Long accounting period calculator: two CT600s

Can I use this calculator in an AI assistant?

Yes. Taxley runs a free Model Context Protocol (MCP) server, so you can add these calculators to Claude, ChatGPT or your own code and ask for a figure in plain English. The assistant gets the same answer as this page, with a link back to the result. There's no key and no sign-up.

How many CT600 returns does a long accounting period need?

Two. A Corporation Tax accounting period can't be longer than 12 months, so accounts covering, say, 15 or 18 months are split into the first 12 months and the rest (CTA 2009 s.10; GOV.UK: Accounting periods). Each part needs its own Company Tax Return and tax computation, and the same accounts go with both returns.

When is Corporation Tax due for each period?

Each accounting period has its own payment date, 9 months and 1 day after that period ends (GOV.UK: Pay your Corporation Tax bill). For 15-month accounts from 1 January 2025 to 31 March 2026, tax for the first 12 months is due on 1 October 2026 and tax for the rest on 1 January 2027.

This worked example is the calculator's default: a company incorporated on 1 January 2025, with first accounts to 31 March 2026 and a trading profit of £150,000.00.

Accounting period	Days	Profit share	Pay by	File by
1 January 2025 to 31 December 2025	365	£120,329.67	1 October 2026	31 March 2027
1 January 2026 to 31 March 2026	90	£29,670.33	1 January 2027	31 March 2027

When must both CT600 returns be filed?

Both returns are due 12 months after the end of the accounts, not 12 months after each accounting period, as long as the accounts cover no more than 18 months (Finance Act 1998 Sch 18 para 14). GOV.UK's example extends a 31 December 2024 year end to 31 March 2025 and files all returns by 31 March 2026.

If you lengthen your company's year, tell HMRC the new accounting period dates before the original filing deadline, or you may get a late filing penalty (GOV.UK). Our guide on changing your company year end covers the steps.

How is the profit split between the two periods?

Trading profits are split by the number of days in each period (CTA 2009 s.52). In the example, £150,000.00 over 455 days gives the first 365 days £120,329.67 and the last 90 days £29,670.33. This calculator rounds the first share to the nearest penny and gives the second the rest, so they add up exactly.

Not everything is split by days. Capital allowances are worked out separately for each period, and the Annual Investment Allowance is reduced for the shorter one. A chargeable gain belongs to the period in which the disposal happens. Each period is taxed at the rates for the financial years it falls in, with the marginal relief limits reduced for a period under 12 months (GOV.UK: Marginal Relief).

When are long first accounts due at Companies House?

First accounts covering more than 12 months must reach Companies House within 21 months of incorporation, or 3 months after the accounting reference date if that is later (Companies House: Life of a company). For the example company, incorporated on 1 January 2025, that is 1 October 2026. Later accounts are due 9 months after the period end.

Our guide explains why first accounts may need two CT600s. If the company started trading after it was incorporated, its first Corporation Tax accounting period starts when trading began, and HMRC writes to confirm the dates.

How does this long accounting period calculator work?

Enter the first and last days of the accounts and, if you like, the profit or loss for the whole period. The calculator splits the period into the first 12 months and the rest, shows each part's days, share of the profit and payment date, and gives the filing deadline and the Companies House deadline.

• It covers accounts of more than 12 and up to 18 months. For 12 months or less, use the Corporation Tax deadline calculator.
• It doesn't work out the tax: the Corporation Tax calculator does that for each period.
• It assumes the company doesn't pay Corporation Tax by quarterly instalments, and that the first accounting period starts on the start date you enter.
• A company can extend its year end to make accounts of up to 18 months, normally no more than once every 5 years (GOV.UK: Change your company's year end).

Which rules does this calculator follow?

Every figure comes from the rules HMRC and Companies House publish on GOV.UK, and from the legislation behind them. The links below go to the pages we checked when we last reviewed this calculator, so you can read each rule in full and check that it applies to your company.

• GOV.UK: Accounting periods for Corporation Tax
• Corporation Tax Act 2009, section 10 (end of an accounting period)
• Corporation Tax Act 2009, section 52 (apportioning trade profits by days)
• Finance Act 1998, Schedule 18, paragraph 14 (filing date)
• GOV.UK: Pay your Corporation Tax bill
• Companies House: Life of a company — accounts (filing deadlines)

Frequently asked questions

Can a company's first accounts cover more than 12 months?

Yes. First accounts run from incorporation to the accounting reference date, normally the end of the month of the first anniversary, so they often cover a little more than 12 months. A company can also extend its year end to up to 18 months. Any period over 12 months is split into two accounting periods for Corporation Tax.

Do I need two sets of accounts for two CT600 returns?

No. You prepare one set of accounts for the whole period and send the same accounts with both Company Tax Returns. Each return has its own tax computation, working out the profits, capital allowances and Corporation Tax for its own accounting period, and each period has its own payment date.

Are the two CT600 filing deadlines different?

No. When the accounts cover more than 12 but no more than 18 months, both returns are due 12 months after the end of the accounts. The payment dates are different, though: each period's Corporation Tax is due 9 months and 1 day after that period ends, so the first payment comes earlier.

How is a loss split in a long accounting period?

The same way as a profit: by the number of days in each period. A £20,000 loss over 395 days, with 365 in the first period and 30 in the second, gives £18,481.01 and £1,518.99. Each period's loss is then used or carried forward under the normal loss relief rules for that period.

Can accounts cover more than 18 months?

Not normally. A company can extend its accounting reference period so its accounts cover up to 18 months, but no longer unless it is in administration, and it can usually extend only once every 5 years. This calculator covers accounts of more than 12 and up to 18 months.

Related free tools

• Corporation Tax calculator: Estimates the Corporation Tax a UK limited company pays on its taxable profits for one accounting period of up to 12 months. Applies the small profits rate, the main rate and marginal relief, with associated companies, short periods and periods that straddle 1 April. A close investment-holding company pays the main rate on all its profits. Returns the tax, the effective rate and the payment and filing dates.
• Corporation Tax deadline calculator: Works out a UK private limited company's Corporation Tax payment date, CT600 filing deadline and Companies House accounts deadline from its accounting period. Accounts longer than 12 months are split into two Corporation Tax periods, each with its own payment date.
• Corporation Tax late filing penalty and interest calculator: Works out HMRC's penalties for filing a UK company's Company Tax Return (CT600) late — the flat £200/£400 penalties, doubled for filing deadlines from 1 April 2026, and the 10% or 20% tax-geared penalty — plus late payment interest on the Corporation Tax at HMRC's published rates.
• All free Corporation Tax tools

Last reviewed 29 September 2026. This calculator gives general guidance, not tax advice.