 55.
Section 3: Turnover and Income (Boxes 145-165)
This is where your headline numbers go.
| Box | What It Means | Example |
|---|---|---|
| 145 | Turnover from trade | Your sales/revenue (trading companies) |
| 155 | Total turnover | All income including non-trading |
| 160 | Trading losses brought forward | Losses from previous years you're using |
| 165 | Net trading profit | Turnover minus cost of sales minus expenses |
Box 155 is the big one — your total income. For a simple company, this is your sales figure from your profit and loss account.
Box 165 is your profit after deducting all business expenses. This feeds into the tax calculation.
Section 4: Income and Deductions (Boxes 170-235)
These boxes cover non-trading income and deductions.
| Box | What It Means |
|---|---|
| 170 | Trading profits (usually same as Box 165) |
| 172 | Trading losses from this period |
| 190 | Income from property |
| 200 | Non-trading loan relationship profits (interest received) |
| 205 | Non-trading loan relationship deficits (interest paid, non-trading) |
| 215 | Annual payments and patent royalties |
| 220 | Charitable donations |
| 225 | Other income |
| 235 | Profits before other deductions and reliefs |
Most small companies only fill in Boxes 155, 165, 170, and 235. The other boxes are for specific situations.
Section 5: Losses and Deficits (Boxes 240-290)
If your company made a loss, or you're using losses from previous years:
| Box | What It Means |
|---|---|
| 240 | Losses brought forward against trading profits |
| 250 | Trading losses carried back to previous period |
| 260 | Non-trade capital allowances |
| 275 | Trading losses carried forward (to use in future years) |
| 280 | Non-trade deficits carried forward |
| 285 | Qualifying charitable donations |
If your company is profitable and you have no losses to use, you can skip most of this section.
Read our guide to carrying losses forward and back →
Section 6: Tax Computation (Boxes 300-440)
This is where the actual tax is calculated. The key boxes:
| Box | What It Means |
|---|---|
| 300 | Profits before charges and group relief |
| 305 | Charges paid (qualifying charitable donations) |
| 310 | Group relief claimed |
| 315 | Profits chargeable to corporation tax |
| 330 | Financial year 1 (the FY your period starts in) |
| 335 | FY1 profit |
| 340 | FY1 tax rate |
| 345 | FY1 tax |
| 380-395 | Same boxes for Financial Year 2 (if your period spans two FYs) |
| 430 | Corporation tax chargeable |
| 435 | Marginal relief for ring fence trades |
| 440 | Net corporation tax chargeable |
Financial Year Splitting
If your accounting period spans 1 April (e.g., January to December), you need to split profits across two financial years. Each FY may have different tax rates.
Example: Period 1 Jan 2024 – 31 Dec 2024
- FY2023 (1 Jan – 31 Mar): 91 days
- FY2024 (1 Apr – 31 Dec): 275 days
- Profits split proportionally by days
Taxpipe handles FY splitting automatically →
Section 7: Tax Payable/Repayable (Boxes 475-525)
The final calculation of what you owe (or what HMRC owes you):
| Box | What It Means |
|---|---|
| 475 | Tax outstanding or overpaid |
| 480 | Tax payable (if you owe HMRC) |
| 485 | Tax repayable (if HMRC owes you) |
| 510 | Tax payable including any payable research credits |
| 515 | Amount of research and development credit payable |
| 520 | Amount of other reliefs payable |
| 525 | Total tax payable |
For most companies: Box 525 = the amount you need to pay HMRC.
Section 8: Supplementary Pages
Depending on your company, you might need additional pages:
| Page | When Needed |
|---|---|
| CT600A | Loans to participators (director's loan account overdrawn) |
| CT600B | Controlled foreign companies |
| CT600C | Group and consortium relief |
| CT600D | Insurance companies |
| CT600E | Charities and community amateur sports clubs |
| CT600F | Tax avoidance schemes |
| CT600J | Residential property developer tax |
Most small companies don't need any supplementary pages. The main CT600 form is sufficient.
Section 9: Declaration (Boxes 960-990)
The person signing the return:
| Box | What It Means |
|---|---|
| 960 | Date of declaration |
| 965 | Name of person making declaration |
| 970 | Their status (e.g., Director, Company Secretary) |
| 975 | Name of person signing |
| 980 | Date signed |
| 985 | Status of signatory |
The declaration confirms the return is correct and complete to the best of your knowledge. A director must sign — you can't delegate this to your accountant (though they can prepare the return).
Boxes Most Small Companies Fill In
For a typical small limited company with straightforward affairs:
- Boxes 1-3, 30-35 — Company details and period
- Boxes 45-55 — What's attached
- Box 155 — Total turnover
- Box 165 — Net trading profit
- Box 235 — Profits before deductions
- Box 300 — Profits before charges
- Box 315 — Profits chargeable to CT
- Boxes 330-345 — FY1 tax calculation
- Box 430 — CT chargeable
- Box 440 — Net CT chargeable
- Box 475 — Tax outstanding
- Box 510 — Tax payable
- Box 525 — Total tax payable
- Boxes 960-985 — Declaration
That's about 15-20 boxes out of 200+. Everything else is for specialist situations.
Let Taxpipe Do the Hard Part
Calculating FY splits, marginal relief, and filling in the right boxes is exactly what Taxpipe is built for. Answer simple questions about your company and income — we handle the CT600 boxes, iXBRL accounts, and HMRC submission.
£59 per filing. No subscription. No jargon.