Self Assessment Tax Bill Calculator UK 2025–26
Estimate your Self Assessment tax bill for 2025–26. Enter your self-employment profit to see income tax, Class 4 National Insurance, and student loan repayments in one place.
How Self Assessment tax works for the self-employed
When you are self-employed, you are responsible for calculating and paying your own tax. Unlike employees whose tax is deducted at source via PAYE, self-employed individuals report their income and expenses via an annual Self Assessment tax return and pay the resulting bill directly to HMRC.
Income tax on self-employment profit
Income tax is charged on your taxable profit — the amount remaining after allowable business expenses are deducted from your income. The personal allowance (£12,570 in 2025–26) is deducted first, and the remainder is taxed at 20% (basic rate, up to £50,270), 40% (higher rate, £50,270–£125,140), or 45% (additional rate, above £125,140). The personal allowance tapers for high earners: it reduces by £1 for every £2 of income above £100,000, reaching zero at £125,140.
Class 4 National Insurance
Self-employed people pay Class 4 NI on profits above the lower profits limit. In 2025–26, Class 4 NI is charged at 9% on profits between £12,570 and £50,270, and 2% on profits above £50,270. Class 4 NI does not count towards your NI record for state pension purposes — Class 2 NI (or voluntary contributions) does that.
Class 2 NI (£3.45/week if profits exceed £12,570) is treated as a voluntary payment from 2024–25 but is still worth paying to protect your state pension entitlement. This calculator does not include Class 2 NI.
Payments on account
HMRC operates a payments on account system for Self Assessment taxpayers with a bill above £1,000. You make two advance payments towards the following year's bill — each equal to 50% of the current year's total. The first payment on account is due by 31 January alongside your tax bill; the second is due 31 July. Any balancing payment (if your actual bill is higher than the payments made) is due 31 January the following year.
In your first year of self-employment, your January bill can therefore be 150% of the year's tax (100% for the year just ended, plus 50% payment on account for the next year). Planning ahead for this is important.
Student loan repayments via Self Assessment
If you have a student loan, repayments are calculated on your total income above the threshold for your plan type. For sole traders, HMRC collects student loan repayments through Self Assessment rather than PAYE. The repayment is included on your Self Assessment return and paid alongside your tax bill.
Frequently asked questions
Who needs to file a Self Assessment tax return?
You must file a Self Assessment return if you were self-employed as a sole trader and earned more than £1,000, if you were a partner in a business partnership, if your income was over £100,000, if you had untaxed income over £2,500 (from renting property, for example), or if you received income from abroad. HMRC also requires returns for those with complex tax affairs, such as those with significant investment income or Child Benefit where income exceeded £60,000.
What is Class 4 National Insurance?
Class 4 NI is the National Insurance paid by self-employed individuals on their profits. It is calculated at 9% on profits between £12,570 and £50,270, and 2% on profits above £50,270. Unlike employed workers who pay Class 1 NI through PAYE, self-employed people pay Class 4 via their Self Assessment return. You may also owe Class 2 NI (£3.45/week) if your profits exceed the Small Profits Threshold of £12,570.
What are payments on account?
Payments on account are advance payments towards your next year's tax bill. HMRC requires two payments on account — one by 31 January and one by 31 July — each equal to 50% of your previous year's Self Assessment bill. This means in your first year of self-employment you pay your full tax bill by 31 January, plus 50% on account for the following year. If your income is likely to be significantly lower in the following year, you can apply to reduce your payments on account.
What expenses can I deduct from self-employment profit?
You can deduct allowable business expenses before calculating your taxable profit. These include office costs, travel expenses (excluding ordinary commuting), stock and materials, marketing, professional fees, and a proportion of home costs if you work from home. Capital purchases (equipment, vehicles) are claimed via the Annual Investment Allowance rather than as direct expenses. HMRC's simplified expenses scheme offers flat rates for mileage, working from home, and living at your business premises.
How does the personal allowance apply to self-employment income?
The standard personal allowance of £12,570 applies to self-employment profit in the same way as employment income. Profit up to £12,570 is free of income tax. Profit between £12,570 and £50,270 is taxed at 20% (basic rate). Profit between £50,270 and £125,140 is taxed at 40% (higher rate). Above £125,140 the rate is 45%. The personal allowance tapers by £1 for every £2 of income above £100,000.
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This calculator provides estimates only. Results are based on the 2025–26 tax year. Credibrate is not a tax adviser. For personalised advice speak to a qualified accountant.