Compute Self-Assessment tax + NI on UK self-employed profit.
Total tax + NI due
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Income tax
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Class 4 NI (main)
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Class 4 NI (add'l)
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Your breakdown
Updates live as you type| Component | Basis | Amount |
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What your January bill is actually made of
For a sole trader, the Self Assessment bill is not one tax. It is income tax on your profit plus Class 4 National Insurance, and this calculator adds both. You enter net profit, the tool subtracts the £12,570 personal allowance, runs the remainder through the 2026/27 income tax bands (20% basic, 40% higher, 45% additional), and then layers Class 4 NI on top: 6% on profit between £12,570 and £50,270, and 2% on anything above. Class 2 NI sits in the background. Since April 2024 it is treated as paid with no cash to hand over, provided your profit clears the small profits threshold, so it adds nothing to the figure here.
A £45,000 profit, line by line
Run the default profit of £45,000. The first £12,570 is covered by the allowance, leaving £32,430 taxed at 20%. All of it sits inside the basic-rate band, so income tax is £6,486. Class 4 NI runs at 6% on the £32,430 between the thresholds, which is £1,946. Nothing reaches the 2% band because profit is under £50,270. Total due is £8,432, and net profit after tax and NI is £36,568.
About 81% of the profit lands in your pocket at this level. That ratio falls fast once profit climbs into the higher-rate band, because the marginal cost there jumps to 40% income tax plus 2% NI.
The payment-on-account surprise
Here is the part that catches new sole traders off guard, and it is the single most common complaint to HMRC about a first tax bill. Self Assessment is due by 31 January after the tax year ends. But once your bill passes £1,000, HMRC also asks for two payments on account toward the following year, each worth half your current bill. On a first return that means you settle the £8,432 in our example and immediately pay another £4,216 on account, so the 31 January cheque is closer to £12,648. A second payment on account follows on 31 July. Budget for roughly 150% of your first calculated bill, not 100%, or the January demand will hurt.
A practical tip on allowable costs
The figure that matters is net profit, not turnover. Before you type a number in, deduct every legitimate business expense: software, professional fees, mileage at HMRC approved rates, use-of-home, and equipment. If you trade through the cash basis, which is now the default for most unincorporated businesses, you record income and costs when money actually moves. Trimming £3,000 of genuine expenses off a £45,000 turnover saves you about £660 here, because each pound of profit at this level carries 20% tax plus 6% NI. Keep digital records as you go rather than reconstructing them in January, both because Making Tax Digital is steadily extending to sole traders and because a shoebox of receipts in the new year is how legitimate deductions get missed.
When is my first Self Assessment return due?
If you started trading in the 2025/26 tax year, you must register for Self Assessment by 5 October 2026 and file online by 31 January 2027. Miss the filing deadline and there is an automatic £100 penalty even if no tax is owed, with daily penalties stacking up after three months.
Does this cover dividends or rental income?
No. This calculator models sole-trader trading profit only. Dividends, rental profit, savings interest and capital gains all go on the same return but follow different rules and rates, so add those separately if they apply to you. Dividends, for instance, have their own £500 allowance and lower rates, and they do not attract National Insurance at all, so blending them into trading profit here would overstate your bill. If your tax affairs combine several income types, treat this figure as the trading slice and stack the others on top.