Corporation Tax

First-year Corporation Tax: the two-return split

Your first accounting period usually runs over 12 months — so it splits into two CT600s. Enter your dates and figures to see both returns, both deadlines and both payment references — the split most free calculators skip.

Your first period
From incorporation to the end of your first accounts.

Over 12 months — two CT600s required

£17,254.79

Total Corporation Tax across the period.

Return 1 of 2

12 June 2025 → 11 June 2026

Taxable profit
£76,041.67
Corporation Tax
£16,401.04
Marginal relief
£2,609.38
Pay by
12 March 2027
File CT600 by
11 June 2027

Return 2 of 2

12 June 2026 → 30 June 2026

Taxable profit
£3,958.33
Corporation Tax
£853.75
Marginal relief
£135.83
Pay by
31 March 2027
File CT600 by
30 June 2027

Why one long year becomes two returns

A Corporation Tax accounting period can't exceed 12 months (CTA 2009 s.10). Your first Companies House period runs from incorporation to your accounting reference date — the month-end a year later — so it's almost always 12 months and a few days. HMRC splits that into a 12-month period and a short stub, and each gets its own CT600.

One set of accounts covers both, attached to the first return. But the tax is computed twice, once per period — and because the rate bands and marginal-relief limits are scaled to each period's length, the two halves don't simply split the tax in half.

Two deadlines, two payment references

  • Two payment deadlines — Corporation Tax is due 9 months and 1 day after each period ends, so the two dates sit about a month apart.
  • Two 17-character payment references — each period has its own (your 10-digit UTR plus a period suffix). Paying both with the same reference is a common, costly slip.
  • One set of accounts — the iXBRL accounts attach to the first return only; the second return references them.

The stub period is where the tax gets fiddly

A short stub gets only its day-count share of the profit limits. So a modest profit in a 30-day stub can land in the marginal band and be taxed at a higher effective rate than the main period — which is exactly why guessing the split by eye goes wrong.

Common questions

Why does my first year need two Corporation Tax returns?

Because a Corporation Tax accounting period can never exceed 12 months (CTA 2009 s.10), but your first Companies House accounting period usually runs a little longer than a year. HMRC therefore splits it into a 12-month period and a shorter 'stub', and each period needs its own CT600 return.

How is the profit divided between the two periods?

By default, turnover and profit are apportioned across the two periods by the number of days in each — HMRC's time basis (CTM01405). If you know the real attribution — for example the trade only started partway through — you can allocate on that actual basis instead, which is usually more accurate.

Do I get two payment deadlines?

Two. Corporation Tax is due 9 months and 1 day after the end of each period, and each period has its own 17-character payment reference (your UTR plus a period suffix). Paying both with the same reference is a common way for a payment to land against the wrong period.

Why isn't the tax just split in half?

The tax can differ because rates and the marginal-relief thresholds are scaled to the length of each period. A short stub period gets a small slice of the profit limits, so a modest profit there can fall into the marginal band and be taxed at a higher effective rate than you would expect.

General information about UK Corporation Tax, not tax advice. Covers straightforward trading cases — no reliefs, losses or ring-fence profits. Rates verified through FY2026.