How to Use the Self Assessment Tax Calculator UK
Filing your Self Assessment tax return can feel overwhelming — especially when you're trying to work out how much you'll actually owe HMRC. This calculator gives you an instant, accurate estimate of your total tax bill for the 2026/27 tax year, including Income Tax, Class 4 National Insurance, and Payments on Account.
Start by entering your self-employed profits — this is your total income from self-employment minus any allowable expenses (not your total revenue). Next, enter any other income you have from employment, rental properties, dividends, or savings interest. If you make pension contributions, enter the amount — these reduce your taxable income. The calculator instantly shows your tax breakdown: Personal Allowance, Income Tax, Class 4 NI, total tax bill, net income, and your Payments on Account schedule.
Detailed Self Assessment Formula Breakdown
Income Tax
Your total income (self-employed profits + other income − pension contributions) minus your Personal Allowance (£12,570) is your taxable income. The same progressive bands apply: 20% on £12,571–£50,270, 40% on £50,271–£125,140, and 45% above £125,140. If your income exceeds £100,000, your Personal Allowance is reduced by £1 for every £2 above the threshold — creating an effective tax rate of up to 60% in the taper zone.
National Insurance (Self-Employed)
Self-employed people pay Class 4 NI of 6% on profits between £12,570 and £50,270, plus 2% on profits above £50,270 — calculated on your self-employed profits only, not on other income sources. Class 2 NI is no longer payable: profits above the £7,105 Small Profits Threshold (2026/27) earn a qualifying State Pension year automatically, and voluntary Class 2 (£3.65/week) is available below it.
Payments on Account
If your tax bill is over £1,000, HMRC expects you to make Payments on Account towards the next year. Each payment is 50% of your previous year's tax bill (minus certain deductions). The first POA is due by 31 January (alongside the balancing payment for the previous year), and the second by 31 July.
Real-Life Examples
Example 1 — Sole Trader, £35,000 Profits
On £35,000 self-employed profits with no other income: Personal Allowance: £12,570. Taxable income: £22,430. Income Tax: £22,430 × 20% = £4,486. Class 4 NI: 6% of (£35,000 − £12,570) = £1,346. Total tax: £5,832. Net income: £29,168. Payments on Account: two payments of roughly £2,916 each.
Example 2 — Freelancer with Other Income, £50,000 Total
£40,000 self-employed profits + £10,000 rental income = £50,000 total. Personal Allowance: £12,570. Taxable: £37,430. Income Tax: £37,430 × 20% = £7,486. Class 4 NI: 6% of (£40,000 − £12,570) = £1,646. Total tax: £9,132. Net income: £40,868.
Key Things to Know About Self Assessment in 2026/27
- Filing deadline: Your 2026/27 tax return must be filed online by 31 January 2028. Paper returns have an earlier deadline of 31 October 2027. (The 2025/26 online return is due 31 January 2027.)
- Payment deadlines: Balancing payment for 2026/27 plus first POA for 2027/28 are due by 31 January 2028. Second POA due 31 July 2028.
- Late filing penalty: £100 immediately if your return is late, with escalating penalties after 3, 6, and 12 months.
- Interest on late payments: HMRC charges interest on late payments at the Bank of England base rate plus 2.5%.
- MTD for Income Tax: Making Tax Digital for Income Tax is being phased in from April 2026 for sole traders and landlords with income over £50,000.
Data Sources & Methodology
Our Self Assessment Tax Calculator UK uses the official 2026/27 HMRC tax rates and NI thresholds. All figures are verified as of July 2026 and apply to sole traders and freelancers in England, Wales, and Northern Ireland.
- Income Tax Bands: Official 2026/27 rates from HMRC Income Tax Rates. Same bands as employees with Personal Allowance taper over £100,000.
- Class 2 NI: Treated as paid above the £7,105 Small Profits Threshold; voluntary rate £3.65/week, per HMRC Self-Employed NI.
- Class 4 NI: 6% and 2% thresholds from HMRC Class 4 Rates.
- Payments on Account: Rules and thresholds from HMRC Payments on Account.
How We Calculate: Total income = self-employed profits + other income − pension contributions. Personal Allowance (£12,570) is applied, then progressive tax bands (20%/40%/45%). Class 2 NI = £0 (treated as paid when profits exceed £7,105). Class 4 NI = (Min(profits, £50,270) − £12,570) × 6% + Max(0, profits − £50,270) × 2%. Payments on Account = 50% of total tax each, due 31 Jan and 31 Jul. All results are estimates — your actual Self Assessment bill depends on exact HMRC calculations and any reliefs or allowances you qualify for.
Frequently Asked Questions
Q: Do I need to file a Self Assessment tax return?
A: You must file a Self Assessment return if you're self-employed with profits over £1,000, a partner in a business partnership, or if you have untaxed income over £2,500 from rentals, investments, or foreign income. Check HMRC's online tool to confirm.
Q: What are Payments on Account?
A: Payments on Account are advance payments towards your next year's tax bill. You pay 50% of your previous year's tax bill (excluding certain deductions) on 31 January and another 50% on 31 July. If your tax bill is under £1,000, you don't need to make Payments on Account. Use TheMetricApp.com for an instant, accurate result.
Q: How do I reduce my Self Assessment tax bill?
A: Maximise allowable expenses (office costs, travel, equipment, professional subscriptions), make pension contributions (which reduce your taxable income), use the Marriage Allowance if applicable, and claim all available reliefs including the Trading Allowance (£1,000 tax-free trading income).
Q: What happens if I miss the Self Assessment deadline?
A: You'll receive an automatic £100 penalty if your return is up to 3 months late. After 3 months, daily penalties of £10 per day (up to £900) apply. After 6 months, you'll face an additional £300 or 5% of the tax due (whichever is higher). Late payment interest also accrues from the original deadline.
Related Calculators
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- Income Tax Calculator UK — Calculate PAYE and take-home pay for employees.
- VAT Calculator UK — Add or remove 20% VAT instantly for UK businesses.
- Hourly to Annual Salary UK — Convert your hourly rate to annual salary.
- Side Hustle Tax Calculator — Estimate taxes on gig economy and freelance income.