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: About This Return (Boxes 40-90)
These boxes flag special circumstances about the return itself. For a standard filing, most of them stay blank.
| Box | What It Means |
|---|---|
| 40 | A repayment is due for this return period — tick if you expect HMRC to owe your company money |
| 45 | Claim or relief affecting an earlier period — tick if you're carrying a loss back to a previous year |
| 50 | Making more than one return for this company now — tick only if filing multiple returns simultaneously |
| 55 | This return contains estimated figures — tick only if any figures are estimates, not final |
| 80 | Accounts and computations for this period are attached — tick for a standard filing |
| 85 | Accounts and computations for a different period are attached — use instead of box 80 if your accounts cover a different period |
| 90 | If not attaching accounts and computations, explain why |
For a standard first-time filing, you'll tick box 80 (accounts and computations attached for this period). Boxes 40, 45, 50, and 55 are each for specific circumstances: ticking box 40 tells HMRC to expect a repayment; box 45 means you're carrying a loss back to a prior year; box 50 means you're filing multiple returns at once; box 55 means some figures are estimates. A straightforward filing leaves all four blank.
Section 3: Turnover and Trading Profits (Boxes 145-165)
This is where your headline trading numbers go.
| Box | What It Means | Example |
|---|---|---|
| 145 | Total turnover from trade | Your sales/revenue figure from the profit and loss account |
| 155 | Trading profits | Your tax-adjusted trading profit — not the same as turnover or accounting profit |
| 160 | Trading losses brought forward set against trading profits | Old losses from previous years being applied this year |
| 165 | Net trading profits (box 155 minus box 160) | Trading profit after any brought-forward losses |
Box 145 is your company's total trading turnover — the top-line revenue figure before any deductions.
Box 155 is your tax-adjusted trading profit. This is not your turnover or your accounts profit. Start with accounting profit, add back disallowable expenses (depreciation, client entertaining, fines), then deduct capital allowances. The result is the taxable trading profit. It can differ substantially from the profit and loss figure in your accounts.
Box 165 is what remains after using any brought-forward trading losses (box 160). This feeds into the rest of the return.
Section 4: Other Income and Gains (Boxes 170-235)
These boxes cover income from sources other than your main trade.
| Box | What It Means |
|---|---|
| 170 | Bank, building society or other interest, and profits from non-trading loan relationships |
| 175 | Annual payments not otherwise charged to corporation tax (without income tax deducted) |
| 180 | Non-exempt dividends or distributions from non-UK resident companies |
| 185 | Income from which income tax has been deducted |
| 190 | Income from a property business (rental income, net of allowable expenses) |
| 205 | Income not falling under any other heading (miscellaneous taxable income) |
| 210 | Gross chargeable gains (total capital gains before losses) |
| 215 | Allowable losses including losses brought forward |
| 220 | Net chargeable gains (box 210 minus box 215) |
| 235 | Profits before other deductions and reliefs (the running subtotal) |
Box 170 is for interest your company earned on bank accounts and other non-trading loan relationship credits — for most small companies this is simply bank interest received.
Box 190 is net rental income from property the company owns. Enter the amount after deducting allowable costs (repairs, insurance, agent fees); if the property business made a loss, enter the loss in box 805 instead.
Boxes 210–220 apply when the company sold a capital asset — for example, a commercial property, shares in another company, or goodwill. Enter the gross gains in box 210, deduct allowable losses in box 215, and the taxable net gain flows to box 220.
Most small trading companies only fill in box 170 (bank interest) and possibly box 190 (rental income). Box 235 is the subtotal of all income.
Section 5: Losses and Deductions (Boxes 240-315)
This section reduces your taxable profits through losses, specific reliefs, and charitable donations.
| Box | What It Means |
|---|---|
| 240 | Losses on unquoted (unlisted) shares — Share Loss Relief under s.68 CTA 2010 |
| 250 | UK property business losses for this or a previous accounting period |
| 260 | Non-trade deficits on loan relationships for this period (e.g. net interest paid on non-trading borrowings) |
| 263 | Carried-forward non-trade deficits on loan relationships |
| 275 | Total trading losses of this or a later period set against total profits |
| 280 | Tick if box 275 includes amounts carried back from a later period |
| 285 | Old trading losses (post-April 2017) carried forward and claimed against total profits |
| 290 | Non-trade capital allowances |
| 295 | Total of all deductions and reliefs |
| 300 | Profits before qualifying donations and group relief (box 235 minus box 295) |
| 305 | Qualifying charitable donations paid by the company |
| 310 | Group relief |
| 315 | Profits chargeable to corporation tax |
Box 275 is for trading losses your company made this period that you are setting against total profits. It can also include losses carried back from a future period — tick box 280 in that case. Box 285 is for earlier-year trading losses (post-April 2017) being carried forward and used against all income now.
Box 305 is for genuine charitable donations to registered charities or Community Amateur Sports Clubs — not for sponsorship or services, which are normal business expenses.
If your company is profitable with no losses to claim and no charitable donations, you can skip most of this section. Box 315 will simply equal box 235.
Read our guide to carrying losses forward and back →
Section 6: Tax Calculation (Boxes 326-440)
This is where the actual tax is calculated. The key boxes:
| Box | What It Means |
|---|---|
| 326 | Number of associated companies in the period |
| 329 | Tick if the company qualifies for the small profits rate or marginal relief |
| 330 | Financial year 1 |
| 335 | FY1 amount of profit |
| 340 | FY1 rate of tax (%) |
| 345 | FY1 tax |
| 380–425 | The same profit/rate/tax rows for Financial Year 2 (if the period spans two FYs) |
| 430 | Corporation tax — total of all FY tax rows |
| 435 | Marginal relief |
| 440 | Corporation tax chargeable (box 430 minus box 435) |
Associated Companies and Box 326
Box 326 is the number of companies associated with yours during the period (do not include your own company in the count). Two companies are associated if the same person or group controls both. This matters because the small profits rate thresholds — £50,000 and £250,000 — are divided by (associated companies + 1). A single standalone company uses the full thresholds; a company with two associates uses only a third.
Marginal Relief and Box 435
Box 435 is the marginal relief deduction. If your augmented profits fall between £50,000 and £250,000 (adjusted for associated companies and short periods), you pay the 25% main rate on the full amount but claim marginal relief to taper your effective rate back towards 19%. The relief is calculated using the standard 3/200 fraction and reduces the gross CT figure in box 430. Don't overlook this box — it can be a meaningful reduction.
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 and marginal relief automatically →
Section 7: Tax Payable (Boxes 475-605)
The final calculation of what you owe (or what HMRC owes you):
| Box | What It Means |
|---|---|
| 475 | Net Corporation Tax liability (box 440 minus any double taxation relief and similar credits) |
| 480 | Tax on loans and arrangements to participators — the S455 charge (from CT600A) |
| 485 | Tick if you completed box A70 on the CT600A supplementary page |
| 510 | Total tax chargeable (box 475 + box 480 + any supplementary levies) |
| 515 | Income tax deducted from income received by the company — a credit against Corporation Tax |
| 520 | Income tax repayable to the company (where box 515 exceeds box 510) |
| 525 | Self-assessment of tax payable (box 510 minus box 515) |
| 595 | Tax already paid and not yet repaid |
| 600 | Tax outstanding — amount still owed to HMRC |
| 605 | Tax overpaid — amount HMRC owes you |
For most small companies: Box 525 is the amount you need to pay HMRC by 9 months and 1 day after your period end.
Box 480 applies only if your company is a close company that lent money to a director or shareholder (the S455 charge, currently 33.75% of the outstanding loan). This requires the CT600A supplementary page, which Taxpipe doesn't support — you'd need specialist tax software or an accountant to file a return that includes a director's-loan / S455 charge. The tax is temporary — HMRC repays it once the loan is repaid to the company.
Box 515 is a credit. If someone deducted Income Tax from a payment made to your company (e.g. patent royalties), enter the gross amount of that income in the income section and the tax deducted here. Most small companies have no entries in box 515.
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 at year end) |
| CT600B | Controlled foreign companies |
| CT600C | Group and consortium relief |
| CT600D | Insurance companies |
| CT600E | Charities and Community Amateur Sports Clubs |
| CT600F | Tonnage tax (shipping companies electing the tonnage tax regime) |
| CT600J | Disclosure of Tax Avoidance Schemes (DOTAS) |
| CT600L | Research and development credits |
| CT600M | Freeports and Investment Zones |
| CT600N | Residential Property Developer Tax |
Most small companies don't need any supplementary pages. The main CT600 form is sufficient.
Section 9: Declaration (Boxes 975-985)
The person signing the return:
| Box | What It Means |
|---|---|
| 975 | Name of the person making the declaration |
| 980 | Date of the declaration |
| 985 | Status (e.g., Director, Company Secretary) |
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).
Note on boxes 920-970: The section above the declaration covers bank account details for any tax repayment (boxes 920-940) and a nominee authority (boxes 945-970) that lets you direct a repayment to someone other than the company — for example, your accountant acting as nominee. Most companies leave these blank.
Boxes Most Small Companies Fill In
For a typical small limited company with straightforward affairs:
- Boxes 1-3, 30-35 — Company details and period
- Box 80 — Accounts and computations attached
- Box 145 — Total turnover from trade
- Box 155 — Trading profits (tax-adjusted)
- Box 165 — Net trading profits
- Box 235 — Profits before deductions and reliefs
- Box 300 — Profits before donations and group relief
- Box 315 — Profits chargeable to corporation tax
- Boxes 330-345 — FY1 tax calculation (plus FY2 rows if the period spans 1 April)
- Box 430 — Corporation tax
- Box 435 — Marginal relief (if profits are between £50,000 and £250,000)
- Box 440 — Corporation tax chargeable
- Boxes 475, 510, 525 — Tax payable chain
- Boxes 975-985 — Declaration
That's about 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 main CT600, iXBRL accounts (FRS 105 micro-entity / dormant), and HMRC submission. Returns that need a supplementary page — CT600A (director's loans / S455), CT600C (group & consortium relief), CT600L (R&D) — or a loss carry-back claim aren't supported; for those you'll need specialist software or an accountant.
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