CT600 example: a completed company tax return, box by box

In short: This worked example follows a fictional company, Example Trading Ltd, through a 12-month period to 31 March 2026. Accounts profit of £39,535 becomes taxable profit of £36,250 after two add-backs and £6,000 of capital allowances, so the 19% rate gives £6,887.50. At taxable profit of £100,000, marginal relief makes the tax £22,750.00. The CT600 boxes, the iXBRL accounts and computation, and the deadlines follow.

The company, its figures and its details are invented to show how the numbers flow. They are not advice for your own company, and your own return depends on your own records. The rates and limits below come from GOV.UK: Corporation Tax rates and allowances.

What does a completed CT600 look like for a small company?

A completed CT600 for a small trading company is a short form: company details and period, turnover and trading profit, taxable profit, the tax calculation, any capital allowances claimed, and a declaration. The detail sits in two attachments, the accounts and the tax computation, which explain how each figure was reached.

The example below uses Example Trading Ltd, a fictional UK trading company. Its accounting period runs from 1 April 2025 to 31 March 2026, which is 12 months. It prepares micro-entity (FRS 105) accounts, has no associated companies, trades only in the UK and has no loans to its director.

What are Example Trading Ltd's accounts figures?

The accounts show turnover of £96,000, costs of £56,465 including £1,500 of depreciation and £1,215 of client entertaining, and a profit before tax of £39,535. These are the figures that appear in the profit and loss account, before any tax adjustments, and they are the starting point for the tax computation.

During the year the company also bought £6,000 of equipment, such as computers and workshop tools, for use in the business. The depreciation of £1,500 in the accounts relates to that and to earlier equipment.

How do you turn the accounts profit into taxable profit?

Start with the accounts profit, add back costs the tax rules do not allow, and deduct capital allowances. Here depreciation of £1,500 and client entertaining of £1,215 are added back, and the Annual Investment Allowance gives a deduction of £6,000, leaving taxable trading profit of £36,250.

Depreciation is added back because tax relief for equipment comes through capital allowances instead. GOV.UK: Corporation Tax expenses lists entertaining clients among the expenses that cannot be deducted. The Annual Investment Allowance lets a company deduct the full value of qualifying plant and machinery, up to £1 million, from its profits. Our guide to equipment, depreciation and capital allowances covers what qualifies.

How much Corporation Tax does Example Trading Ltd pay?

Example Trading Ltd pays £6,887.50. Taxable profit of £36,250 is below the £50,000 lower limit, so the whole amount is charged at the 19% small profits rate and no marginal relief applies. The same 19% rate applies to the financial year starting 1 April 2025, which covers this whole period.

Check: 10% of £36,250 is £3,625.00 and 9% is £3,262.50, and the two add up to £6,887.50. The limits of £50,000 and £250,000 fall if the period is shorter than 12 months or the company has associated companies, so check both before relying on the 19% rate (GOV.UK: Corporation Tax Marginal Relief).

What changes if taxable profit is £100,000?

Between £50,000 and £250,000 the company pays 25% less marginal relief. At taxable profit of £100,000, with no exempt distributions and no associated companies, tax at 25% is £25,000.00, marginal relief is £2,250.00 and the Corporation Tax chargeable is £22,750.00, an effective rate of 22.75%.

HMRC's formula is the marginal relief fraction of 3/200 multiplied by the upper limit less augmented profits, scaled by taxable profits divided by augmented profits (HMRC Company Taxation Manual CTM03925). With no exempt distributions, augmented profits equal taxable profits, so the scaling factor is 1.

Check by a second route: the first £50,000 at 19% is £9,500.00 and the next £50,000 at the 26.5% effective marginal rate is £13,250.00, which also gives £22,750.00. In the CT600 the £25,000.00 goes in box 430, the £2,250.00 in box 435 and £22,750.00 in box 440. The rates and marginal relief guide covers the other cases.

Which CT600 boxes does Example Trading Ltd fill in?

The main example, at £36,250 taxable profit, fills in the boxes below. The box numbers and labels follow HMRC's Company Tax Return guide. Boxes that do not apply to Example Trading Ltd, such as losses, group relief and loans to participators, are left blank.

Box 155 is the accounts profit after the tax adjustments and capital allowances, which is why it matches the computation, not the £39,535 in the accounts. Box 326 must be completed when the company pays the small profits rate or is entitled to marginal relief. For the £100,000 variant, boxes 155, 165, 235, 300, 315 and 335 become £100,000, box 340 is 25%, boxes 345 and 430 are £25,000.00, box 435 is £2,250.00 and boxes 440, 475, 510, 525 and 528 become £22,750.00.

What goes with the CT600?

The CT600 goes to HMRC with the company's accounts and its tax computation. HMRC's guidance says the computations and accounts must be in Inline eXtensible Business Reporting Language (iXBRL), which you produce with software. A PDF or a spreadsheet on its own is not the normal online filing package.

The computation is the table in the earlier section, with each adjustment explained. The accounts are the statutory accounts, here micro-entity accounts under FRS 105. The accounts are also filed at Companies House, which is a separate filing with its own deadline. The same figures should appear in both, as our guide to whether the CT600 and company accounts have to match explains. For the wider process, see how to file a CT600 yourself and the HMRC guidance on what a return includes.

When must the tax be paid and the return filed?

For Example Trading Ltd, whose period ends on 31 March 2026, the Corporation Tax of £6,887.50 is due by 1 January 2027, which is 9 months and 1 day after the period end. The Company Tax Return is due within 12 months of the period end, by 31 March 2027.

The payment date comes first, and it is not extended by filing later. GOV.UK: Pay Corporation Tax says that if the deadline falls on a weekend or bank holiday, the payment must reach HMRC by the last working day before, except for Faster Payments. 1 January is a bank holiday, so most payment methods would need to clear by 31 December 2026. See Corporation Tax deadlines explained and GOV.UK: Company Tax Returns for the rules.

What records should you keep?

Keep the invoices, bank statements, receipts and workings behind every figure, plus the final accounts, the tax computation and the filed return. GOV.UK: Company and accounting records says to keep records for 6 years from the end of the last company financial year they relate to.

For Example Trading Ltd, keep the purchase invoice for the £6,000 of equipment, a note of why the £1,215 of client entertaining was added back, and a list of the assets behind the £1,500 depreciation. Some records must be kept longer, for example those covering equipment expected to last more than six years, so check the GOV.UK page for your own case.

How does Taxley produce this for you?

Taxley builds the CT600, the computation and micro-entity (FRS 105) or small-company (FRS 102 Section 1A) accounts in iXBRL from the figures you enter. It files to HMRC with the company's own Government Gateway login, then files eligible accounts it prepares at Companies House from the same filing page at no extra cost (the pricing page lists the exceptions).

To see the boxes in HMRC's order, with what each one holds, use the CT600 boxes reference. To find out whether your company is the sort Taxley files for, check whether Taxley suits your company.

Rates and limits checked against GOV.UK on 27 October 2026. The company, figures, registration number and UTR in this example are invented. General information, not tax advice.

Frequently asked questions

Is the profit in my accounts the same as the profit on my CT600?

Usually not. The CT600 uses taxable profit, which starts from the accounts profit, adds back depreciation and costs the tax rules disallow, and deducts capital allowances. In this example £39,535 of accounts profit becomes £36,250 of taxable profit.

Do I attach the tax computation to the CT600?

Yes. A Company Tax Return includes the accounts and the tax computation, and HMRC requires both in iXBRL. The computation shows how the accounts profit became the taxable profit in box 155, so HMRC can follow each adjustment.

Is the 19% rate available to every small company?

No. It applies when profits are £50,000 or less, and that limit is divided by the number of associated companies plus one, and cut for periods shorter than 12 months. Above the limit, marginal relief applies up to £250,000, and 25% applies beyond that.

What if the company has no profit to tax?

A return is still due if HMRC has issued a notice to file, even when the result is a loss or no tax. A trading loss is reported in its own box rather than in box 155. Our guide on loss-making companies covers the loss boxes.