Estimate your Self-Assessment bill as a sole trader, freelancer, or partner. Enter your annual profits (turnover minus allowable expenses) and the calculator works out Income Tax via the standard bands plus National Insurance Class 2 (voluntary) and Class 4. It also tells you what each payment on account will be in January and July.
Income Tax is calculated on (profits − Personal Allowance) using the standard England/Wales/NI bands: 20% on the first £37,700 of taxable income, 40% up to £125,140, and 45% above. National Insurance Class 4 is paid through Self Assessment: 6% on profits between £12,570 and £50,270, then 2% above. Class 2 is voluntary post-2024 reform but adds £3.45/week (£179.40/year) for the contribution-record benefit once profits exceed £6,725. Payments on account are 50% of your total tax + NI bill, due 31 January (current year) and 31 July (following year).
Anyone who is self-employed (sole trader or partner), earned more than £1,000 from side gigs, received untaxed income (rent, dividends above £500), is a higher-rate taxpayer claiming pension relief, has capital gains above the annual exempt amount, or HMRC has specifically asked them to. The threshold for sole-trader registration is £1,000 of trading income.
If your last Self-Assessment bill was over £1,000 (and less than 80% was collected at source), HMRC requires two advance payments toward next year's bill: 50% on 31 January and 50% on 31 July. They're a forecast — when you file the actual return, any difference is a balancing payment (or refund). First-year filers don't have payments on account to pay; second year is when they kick in.
Wholly and exclusively business expenses: office costs, travel and subsistence, professional fees, marketing, equipment (with capital allowances for big-ticket items), use-of-home calculations, training relevant to your existing trade. Personal/private expenses, business entertaining, and capital expenditure (claimed via capital allowances instead) are not deductible. Keep records for at least 5 years after the 31 January following the tax year.
Register with HMRC by 5 October following the end of the tax year you started self-employment. The deadline to file online is 31 January after the end of the tax year (paper filers must submit by 31 October). Late filing is an automatic £100 penalty plus interest on unpaid tax.
No — HICBC is not modelled here. If your adjusted net income is between £60,000 and £80,000 (thresholds raised from April 2024), you pay back some or all of any Child Benefit you receive via Self Assessment. Factor this in separately if relevant.
Carry forward lets you contribute more than the £60,000 annual pension allowance in a single tax year by using unused allowance from the previous three years. Here's exactly how it works in 2026/27, who qualifies, and how the tapered allowance changes the maths.
How UK National Insurance works in 2026/27: Class 1 employee and employer rates, Class 2 and Class 4 for the self-employed, and the thresholds that drive each.
Last updated: May 2026 · Rates sourced from HMRC
