Free Corporation Tax calculator: your CT600, box by box
Enter your company's income and costs for the year. You'll get the Corporation Tax bill with marginal relief worked out, the figures for each CT600 box, your capital allowances, every deadline, and the salary and dividend split that leaves you the most.
For accounting periods ending from April 2023 onwards. HMRC's free filing service closed on 31 March 2026, so you'll file with software or an accountant. Use these figures to check them.
Made for you if your company...
- is a UK private limited company that trades
- is small (profits under £1.5 million)
- has one or a few directors and shareholders
Have these to hand
- your profit and loss account, or bank records for the year
- a list of equipment, vans and cars bought or sold
- last year's CT600 (losses and capital allowance pools)
- your accounting period dates
What you get
The figure for every CT600 box
Turnover, trading profits, losses, interest, the tax calculation lines, marginal relief and capital allowances, each with its box number and a copy button.
Marginal relief done for you
19%, 25% or somewhere between. Limits are cut for short periods and shared with associated companies, and periods that span two financial years are split.
Capital allowances
Annual Investment Allowance, 100% for new electric cars, and the main and special rate pools, including the hybrid rate for periods spanning 1 April 2026.
Salary or dividends?
Tries every salary in £10 steps, with employer and employee NI, Corporation Tax and dividend tax, and shows the split that leaves you the most.
Guides
- →Corporation Tax marginal relief explained, with worked examples
- →The best director salary for 2026-27, and how much dividend
- →What a limited company can claim (and what it can't)
- →Company deadlines and late filing penalties
Common questions
What is the Corporation Tax rate?
19% on profits up to £50,000 and 25% on profits over £250,000. In between, you pay 25% less marginal relief, which works out at an effective 26.5% on each pound between £50,000 and £250,000. The limits are shared between associated companies and cut for accounting periods shorter than 12 months.
Can I still file my CT600 for free on GOV.UK?
No. HMRC's free Company Accounts and Tax Online service closed on 31 March 2026. Companies now file with commercial software (some is free for very small companies) or through an accountant. Company Tax Helper gives you the figures to type in or to check what your software says.
When is Corporation Tax due?
Small and medium companies pay 9 months and 1 day after the end of the accounting period. The CT600 is due 12 months after the period ends, and accounts go to Companies House 9 months after the year end. See deadlines and penalties.
Is it free?
Yes. There's no sign-up and no charge. Everything is worked out in your browser, and your figures stay on your device.
Your company
Accounting period
Usually your financial year, as in your accounts. A Corporation Tax period can't be longer than 12 months. If your accounts cover more, it splits into two returns: put the first 12 months in here, then the rest separately.
Other companies controlled by the same people (including close family, such as a spouse or child, in many cases). Dormant companies and passive holding companies don't count. Most one-company owners enter 0. The £50,000 and £250,000 limits are shared out between associated companies.
Income for the period
From your profit and loss account. Without VAT if you're VAT registered.
Everything you invoiced for the period, whether paid yet or not.
Grants, insurance payouts for lost stock, sale of scrap.
Interest on the company's bank and savings accounts.
If the company lets out property: rent less letting costs.
Running costs
Costs in your profit and loss account for the period. Don't include equipment, vans or cars you bought: they go on the next page. Not sure what counts? See what a company can claim.
Costs in your accounts that aren't allowed for tax
If these are in your accounts, enter them here. They're taken off your accounts profit, then added back for tax.
Equipment, vans and cars
What the company bought in the period. These get capital allowances instead of going in costs.
Gets 100% Annual Investment Allowance up to £1 million a year. Include second-hand items.
100% first-year allowance. A used electric car goes in the box below.
Pools from last year and things you sold
From last year's capital allowances: the written-down value carried forward in each pool. Sales: what you got for items you sold, up to what they cost.
Losses, donations and tax already paid
Unused trading losses from earlier periods (last year's CT600 box 780 or your computation). They're set against this period's trading profits.
Money the company gave to UK charities. Companies don't use Gift Aid: the donation comes off profits.
Not included: capital gains on selling property or shares, R&D relief, group relief, losses carried back to earlier years, overseas income, structures and buildings allowance, the 40% first-year allowance and full expensing (most small companies don't need them because the £1 million AIA covers their spending), loans to directors (s455) and large-company quarterly instalments. If any apply, ask an accountant.