Every year, millions of people in the UK pay income tax without fully understanding how the number on their bill was reached. They trust that HMRC (His Majesty’s Revenue and Customs, the UK tax authority) got it right, pay what they owe, and move on. That trust is sometimes well-placed. But not always.
Overpaid tax, missed reliefs, and unexpected bills are common precisely because the calculation process remains a mystery. This guide removes that mystery. Once you understand the structure of an income tax calculation, you can check your own position, spot errors, and make smarter decisions about how you earn and what you claim.
Why this matters
Understanding how your tax bill is built is not about becoming an accountant. It is about having enough knowledge to ask the right questions and recognise when something does not look right.
What Income Tax Actually Taxes
Income tax in the UK does not apply to every pound you earn in the same way. HMRC separates income into three distinct categories, each with its own set of rules.
Non-savings income is the broadest category. It covers salary from employment, profits from running a business as a sole trader (called trading income), rental income from property, and certain pension income. This is where most people’s income lands.
Savings income covers interest earned on bank and savings accounts, bonds, and certain other financial products. It gets its own column in the calculation because different rules can apply, including a personal savings allowance that may mean no tax is due on modest amounts of interest.
Dividend income applies to shareholders who receive payments from company profits. If you own shares in a limited company and pay yourself via dividends, this is where that income sits. Dividends have their own tax rates, which are lower than the standard rates that apply to employment income.
The Building Blocks: From Gross Income to What HMRC Taxes
Think of the income tax calculation as a structured template with several steps. Each step either reduces or confirms the amount of income that will ultimately face tax.
Step 1: Compute Your Income
You start by adding up all the income you have received across each category. For someone running a small business who also owns a rental property and has some savings interest, that means adding trading profit, rental income, and interest income separately.
Step 2: Deduct Qualifying Payments
Certain payments you make during the tax year can be subtracted from your total income before tax is even considered. These are called deductible payments. The most common examples are personal pension contributions paid under certain arrangements and gift aid donations to charity.
This step reduces your net income, the figure that sits between gross income and the final taxable amount. A lower net income can keep you in a lower tax band and, as we will see shortly, can protect your personal allowance.
Step 3: Apply the Personal Allowance
The personal allowance is the amount of income everyone in the UK can earn completely free of tax each year. For the 2025/26 tax year, that figure is £12,570.
This allowance is deducted from your net income, starting with non-savings income. Only the income remaining after this deduction, your taxable income, will actually be charged any tax.
A straightforward example: if your net income is £40,000 and your personal allowance is £12,570, your taxable income is £27,430. Tax is calculated on that £27,430, not on the full £40,000.
Income tax thresholds: frozen until 2028/29
The personal allowance (£12,570), the basic rate band (£37,700), and the higher rate threshold (£125,140) have been frozen since 2022/23 and are set to remain unchanged until 2028/29. The rates and figures in this article apply to 2025/26 and 2026/27.
The Personal Allowance Trap: What Happens Above £100,000
Here is the part that surprises a great many people when they first learn about it.
The personal allowance is not a fixed benefit available to everyone regardless of earnings. Once your adjusted net income exceeds £100,000, your personal allowance starts to be taken away. Adjusted net income is your net income reduced by any pension contributions or gift aid payments that qualify for relief.
The rule is simple but the effect is severe: for every £2 your adjusted net income is above £100,000, you lose £1 of personal allowance. Someone with adjusted net income of £110,000 loses £5,000 of personal allowance. Someone earning £125,140 or more loses the allowance entirely. It reduces to zero.
Between £100,000 and £125,140, the effective income tax rate is 60%.
Each extra pound earned is taxed at 40% in the higher rate band.
At the same time, each extra pound triggers a withdrawal of £0.50 of personal allowance.
That exposed £0.50 is also taxed at 40%, adding a further 20p of tax per pound earned.
The combined effect: 60p of tax on every additional pound in this range.
This is why pension planning and charitable giving become especially valuable for people with income near or above £100,000.
How Tax Is Actually Calculated: The Three Rates
Once you know your taxable income, HMRC applies tax at one of three rates, depending on which portion of income you are looking at. Notice that the basic rate band covers the first £37,700 of taxable income, not total income. The personal allowance comes first, and tax bands apply to what remains.
Income tax rates and bands: 2025/26 tax year (England, Wales and Northern Ireland)
| Tax Band | Taxable Income Range | Rate |
| Basic Rate | First £37,700 of taxable income | 20% |
| Higher Rate | Next £87,440 (up to £125,140) | 40% |
| Additional Rate | Everything above £125,140 | 45% |
Worked Example A: Standard Earner with a Deductible Payment
Mr Green runs a small consultancy. His trading income for 2025/26 is £47,500. He makes a pension contribution of £900 that qualifies as a deductible payment.
Tax calculation for Mr Green | 2025/26
| Description | Amount |
| Trading income | £47,500 |
| Less: deductible payment (pension) | (£900) |
| Net income | £46,600 |
| Less: personal allowance | (£12,570) |
| Taxable income | £34,030 |
| £34,030 at 20% (basic rate) | £6,806 |
| Tax liability | £6,806 |
Mr Green’s taxable income of £34,030 falls entirely within the basic rate band of £37,700. All of it is taxed at 20%. His pension contribution of £900 reduced net income from £47,500 to £46,600, saving him £180 in tax (£900 at 20%).
Worked Example B: High Earner with Personal Allowance Withdrawn
Ms Carter has trading income of £165,000 in 2025/26. She makes no deductible payments. Her adjusted net income exceeds £125,140, so her personal allowance is reduced to zero.
Tax calculation for Ms Carter | 2025/26
| Description | Amount |
| Trading income | £165,000 |
| Less: deductible payments | £0 |
| Net income | £165,000 |
| Less: personal allowance (nil: income above £125,140) | £0 |
| Taxable income | £165,000 |
| £37,700 at 20% (basic rate) | £7,540 |
| £87,440 at 40% (higher rate) | £34,976 |
| £39,860 at 45% (additional rate) | £17,937 |
| Tax liability | £60,453 |
Ms Carter’s income sits across all three bands. The basic rate band covers £0 to £37,700 of taxable income (£37,700 x 20% = £7,540). The higher rate band covers £37,701 to £125,140, a range of £87,440 (£87,440 x 40% = £34,976). The additional rate applies to the remainder: £165,000 minus £125,140 = £39,860 (£39,860 x 45% = £17,937). Total: £60,453.
Key Figures to Remember
| Item | Amount |
| Personal allowance | £12,570 |
| Income at which allowance begins to reduce | £100,000 |
| Income at which allowance reaches zero | £125,140 |
| Basic rate (20%) band | £0 to £37,700 of taxable income |
| Higher rate (40%) band | £37,701 to £125,140 |
| Additional rate (45%) | Above £125,140 |
Figures are for 2025/26. Thresholds are frozen and unchanged for 2026/27.
See you soon.
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