Self Assessment Tax Calculator
The first £12,570 of self-employed profit is tax-free; above that, Income Tax is 20%, 40% or 45%, plus Class 4 National Insurance at 6% and 2% (gov.uk). This calculator estimates your 2024/25–2026/27 Self Assessment bill instantly.
Who this calculator is for
If you're a sole trader, this calculator estimates the two taxes HMRC charges on your trading profit through Self Assessment: Income Tax and Class 4 National Insurance. It uses the same taxable-income-band method HMRC applies, and the same frozen thresholds published for the 2024/25 through 2026/27 tax years.
How the Income Tax figure is worked out
Every taxpayer gets a tax-free Personal Allowance, currently £12,570, before Income Tax applies. Above £100,000 of profit that allowance is withdrawn at £1 for every £2 of profit over the threshold, until it reaches £0. What's left of your profit after the allowance is your taxable income, and it's taxed in bands: 20% up to the basic-rate limit, 40% on the next slice up to the higher-rate limit, and 45% above that.
How Class 4 National Insurance is worked out
Self-employed profit above the Class 4 lower profits limit (aligned with the Personal Allowance) is charged National Insurance at 6% up to the upper profits limit, and 2% on anything above that. Class 4 NI is calculated on profit, not on taxable income after your Personal Allowance — so it starts as soon as your profit crosses the lower limit, even though your Income Tax hasn't.
What this calculator doesn't do
It only covers trading profit — it doesn't add other income (employment, dividends, rental) that would affect your overall tax band, and it doesn't include Class 2 NI, student loan repayments, or any reliefs and allowances specific to your circumstances (marriage allowance, pension contributions, Gift Aid). For a full picture that includes all your income sources, use ac-co's Self Assessment filing service, which runs the complete calculation HMRC will apply to your return.
Why the numbers are frozen across three tax years
The Personal Allowance, basic and higher-rate thresholds, and the NI limits shown here have been frozen by the government since 2021/22, and stay frozen through the 2026/27 tax year covered by this calculator. That's a policy choice (a 'stealth' tax rise as wages grow into higher bands), not an error in the calculator — the same fixed numbers apply whichever of the three years you select.
A worked example
Take a sole trader with £45,000 of annual profit and no other income. The first £12,570 is tax-free under the Personal Allowance, leaving £32,430 taxable. All of that falls within the basic-rate band, so it's taxed at 20% — £6,486 of Income Tax. Class 4 NI is charged on profit above £12,570 up to £50,270 at 6%, so on £32,430 of profit above that threshold the NI comes to £1,945.80. Together, roughly £8,432 is owed on £45,000 of profit, leaving take-home profit of around £36,568 before any other reliefs.
Common mistakes to avoid
The most common error is entering turnover instead of profit — this calculator, like HMRC, taxes profit after allowable business expenses, not the total amount your business took in. The second is forgetting payments on account: if your tax bill is over £1,000 and less than 80% of it is collected at source, HMRC usually asks for two advance payments toward next year's bill alongside your current one, which can roughly double what's due on 31 January. The third is applying this year's calculation to a different tax year without checking — the thresholds shown here are frozen across 2024/25 to 2026/27, but that freeze is a specific policy decision, not a permanent feature of the tax system.
Filing the actual return
This calculator gives you a same-day estimate so you can plan for your tax bill, including the two payments on account HMRC may ask for in advance. When you're ready to file, ac-co's Self Assessment service turns your bank transactions and documents into a submitted SA100, with the same Income Tax and Class 4 NI calculation running underneath, so the number you see here should match what you eventually owe.
Questions about the self assessment tax calculator
Do I pay Income Tax and National Insurance on the same profit?
Yes — they're calculated separately but both apply to your self-employed trading profit. Income Tax uses your Personal Allowance and rate bands; Class 4 NI has its own lower and upper profit limits.
Is this the same calculation HMRC uses for my SA100?
It uses the same rates and thresholds HMRC publishes for Income Tax and Class 4 NI, but your actual Self Assessment bill also depends on any other income, reliefs and allowances you're entitled to, which this calculator doesn't include.
Why did my tax rate jump so much above £100,000?
Above £100,000 of profit, £1 of your Personal Allowance is withdrawn for every £2 you earn, so more of your income becomes taxable at the same time as it moves into higher bands — the combined effect is often called the '60% tax trap'.
Does this include Class 2 National Insurance?
No. Class 2 NI is a separate, smaller flat-rate charge with its own small-profits threshold — this calculator only covers Income Tax and Class 4 NI on trading profit.
What if I also have a job as well as self-employment?
This calculator only models self-employed profit in isolation. Employment income uses your Personal Allowance first under PAYE, which changes how much of that allowance is left for your self-employed profit — file with ac-co for a calculation across all your income.