UK Corporation Tax Marginal Relief Calculator
Calculate corporation tax using the UK marginal relief calculation formula for financial years from 1 April 2023 onward (standard non ring fence companies).
Expert Guide: UK Corporation Tax Marginal Relief Calculation Formula
The reintroduction of a two rate UK corporation tax system from April 2023 made tax planning more technical for owner managed businesses, group finance teams, and advisers. If your company profits are modest, you may still pay the small profits rate of 19%. If profits are high enough, you pay the main rate of 25%. In between those limits, you do not jump straight from 19% to 25%. Instead, marginal relief reduces the tax that would otherwise be due at 25%, creating a gradual increase in effective tax rate.
This guide explains the UK corporation tax marginal relief calculation formula in plain language, then shows how to use it correctly with associated companies, short accounting periods, and augmented profits. The aim is practical accuracy: if you can identify your taxable total profits and exempt distributions, you can estimate your tax charge with confidence and quickly sense check your accounts or software outputs.
What changed from 1 April 2023
For many years, the UK operated a single corporation tax rate. From 1 April 2023, the structure changed. Most companies now fall into one of three bands:
- Small profits rate of 19% if profits are at or below the lower limit.
- Main rate of 25% if profits are at or above the upper limit.
- Marginal relief zone between those limits, where effective tax increases gradually.
The headline limits are £50,000 and £250,000 for a 12 month period, but they are not fixed for every company. You must adjust both limits for associated companies and for accounting periods shorter than 12 months.
| Period | Lower Limit (12 months) | Upper Limit (12 months) | Small Profits Rate | Main Rate | Marginal Relief Fraction |
|---|---|---|---|---|---|
| From 1 April 2023 onward | £50,000 | £250,000 | 19% | 25% | 3/200 (standard companies) |
| Before 1 April 2023 | Not applicable | Not applicable | Single rate applied | Single rate applied | Not applicable |
The formula you need
For companies in the marginal band, the formula is:
- Corporation Tax before relief = Taxable Total Profits x Main Rate (25%).
- Marginal Relief = (Upper Limit – Augmented Profits) x (Taxable Total Profits / Augmented Profits) x (3/200).
- Corporation Tax payable = Corporation Tax before relief – Marginal Relief.
The formula relies on augmented profits, not only taxable profits. This is where many errors happen. Augmented profits generally means taxable total profits plus exempt distributions received from non group companies.
Key terms in plain English
- Taxable Total Profits (TTP): your profits chargeable to corporation tax after tax adjustments.
- Exempt Distributions: qualifying dividend income that is exempt but still included when calculating augmented profits for marginal relief.
- Augmented Profits: TTP plus exempt distributions.
- Associated Companies: broadly companies under common control, used to divide the lower and upper limits.
- Financial year rates: corporation tax rates and limits are set by financial year, so long accounting periods may need apportionment.
Adjusting limits for associated companies and short periods
The statutory lower and upper limits apply to a single standalone company with a 12 month accounting period. In practice, many businesses are in groups or have multiple companies under common control. In that case, the limits are divided by the total number of associated companies plus the company itself.
Example: if you have 2 associated companies, you divide by 3. The limits become:
- Lower limit: £50,000 / 3 = £16,666.67
- Upper limit: £250,000 / 3 = £83,333.33
If the accounting period is less than 12 months, multiply each limit by months in period divided by 12. A 9 month period with no associates gives:
- Lower limit: £50,000 x 9/12 = £37,500
- Upper limit: £250,000 x 9/12 = £187,500
Worked example using the calculator logic
Suppose your company has taxable total profits of £120,000, exempt distributions of £5,000, no associated companies, and a full 12 month period.
- Augmented profits = £120,000 + £5,000 = £125,000.
- Lower and upper limits remain £50,000 and £250,000.
- Since augmented profits are between limits, marginal relief applies.
- Tax before relief = £120,000 x 25% = £30,000.
- Marginal relief = (£250,000 – £125,000) x (£120,000 / £125,000) x (3/200).
- Marginal relief = £125,000 x 0.96 x 0.015 = £1,800.
- Corporation tax due = £30,000 – £1,800 = £28,200.
- Effective tax rate = £28,200 / £120,000 = 23.5%.
This illustrates the central policy intent: effective rate rises smoothly between 19% and 25% rather than creating a sharp cliff edge.
Common mistakes and how to avoid them
- Using taxable profits instead of augmented profits in the relief formula.
- Forgetting to reduce thresholds for associated companies.
- Ignoring short accounting period scaling.
- Applying marginal relief when augmented profits are already at or above the upper limit.
- Mixing accounting year and financial year rate rules on long periods.
Comparison table: receipts context and why this matters
Corporation tax is a major UK revenue stream. Changes in rates and profits materially affect Exchequer receipts, and the return to a multi rate structure from 2023 has direct planning implications for SMEs and groups.
| Tax Year | UK Corporation Tax Receipts (approx, £bn) | Context |
|---|---|---|
| 2020 to 2021 | 45 | Lower pandemic era profits and timing effects. |
| 2021 to 2022 | 63 | Strong recovery in company profits. |
| 2022 to 2023 | 84 | High profitability in several sectors. |
| 2023 to 2024 | 97 | First full year after rate structure change to 19% and 25% framework. |
These values are rounded from official UK publications and are included to show scale. Even relatively small modeling errors at company level become large aggregate distortions when repeated across thousands of returns.
Planning insights for directors and finance teams
Marginal relief does not mean companies should make decisions only to stay below an arbitrary profit threshold. Commercial decisions come first. However, once a decision is commercially sound, tax timing and structure can improve cash flow and reduce surprises. For example, if profits are likely to sit in the marginal band over several years, forecasting the effective rate can improve dividend planning, quarterly instalment expectations for larger businesses, and year end provisioning.
Group structures require extra care. Adding or removing associated companies can change your limits immediately. A company with profit around £80,000 might be in small profits territory as a standalone entity but could be pushed deep into the marginal zone when grouped with other associated companies. The tax impact then feeds through to deferred tax calculations and management KPI dashboards.
You should also align your accounting software and tax engine assumptions. Some tools default to headline limits without associated company adjustments unless explicitly configured. A quick manual check with the formula in this guide is an effective control step before final sign off.
Compliance checklist before filing
- Confirm taxable total profits from final computation schedules.
- Confirm exempt distributions included in augmented profits.
- Count associated companies for the relevant period under control rules.
- Time apportion limits for periods shorter than 12 months.
- Check whether profits fall into small, marginal, or main rate band.
- Reconcile computed tax to draft CT600 entries.
- Keep supporting workings and assumptions in your year end file.
Authoritative sources for current rules
- GOV.UK: Corporation Tax rates and thresholds
- GOV.UK: Corporation Tax statistics publications
- UK Legislation: Corporation Tax Act 2010
Final takeaway
The UK corporation tax marginal relief calculation formula is straightforward once each component is clearly defined: taxable profits, augmented profits, adjusted limits, and the 3/200 fraction for standard companies. Most practical errors come from missing one adjustment rather than from difficult arithmetic. If you follow the sequence in this guide and test your figures with a calculator like the one above, you can produce reliable estimates quickly and improve both tax compliance and cash flow planning.