The UK tax year starts on 6 April, not 1 January, and the reason why is genuinely strange. Here is what the dates actually mean for your tax bill.
Most countries align their tax year with the calendar year. The UK does not. Ours begins on 6 April and ends on 5 April the following year, a quirk that dates back to calendar reform in 1752 when Britain switched from the Julian to the Gregorian calendar and lost 11 days. The Treasury, reluctant to lose 11 days of tax revenue, shifted the financial year accordingly, and the date has stayed ever since. Knowing this does not change your tax bill by a penny, but it does explain why your payslip, P60, and pension statements all reset at a point that feels arbitrary.
The most common practical mistake sole traders make is mixing up calendar-year figures with tax-year figures. If you started freelancing in October 2024 and used your bank statements for the period January to December 2025 to estimate your tax, you would have the wrong numbers entirely. HMRC wants your income and expenses for 6 April 2024 to 5 April 2025 (the 2024/25 tax year), not the calendar year.
The same confusion hits people who switch jobs mid-year, invest in an ISA in January thinking they still have time (they do, until 5 April), or try to claim expenses from the previous calendar year without checking whether those costs actually fell in the correct tax year.
Here are the dates that actually matter if you are filing, paying or planning in the current tax year.
| Date | What happens |
|---|---|
| 6 April 2025 | 2025/26 tax year begins; new allowances and thresholds apply |
| 31 July 2025 | Second payment on account due (if you pay self-employed tax in instalments) |
| 5 October 2025 | Deadline to register for Self Assessment if you are newly self-employed |
| 5 April 2026 | 2025/26 tax year closes; ISA allowance resets |
| 31 January 2027 | Online Self Assessment return and tax payment for 2025/26 must reach HMRC |
The gap between the tax year closing (5 April 2026) and the payment deadline (31 January 2027) is nine months. That sounds generous until January arrives and you realise you have not saved anything.
HMRC refers to tax years by straddling both calendar years: 2025/26 means 6 April 2025 to 5 April 2026. You will see both years on official documents, payslips and P60s. When a tax code, threshold or rate is quoted as "2025/26", it applies for that entire 12-month window, not just the first calendar year mentioned.
This naming convention matters when you are looking up allowances. The Personal Allowance for 2025/26 remains £12,570, meaning the first £12,570 of income you earn between 6 April 2025 and 5 April 2026 is free from income tax. An income figure from March 2025 belongs to the 2024/25 tax year, even though the calendar says 2025.
Sarah shoots weddings and went self-employed on 1 September 2024. Her first full tax year with HMRC is 2024/25, covering 6 April 2024 to 5 April 2025. She invoiced £28,000 and spent £4,200 on kit, travel and software between those two dates.
Sarah's combined bill is approximately £3,257 for the tax year, due 31 January 2026. She files her Self Assessment return for 2024/25 by the same date. If she had mistakenly used her September to August figures, she would have filed for the wrong period and potentially paid the wrong amount.
To avoid Sarah's near-miss, TapTax's quarterly tax planner lets you map income and expenses across the actual tax year so your running tax estimate always reflects what HMRC will see, not what your bank app shows.
The tax year boundary has real consequences beyond income tax. Your ISA allowance (£20,000 in 2025/26) resets on 6 April each year; any unused portion cannot be carried forward. Capital gains, dividends and pension contributions are also measured within the tax year, each with their own annual allowances that expire on 5 April.
For sole traders and freelancers reading the TapTax blog, one of the highest-value habits is a simple tax-year review a few weeks before 5 April: have you used your ISA allowance? Have you made pension contributions that reduce your tax bill for this year rather than next? These decisions have a hard deadline.
The UK tax year does not care about your calendar. Everything resets on 6 April, and every allowance you do not use by 5 April is gone.
Not everything changes annually, but you should check each April for updates to: income tax bands and the Personal Allowance; National Insurance thresholds and rates; the ISA subscription limit; capital gains and dividend allowances; and pension annual allowance. HMRC confirms the new figures in the Spring Budget, typically held in March, giving taxpayers a few weeks to plan before the new year opens.
TapTax connects to your bank, categorises expenses automatically, and submits quarterly updates to HMRC. Free plan, no card required.
