Corporation Tax (CT600) Calculator
Turn your accounting profit into a CT600 corporation tax computation: add back disallowable items, deduct allowances and reliefs, and apply the 2025/26 main rate with marginal relief.
Data verified · July 2026
Why is my effective rate not exactly 19% or 25%?
Marginal relief blends the two rates for taxable profits between £50,000 and £250,000 (divided by associated companies), so the effective rate rises smoothly rather than jumping.
Source: HMRC — Corporation Tax rates and reliefs (gov.uk/corporation-tax-rates); Marginal Relief for Corporation Tax (gov.uk/guidance/corporation-tax-marginal-relief), 2025/26. · updated 2026
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Everything about Corporation Tax (CT600) Calculator
📋Overview+
Starts from your accounting profit and adjusts it into taxable total profits: depreciation is added back (it isn't a tax-deductible cost) and replaced by capital allowances, disallowable expenses (client entertaining, fines) are added back, and R&D deductions and losses brought forward reduce the taxable figure. The result is taxed at 19% up to £50,000, 25% above £250,000, with marginal relief tapering the rate in between — both limits divided by the number of associated companies.
💡 Best practices
- Capital allowances (full expensing, the Annual Investment Allowance) often replace most of the depreciation add-back — check you haven't double-counted.
- Marginal relief is automatic on the CT600 — HMRC's own calculator can cross-check the taper.
- Group and associated-company structures should be reviewed annually — the effective rate can jump if a new company joins the group.
🔢 Concrete example
£120,000 accounting profit, £10,000 depreciation add-back, £15,000 capital allowances: £115,000 taxable profit taxed with marginal relief between the two thresholds, giving roughly a 22.75% effective rate.
📖User guide+
How to use this calculator
- 1
Enter your accounting profit for the period.
- 2
Add back depreciation and any disallowable expenses.
- 3
Enter capital allowances, R&D deduction and losses brought forward.
- 4
Add the number of associated companies if you have any — it lowers the small-profits and main-rate thresholds.
📚Glossary+
- Marginal Relief
- A tapering mechanism that smooths the jump from the 19% small-profits rate to the 25% main rate for profits between £50,000 and £250,000 (thresholds divided by associated companies).
- Associated companies
- Companies under common control. Each associated company divides the £50,000 and £250,000 thresholds, pushing more profit into higher-rate bands sooner.
ℹ️Sources & updates+
Last data update
July 7, 2026
Sources and references
HMRC — Corporation Tax rates and reliefs (gov.uk/corporation-tax-rates); Marginal Relief for Corporation Tax (gov.uk/guidance/corporation-tax-marginal-relief), 2025/26.
The data in this calculator is updated regularly to reflect the latest official rates. When in doubt, consult the official sources listed above.
FAQ — Corporation Tax (CT600) Calculator
Why is my effective rate not exactly 19% or 25%?+
Marginal relief blends the two rates for taxable profits between £50,000 and £250,000 (divided by associated companies), so the effective rate rises smoothly rather than jumping.
Do I add back all expenses in my accounts?+
No — only items that are not tax-deductible (client entertaining, most fines, depreciation) are added back. Ordinary trading expenses remain deductible.