Self Assessment is the UK system for declaring untaxed income to HMRC and paying the income tax and Class 4 National Insurance due. About 12 million people file a Self Assessment return each year. For 2026/27 the filing deadline for online returns is 31 January 2028, and the same date is when the tax for 2026/27 is due plus the first payment on account towards 2027/28. This calculator estimates your Self Assessment bill from the income, expense and pension figures you enter, including Class 4 NI, dividend tax, payments on account and any high-income child benefit charge.
Who needs to file Self Assessment
You usually need to file if any of the following apply for a tax year:
- You were self-employed as a sole trader and earned more than £1,000 of trading income.
- You were a partner in a business partnership.
- You earned over £150,000 of taxable income from any source.
- You received over £10,000 of income from dividends or savings interest.
- You had any untaxed property income (rental income above the £1,000 property allowance).
- You received tips or commission that wasn’t taxed at source.
- You received foreign income that you need to pay UK tax on.
- You or your partner received Child Benefit and either of you had income above £60,000 (the High Income Child Benefit Charge threshold).
- You sold or disposed of an asset and have CGT to pay (though residential property CGT has its own 60-day reporting route).
- You’re a director of a limited company who has received untaxed income (dividends, untaxed benefits, etc.).
The list isn’t exhaustive — HMRC publishes a check tool at gov.uk/check-if-you-need-tax-return. If you’re sent a notice to file, you must file even if you think you don’t owe tax.
The 2026/27 deadlines
| Date | What’s due |
|---|---|
| 5 April 2027 | End of the 2026/27 tax year |
| 6 April 2027 | Earliest you can file the 2026/27 return |
| 5 October 2027 | Deadline to register for Self Assessment if you didn’t file last year |
| 31 October 2027 | Paper filing deadline (almost no one files on paper) |
| 31 January 2028 | Online filing deadline AND payment deadline for 2026/27 tax AND first payment on account for 2027/28 |
| 31 July 2028 | Second payment on account for 2027/28 |
Missing the 31 January deadline triggers an automatic £100 late filing penalty even if you owe no tax. Further penalties accrue at 3 months (£10/day for 90 days), 6 months (5% of tax due, minimum £300), and 12 months (another 5% or up to 100% in serious cases). Late payment of tax attracts interest from 1 February plus a 5% surcharge after 30 days, 6 months and 12 months.
What goes on the return
A typical sole trader’s Self Assessment return includes:
- Self-employment pages (SA103): income, allowable expenses (or simplified flat-rate expenses), profit, capital allowances on assets purchased.
- Main tax return (SA100): salary and pension from any employment, savings and dividend income, charitable donations, pension contributions for higher-rate relief claim, untaxed interest.
- Property pages (SA105) if you have rental income.
- Capital Gains Tax pages (SA108) if you have made disposals other than residential property reported separately.
- Foreign income pages (SA106) if you have foreign income.
The calculator on this page combines all income types into the bill: trading profit + employment + savings + dividends + property − pension contributions = taxable income. Tax bands apply, then Class 4 NI on self-employed profit, then dividend tax on dividends, minus PAYE already paid, gives the balance due.
Worked example: sole trader with £45,000 profit
A sole trader with £45,000 trading profit, £2,000 of bank interest and a £2,000 personal pension contribution in 2026/27:
- Taxable income before pension: £45,000 + £2,000 = £47,000
- Pension contribution (relief at source, gross £2,500): basic-rate band extended by £2,500
- Income tax: (£12,570 to £52,770) Ã, 20% basic = (£47,000 − £12,570) Ã, 20% = £6,886
- Class 4 NI: (£45,000 − £12,570) Ã, 6% = £1,946 (only the trading profit, not the interest)
- Total Self Assessment bill: £8,832
- First payment on account for 2027/28: £4,416 due 31 January 2028
- Second payment on account: £4,416 due 31 July 2028
The first year of self-employment is the worst for cash flow because the January 2028 bill is £8,832 + £4,416 = £13,248 — almost two years of income tax in one payment. After year one, payments on account smooth the cycle.
Making Tax Digital from April 2026
From 6 April 2026, sole traders and landlords with qualifying income above £50,000 must keep digital records and submit quarterly updates to HMRC under Making Tax Digital for Income Tax (MTD ITSA). The annual Self Assessment deadline still applies, but the four quarterly updates plus an end-of-period statement are new. The £30,000 threshold follows in April 2027, and below £30,000 will phase in by 2030.
Most affected taxpayers need MTD-compatible software — Xero, QuickBooks, FreeAgent, Sage, FreshBooks and several others meet HMRC’s requirements. The calculator on this page works for any tax year, but the operational reality of MTD ITSA is more frequent reporting alongside the same annual reconciliation.
Key takeaways
- Self Assessment is required for sole traders, landlords with income above £1,000, directors with untaxed income and several other categories.
- The 31 January 2028 deadline covers filing the 2026/27 return, paying the 2026/27 tax and making the first payment on account towards 2027/28.
- Late filing triggers an automatic £100 penalty even when no tax is owed; further penalties accrue rapidly thereafter.
- Year-one Self Assessment is roughly 1.5x the regular annual bill because of the first payment on account.
- MTD for Income Tax begins April 2026 for income above £50,000 and is the biggest operational change to Self Assessment in two decades.
Frequently asked questions
What’s the penalty for missing the 31 January deadline? £100 automatic, regardless of whether you owe tax. Further penalties: £10 per day for 90 days starting 1 May (so up to £900 more), then 5% of the tax due (minimum £300) at 6 months, then another 5% or up to 100% at 12 months. Late payment surcharges of 5% apply at 30 days, 6 months and 12 months past the payment due date.
Can I file my Self Assessment early? Yes. You can file from 6 April after the tax year ends. Filing early gives you certainty on your bill and time to plan the cash flow before 31 January. The tax is still due on 31 January regardless of when you file.
Do I have to pay payments on account? You make payments on account if your previous Self Assessment bill was over £1,000 and less than 80% of your tax was deducted at source. Each payment is half of last year’s tax bill. If you expect lower income this year, you can apply to reduce them — see Payments on Account.
What if I owe tax but can’t pay? Contact HMRC as soon as possible. They offer Time to Pay arrangements that spread the bill over up to 12 months for most simple cases (online via your Self Assessment account if the bill is under £30,000). Don’t ignore the deadline — surcharges and interest accrue regardless.