CT600 Tax Calculation Section (Boxes 430-525): How Your Tax Is Computed
The tax calculation section is where the CT600 turns your profits into a tax bill. Here's what every box means.
Box 430: Corporation Tax (Gross)
This is the total Corporation Tax before any reliefs or deductions. It's calculated by applying the appropriate tax rate(s) to your chargeable profits.
For accounting periods starting on or after 1 April 2023:
- Small profits rate: 19% (profits up to £50,000)
- Main rate: 25% (profits over £250,000)
- Marginal rate: Effective rate between 19% and 25% (profits £50,000-£250,000)
If your accounting period straddles two financial years with different rates, the profit is apportioned by days.
Box 435: Marginal Relief
If your profits fall between £50,000 and £250,000, you get marginal relief — a deduction that smoothly transitions the tax rate from 19% to 25%.
The formula:
Marginal relief = Fraction × (Upper limit - Profits) × (Profits / Profits)
The fraction for FY2023 onwards is 3/200.
The thresholds are divided by the number of associated companies plus one. So if you have one associated company, the thresholds become £25,000 and £125,000.
Box 440: Corporation Tax Net of Marginal Relief
Box 430 minus Box 435. This is your actual Corporation Tax charge.
Box 475-510: Additional Charges and Reliefs
- Box 475: Corporation Tax chargeable
- Box 510: Tax payable after all reliefs
- Box 515: Tax already paid (CIS deductions, payments on account)
- Box 520: Tax outstanding or overpaid
- Box 525: Tax payable
Box 525: The Bottom Line
This is the amount you owe HMRC (or the refund you're due). It's:
Box 510 (tax payable) minus Box 515 (tax already paid) = Box 525
If negative, you're due a repayment.
Associated Companies (Box 326)
The number of associated companies affects your marginal relief thresholds. Associated companies are:
- Companies under common control (51% ownership)
- This includes companies controlled by the same person or group of persons
- Dormant companies are excluded
Enter the number (not counting your own company). If you have no associated companies, enter 0.
Financial Year Split
If your accounting period spans two financial years (e.g., year ending 31 December spans FY2023 and FY2024), the tax calculation splits your profits proportionally:
| Financial Year | Days | Rate |
|---|---|---|
| FY2023 (1 Apr 2023 - 31 Mar 2024) | X days | 19%/25% |
| FY2024 (1 Apr 2024 - 31 Mar 2025) | Y days | 19%/25% |
Taxpipe handles this split automatically.
Don't calculate Corporation Tax by hand. Use Taxpipe — we compute marginal relief, FY splits, and everything else for £59.