What is the marginal relief fraction for the 2023/24 tax year?

The standard marginal relief fraction for the 2023/24 tax year is 3/200 (or 0.015 in decimal form). This fraction is used to calculate the amount of relief deducted from the main 25% tax rate.

What counts as an associated company for corporation tax?

A company is generally associated with another if one has control of the other, or if both are under the control of the same person or persons. This includes non-UK resident companies but excludes dormant companies.

How does a short accounting period affect the tax thresholds?

If your company's accounting period is less than 12 months, the lower (£50,000) and upper (£250,000) limits are scaled down proportionally. For example, for a 6-month accounting period, the limits become £25,000 and £125,000.

Are capital allowances deducted before calculating corporation tax?

Yes. Capital allowances (such as the Annual Investment Allowance or Full Expensing) are deducted from your gross profits to arrive at your Profit Chargeable to Corporation Tax (PCTCT) before the tax rates are applied.

UK Corporation Tax Calculator: Rates & Marginal Relief Guide

The UK corporate tax landscape underwent its most significant transformation in a decade on April 1, 2023. The historically straightforward flat-rate system of 19% was replaced by a tiered, marginal rate structure. For engineers, STEM professionals, and business owners, this change introduces non-linear tax dynamics that directly affect cash flow projections, investment strategies, and financial modeling.

Understanding how your taxable profits translate into actual tax liabilities now requires more than a simple multiplication step. This guide breaks down the mathematical mechanics of the 2023/24 UK Corporation Tax regime, demonstrates how to calculate marginal relief, and explains how our free UK Corporation Tax Calculator can streamline your financial planning.

1. The 2023/24 UK Corporation Tax Framework

Under the current framework, the tax rate applied to your company’s taxable profits (also known as PCTCT—Profits Chargeable to Corporation Tax) depends on which threshold your profits fall into. The system is split into three distinct zones:

The Small Profits Rate (19%): Applies to companies with taxable profits of £50,000 or less. If your profits fall within this band, your tax liability is calculated at a flat 19%.
The Main Rate (25%): Applies to companies with taxable profits exceeding £250,000. All profits are taxed at a flat 25% in this bracket.
The Marginal Relief Zone (Sliding Scale): If your profits fall between £50,000 and £250,000, your company is subject to the main rate of 25%, but this is mitigated by Marginal Relief. This relief gradually scales up your effective tax rate from 19% to 25% as your profits increase.

The Effective Marginal Rate

It is vital to distinguish between your average tax rate and your marginal tax rate. Within the £50,000 to £250,000 zone, every additional pound of profit you earn is actually taxed at an effective marginal rate of 26.5%. This mathematical quirk is designed to smoothly bridge the gap between the 19% and 25% bands.

2. Deciphering Marginal Relief: The Mathematics

To calculate your tax liability within the marginal relief zone, HM Revenue & Customs (HMRC) uses a specific algebraic formula. Instead of applying a complex sliding scale directly, the formula calculates the tax at the 25% main rate first, then subtracts the Marginal Relief fraction.

The Marginal Relief Formula

$$\text{Marginal Relief} = (U - A) \times L \times F$$

Where:

$U$ (Upper Limit): £250,000 (adjusted for associated companies and short accounting periods).
$A$ (Augmented Profits): Your taxable profits plus any exempt distributions received from non-group companies.
$L$ (Ratio of Profits): Taxable Profits divided by Augmented Profits (usually 1 if there are no exempt distributions).
$F$ (Standard Marginal Relief Fraction): $3/200$ (or $0.015$ in decimal form).

If your company has no exempt distributions (which is common for most SMEs), the formula simplifies to:

$$\text{Marginal Relief} = (£250,000 - \text{Taxable Profits}) \times 0.015$$

Once you have calculated the Marginal Relief, you subtract it from the Main Rate tax calculation:

$$\text{Net Tax Payable} = (\text{Taxable Profits} \times 25%) - \text{Marginal Relief}$$

3. Practical Examples with Real Numbers

Let's walk through three distinct scenarios to observe how the math behaves across different profit levels.

Scenario A: The Micro-Consultancy (Profits below £50,000)

Taxable Profit: £42,000
Calculation: Because profits are below the £50,000 lower limit, the company qualifies for the Small Profits Rate of 19%.
Tax Liability: $$£42,000 \times 0.19 = £7,980$$
Effective Tax Rate: 19.0%

Scenario B: The Mid-Sized Engineering Firm (Profits in the Marginal Zone)

Taxable Profit: £120,000 (with no associated companies or exempt distributions)
Step 1: Calculate Tax at the Main Rate (25%) $$£120,000 \times 0.25 = £30,000$$
Step 2: Calculate Marginal Relief $$\text{Relief} = (£250,000 - £120,000) \times 0.015$$ $$\text{Relief} = £130,000 \times 0.015 = £1,950$$
Step 3: Calculate Net Tax Payable $$\text{Net Tax} = £30,000 - £1,950 = £28,050$$
Effective Tax Rate: $$\frac{£28,050}{£120,000} \times 100 = 23.375%$$

Analysis: Notice how the effective tax rate of 23.375% sits precisely between 19% and 25%. This demonstrates how the marginal relief mechanism prevents a sharp cliff-edge tax hike at the £50,000 boundary.

Scenario C: High-Growth Tech Enterprise (Profits above £250,000)

Taxable Profit: £310,000
Calculation: Because profits exceed the £250,000 upper limit, no marginal relief is available.
Tax Liability: $$£310,000 \times 0.25 = £77,500$$
Effective Tax Rate: 25.0%

4. The Impact of Associated Companies

A critical factor that many business owners overlook is the Associated Companies Rule. If your company is "associated" with other companies under common control, the lower and upper limits (£50,000 and £250,000) are divided by the total number of associated companies plus one.

The Math of Fragmentation

If you own or control two active companies, the limits for each company are halved:

New Lower Limit: £25,000
New Upper Limit: £125,000

If Company A makes £45,000 in profit, it can no longer claim the flat 19% rate on all of it, because its individual lower limit has dropped to £25,000. It now falls into the marginal relief zone, increasing its tax burden. Our UK Corporation Tax Calculator allows you to input the number of associated companies to instantly adjust these thresholds dynamically.

5. Leverage the DigiCalcs UK Corporation Tax Calculator

Calculating corporation tax manually—especially when factoring in marginal relief fractions, associated companies, or shortened accounting periods—is highly susceptible to human error.

Our UK Corporation Tax Calculator is engineered to eliminate this friction. By entering your taxable profits and specifying any associated companies, the tool instantly computes:

Your gross tax liability at the main rate.
The precise marginal relief deduction.
Your net tax payable.
Your true effective tax rate.

Use this data to optimize dividend vs. salary distributions, plan capital allowance investments (such as the Full Expensing scheme), and ensure your business retains optimal working capital.