Self-Assessment Tax Calculator 2026-27
Calculate your Self Assessment tax bill for 2026/27. Income tax + Class 4 NI (6%/2%). Class 2 abolished. Payments on Account if bill exceeds £1,000.
By Konstantin Iakovlev · Founder, Calks.uk
Last updated: · Verified against HMRC and GOV.UK 2026/27 rates
Quick Answer
Self Assessment for 2026/27 includes income tax (20%/40%/45%) plus Class 4 NI at 6%/2%. Class 2 was abolished in April 2024. Filing deadline: 31 January 2028 for the 2026/27 year. Penalty £100 for late filing.
Disclaimer
This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.
How It Works
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 income over £150,000 or significant untaxed income.
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.
Payment on Account may apply: if your tax bill is over £1,000 and less than 80% was collected at source, you may need to make two advance payments (each 50% of the previous year's bill) on 31 January and 31 July.
Who needs to file Self Assessment 2026/27. Mandatory if: self-employed earning over £1,000/year (trading allowance threshold); company director (most cases); partnership member; high earner £150k+ (was £100k, raised 2024); property income over £1,000 (or rent-a-room over £7,500); untaxed income (foreign, dividends not via PAYE, savings interest over Personal Savings Allowance); capital gains over £3,000 (AEA 2026/27); CGT on residential property (60-day reporting separate); HMRC sent you a notice. Register by 5 October following the tax year if first time.
Deadlines and Payments on Account. Online filing: 31 January following tax year (2026/27: file by 31 Jan 2028). Paper filing: 31 October (earlier deadline). Payments on Account (POA): apply if your tax bill is over £1,000 AND less than 80% is collected at source. POA = 2 advance instalments toward NEXT year's bill: 50% by 31 January (with current year balance), 50% by 31 July. Final balancing payment for that year due 31 January year after. Reduce POA if expecting lower next year — but interest applies if reduced too aggressively.
Self Assessment penalties — getting expensive fast. Late filing: £100 immediate (even if no tax owed); £10/day from 1 May (max 90 days = £900); 5% of tax owed (or £300, whichever higher) at 6 months; another 5%/£300 at 12 months. Maximum late filing penalty: £1,600 + 10% of tax. Late payment: 5% surcharge at 30 days, another 5% at 6 months, 5% at 12 months. PLUS daily interest at HMRC rate (BoE base + 2.5% = 7%+ 2026). Time-to-Pay arrangement: HMRC will agree instalments if you call before deadline (no penalty for genuine cash flow problems).
Allowable expenses for Self-Employed. Wholly-and-exclusively for business: office costs (rent, rates, utilities), travel (not commute), training, professional fees, advertising, equipment, vehicle (mileage 45p/25p OR actual costs). Simplified expenses: 25-£26/month flat rate for working-from-home; mileage 45p first 10k miles. Capital allowances: 100% Annual Investment Allowance up to £1M/year for plant and machinery; cars at writing-down rate 18% (or 6% for high-emission). Private element: apportion percentage (e.g., 30% home for office space).
Common Self Assessment mistakes. Forgetting to include: dividends from ISAs (NOT taxable — common mistake to include), employer pension contributions (already pre-tax), savings interest below £1,000 PSA (still report on form). Wrong reporting: gross vs net (always report gross income); calendar vs tax year; mixing personal and business. Late SA: register late penalty £100 + late filing penalties cascade — total often £600-£1,600 for procrastinators. Best practice: file in May-August after tax year ends — discover problems early, time to fix.
Example: Self-employed, £55,000 profit
- Income tax: £8,432 (PA + Basic + partial Higher)
- Class 4 NI: £2,264 + £94.60 = £2,358.60
- Total Self Assessment bill: £10,970
Frequently Asked Questions
- Who has to file Self Assessment?
- Must file if: self-employed earning over £1,000, company director (some exceptions), partnership member, high income (£150k+), untaxed income over £2,500 (rent, dividends, interest), capital gains over AEA, foreign income, OR HMRC sent you a notice. Even if no tax due, file if HMRC asks. Register by 5 October following the tax year if first time.
- Deadlines and Payments on Account.
- Online deadline: 31 January following tax year (2026/27: file by 31 Jan 2028). Paper: 31 October. POA: if tax bill >£1,000 and <80% collected at source, pay advance instalments — 50% by 31 January, 50% by 31 July. Apply to reduce POA if expecting lower next year — but interest applies if reduced too aggressively.
- Late filing penalties.
- Late filing: £100 immediate (even if no tax owed); £10/day from 1 May (max 90 days); 5% of tax (or £300) at 6 months; another 5%/£300 at 12 months. Late payment surcharges: 5% at 30 days, another 5% at 6 months, 5% at 12 months. Plus daily interest at HMRC's official rate (BoE base + 2.5%). Time-to-Pay arrangements available if you can't afford.