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 marginal relief or other deductions. It is the sum of the tax rows from the Financial Year section (boxes 345, 360, 375, 395, 410 and 425), 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 effective tax rate from 19% towards 25%.
The HMRC formula is:
Marginal relief = Standard fraction × (Upper limit − Augmented profits) × (Profits / Augmented profits)
For most companies, augmented profits equal taxable profits (they only differ when the company receives exempt dividends from non-associated companies), so the formula simplifies to:
Marginal relief = 3/200 × (£250,000 − 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 Chargeable
Box 430 minus Box 435. This is your Corporation Tax charge after marginal relief. The official HMRC label is "Corporation Tax chargeable".
Box 470: Total Reliefs
Before arriving at the net tax liability, certain reliefs can reduce your box 440 figure:
- Box 445: Community Investment Tax Relief (for investment in CDFIs)
- Box 450: Double Taxation Relief (to avoid being taxed twice on foreign income)
- Box 465: Advance Corporation Tax (only relevant for companies with pre-April 1999 ACT)
Box 470 is the total of boxes 445, 450 and 465.
Box 475: Net Corporation Tax Liability
Box 440 minus Box 470. The official HMRC label is "Net Corporation Tax liability". This is box 440 (Corporation Tax chargeable after marginal relief) reduced by the reliefs in boxes 445, 450 and 465 — Community Investment Tax Relief, Double Taxation Relief, and any Advance Corporation Tax. For most small companies with none of these reliefs, box 475 equals box 440.
Box 480: Tax on Loans to Participators (Section 455)
If your company is a close company (typically, any company controlled by 5 or fewer people — which covers most owner-managed companies) and has made loans or advances to directors or shareholders that are outstanding at the period end, a Section 455 tax charge applies at 33.75% of the outstanding balance.
This flows from box A80 of the CT600A supplementary page. It is a temporary charge — repayable to the company when the loan is repaid.
Box 510: Tax Chargeable
The total of boxes 475, 480, 500, 501, 502 and 505. This is the full tax the company is assessed on.
Boxes 490–505 cover specialist charges that apply only to specific company types:
- Box 490: CFC tax (Controlled Foreign Companies)
- Box 495/496: Bank levy and bank surcharge (banks only)
- Box 497: Residential Property Developer Tax (large property developers only)
- Box 501: Energy (Oil and Gas) Profits Levy
- Box 502: Electricity Generator Levy
- Box 505: Ring fence supplementary charge (oil and gas companies)
For most small companies, box 510 equals box 475 (basic CT) plus box 480 if a director loan charge applies.
Box 515: Income Tax Deducted from Gross Income
If your company received income where the payer deducted Income Tax before paying you — for example, certain patent royalties or interest payments received from individuals — enter the total tax deducted here. You enter the gross amount of that income in box 185, and the tax deducted in box 515 acts as a credit against your Corporation Tax bill.
This also applies if your company is a non-UK resident landlord and has had tax withheld under the Non-Residents Landlord Scheme (with the property income entered in box 190).
Important: CIS (Construction Industry Scheme) deductions are specifically excluded from box 515 — HMRC handles those separately through PAYE. Box 515 is also not for Corporation Tax payments on account or quarterly instalment payments; those go in box 595 instead.
Box 520: Income Tax Repayable to the Company
If box 515 exceeds box 510, HMRC owes your company the difference as a repayment of Income Tax. Box 520 = box 515 minus box 510, but only when box 515 is larger. Leave this blank if box 515 does not exceed box 510.
For most small companies, this is blank.
Box 525: Self-Assessment of Tax Payable
This is box 510 minus box 515 — your self-assessment of the Corporation Tax your company needs to pay HMRC, before restitution tax and any coronavirus support scheme overpayments.
If box 515 exceeds box 510, enter 0 — box 525 cannot be negative. The excess Income Tax is repayable via box 520.
Box 525 is your self-assessment before deducting any credits (R&D, creative industry, land remediation) and any Corporation Tax already paid. To find the actual amount still outstanding or the refund due, look further down the return:
- Box 595: Corporation Tax already paid for this accounting period (quarterly instalment payments, or voluntary payments on account made before filing). This is different from box 515 — box 595 is Corporation Tax paid, not Income Tax deducted at source.
- Box 600: Tax outstanding — what you still owe after credits and prior payments (box 525 minus boxes 545, 560, 565 and 595). Cannot be negative.
- Box 605: Tax overpaid — the amount HMRC will repay to you if credits and prior payments exceed your liability.
Associated Companies (Box 326)
The number of associated companies affects your marginal relief thresholds. Associated companies are:
- Companies under common control (typically, owning more than 50% of the shares or voting rights)
- 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.
Example: If you personally own 100% of Company A and 80% of Company B, Company A would enter 1 (one associated company), halving the thresholds to £25,000 and £125,000.
Financial Year Split
If your accounting period spans two financial years (e.g., a 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.