Self-Assessment Tax Calculator 2026-27

Self Assessment is the system HMRC uses to collect income tax from people whose tax is not fully deducted at source. This includes the self-employed, company directors, landlords and anyone with significant untaxed income. There is no longer an income threshold that forces you to file: the old £100,000 test rose to £150,000 and was then removed altogether from the 2024/25 tax year, so PAYE earners are judged on the type of income they receive rather than how much they earn.

This calculator estimates your Self Assessment tax bill by combining income tax, Class 4 NI (6% on profits between £12,570 and £50,270, then 2% above) and any student loan repayments. On £55,000 of self-employed profit that comes out as £9,432 of income tax plus £2,356.60 of Class 4, being £2,262 in the main band and £94.60 above it, giving a bill of £11,788.60. Class 2 adds nothing, having been abolished from 6 April 2024.

Filing is mandatory if you are self-employed earning over £1,000, the trading allowance threshold, a company director in most cases, or a member of a partnership. It also bites with property income over £1,000, or rent-a-room income over £7,500, with untaxed income such as foreign earnings, dividends outside PAYE or savings interest above the Personal Savings Allowance, and with capital gains over the £3,000 annual exempt amount. Gains on residential property carry a separate 60-day reporting duty. First-timers must register by 5 October following the tax year.

The online filing deadline is 31 January following the tax year, so a 2026/27 return is due by 31 January 2028, while paper returns must be in by 31 October. Payment on Account may apply: if your tax bill is over £1,000 and less than 80% was collected at source, you make two advance payments toward the next year's bill, each 50% of the previous year's total, due on 31 January alongside the current balance and again on 31 July. The balancing payment for that year then falls due on 31 January the following year.

Penalties escalate quickly. Late filing costs £100 immediately, even if no tax is owed, then £10 a day from 1 May for a maximum of 90 days, which is £900, then 5% of the tax owed or £300, whichever is higher, at 6 months, and the same again at 12 months. That means at least £1,600 in total, and £1,000 plus 10% of the tax once the tax due passes £6,000. Late payment adds its own 5% surcharge at 30 days, at 6 months and at 12 months, plus daily interest at the HMRC rate, set at Bank of England base rate plus 4% since 6 April 2025, which is 7.75% with Bank Rate at 3.75%.

Different penalties apply once you are inside Making Tax Digital for Income Tax, which became mandatory on 6 April 2026 for sole traders and landlords with qualifying income over £50,000. Late payment there starts earlier, at 3% of the tax owed if you pay 16 to 30 days late, although that charge is waived in your first year. Whichever regime covers you, a Time-to-Pay arrangement is worth asking about, because HMRC will agree instalments if you call before the deadline and a genuine cash-flow problem carries no penalty.

Expenses must be wholly and exclusively for the business, which covers office costs such as rent, rates and utilities, travel that is not your commute, training, professional fees, advertising, equipment, and vehicle costs at mileage rates of 55p and 25p or on actual figures. Simplified expenses offer a flat £25 to £26 a month for working from home and 55p a mile for the first 10k miles. Capital allowances give 100% Annual Investment Allowance up to £1M a year on plant and machinery, while cars sit at a writing-down rate of 14%, or 6% for high-emission models. Anything with a private element is apportioned, so 30% of a home might count as office space.