# Self Assessment Tax Estimator UK 2026/27 – Estimate Your Tax Bill > Estimate your Self Assessment tax bill for 2026/27 before you file. Covers income tax, National Insurance Class 2 & 4, and payments on account. Self-employed or earning outside of PAYE? Estimate your Self Assessment tax bill for 2026/27 — including payments on account — so you're not caught short when the deadline hits. This self assessment tax calculator estimates your bill including Class 4 National Insurance and payments on account — effectively a self employed tax calculator. Self Assessment is how HMRC collects tax from the self-employed, landlords, company directors and anyone with untaxed income. The bill often comes as a shock because it includes two things people forget: Class 4 National Insurance and payments on account. This estimator gives you a realistic figure before you file. ## What your Self Assessment bill includes A typical bill combines: - Income tax on your total taxable income, after the £12,570 allowance - Class 4 National Insurance — 6% on profits between £12,570 and £50,270, then 2% above - Class 2 National Insurance — no longer compulsory since 2024/25: treated as paid if your profits are at least £7,105, otherwise voluntary at £3.65 a week to protect your State Pension record - Tax on dividends, rental income or savings, if applicable ## Payments on account explained If your bill exceeds £1,000, HMRC usually requires payments on account — advance payments towards next year's tax, each equal to half your previous bill. This is why first-year bills often feel double the expected amount. You pay on 31 January and 31 July, with a balancing payment the following January. If your income has dropped, you can ask to reduce them. ## Deadlines and penalties The online filing deadline is 31 January following the end of the tax year. Late filing triggers an immediate £100 penalty, with further charges after 3, 6 and 12 months, plus interest on unpaid tax. Registering late is also a common and avoidable error — you must register by 5 October after your first tax year. Estimate your bill above, then plan your take-home with the [take-home pay calculator](https://salarycal.co.uk/take-home-pay-calculator/).