UK Self-Assessment Tax Estimator

You need to file a self-assessment tax return if you are self-employed and earned more than £1,000, if you earned more than £150,000 in total, if you have untaxed income (such as rental income, dividends over £10,000, or foreign income), or if you are a company director. You also need to file if you want to claim certain tax reliefs or if HMRC has asked you to.

If your only income is from employment and is fully taxed through PAYE, you usually do not need to file. However, higher rate taxpayers with pension contributions or Gift Aid donations often file to claim additional tax relief.

The tax year runs from 6 April to 5 April. For the 2025/26 tax year: you must register by 5 October 2026, file your paper return by 31 October 2026, and file your online return and pay your tax bill by 31 January 2027. Late filing incurs an automatic £100 penalty, rising to £10 per day after 3 months.

If your tax bill for the previous year was over £1,000, HMRC will ask you to make payments on account. These are advance payments towards next year's bill, due on 31 January and 31 July, each equal to half of your previous year's bill.

Payments on account can catch people off guard. If your self-assessment bill is £5,000, HMRC will also require two payments on account of £2,500 each towards the following year's bill. This means your first January payment could be £7,500 (the £5,000 balance plus the first £2,500 payment on account).

You can apply to reduce payments on account if you expect your income to be lower next year. If the payments on account turn out to be too much, HMRC will refund the difference after you file your next return.

Self-employed workers pay two types of NI. Class 2 NI is a flat rate of £3.45 per week (if profits exceed £6,725). Class 4 NI is 6% on profits between £12,570 and £50,270, then 2% above that. Both are collected through your self-assessment tax return.

Class 2 contributions count towards your State Pension entitlement. Even if your profits are below the threshold, you can choose to pay voluntarily to protect your pension record.