Self Assessment Tax Calculator

This calculator computes your Self Assessment tax bill for the 2025/26 tax year (6 April 2025 to 5 April 2026). It covers income tax on your total income, Class 4 National Insurance on self-employed profits, and the payment on account amounts you may owe HMRC in January and July.

The calculator works by first determining your total income — the sum of your self-employed profit after allowable expenses plus any other income such as rental income, dividends, or savings interest. It then subtracts any pension contributions you make to a personal pension, since these attract tax relief at source and reduce your taxable income accordingly.

Your personal allowance of £12,570 is then applied. For incomes above £100,000, the personal allowance is tapered — reduced by £1 for every £2 of income above the £100,000 threshold — which creates an effective 60% marginal tax rate between £100,000 and £125,140 where the allowance is fully withdrawn.

Income tax is then charged at 20% on taxable income up to £37,700 (the basic rate band), 40% on income from £37,701 to £112,570 (the higher rate band), and 45% on any income above £112,570 (the additional rate band). These are the England, Wales, and Northern Ireland rates; Scotland has different rates.

Class 4 National Insurance is charged only on your self-employed profits. Class 2 NI was abolished from 6 April 2024, so there is no longer a flat weekly Class 2 charge. Class 4 is charged at 6% on profits between £12,570 and £50,270, and 2% on profits above £50,270.

If you also receive employment income taxed under PAYE, any income tax already deducted through your tax code is subtracted from your total liability to arrive at your final SA bill due. Pension contributions into a personal pension (not salary sacrifice) are made gross or with basic-rate relief at source, and the calculator allows for this deduction from your taxable income.

The SA bill due figure is the amount you owe HMRC for the tax year after accounting for any PAYE tax already deducted. This is not necessarily what you pay in one lump sum — HMRC operates a system of payments on account that spreads your liability across two advance instalments.

Payments on account are required when your Self Assessment tax bill exceeds £1,000 and less than 80% of that bill was collected at source through PAYE. They are calculated as 50% of the prior year's SA bill — not the current year's — which means they may over- or under-estimate your actual liability. If you have overpaid, HMRC issues a refund; if you have underpaid, you pay a balancing payment by 31 January following the tax year.

One of the most effective ways to reduce your SA bill is to increase pension contributions. Personal pension contributions made to a registered pension scheme extend your basic rate band and can restore your personal allowance if your income is in the £100,000–£125,140 taper zone. A contribution of £10,000 gross into a pension when your income is £105,000 can save up to £6,000 in income tax by recovering £5,000 of personal allowance and paying 40% on the remaining £5,000 that shifts back to basic rate.

Reducing your taxable profit through allowable business expenses is the other primary lever. HMRC allows deductions for costs wholly and exclusively incurred for the purposes of the trade — including office costs, travel, equipment, professional subscriptions, and a proportion of home working costs. The trading allowance of £1,000 means sole traders with gross income below this threshold owe no tax and do not need to file.

If you cannot pay your SA bill in full, HMRC offers a Time to Pay arrangement, typically allowing monthly instalments. You should contact HMRC before the payment deadline to agree a plan and avoid late payment interest (currently 7.25% per annum). A late filing penalty of £100 applies automatically if your return is filed after 31 January, with further daily penalties of £10 per day after three months.