Why does the UK tax year start on 6 April?
The 6 April start date is a quirk of British tax history. Until 1752 the new year began on 25 March (Lady Day) under the Julian calendar. When Britain adopted the Gregorian calendar in September 1752, 11 days were dropped to align with Europe. To avoid losing 11 days of tax revenue, Treasury moved the new tax year to 5 April (25 March + 11 days). A further day was added in 1800 (a Julian leap year that wasn't a Gregorian leap year), giving us 6 April — and there it has stayed for 224 years.
Other countries align differently: Ireland, USA, France, Germany, Italy use the calendar year. Australia uses 1 July to 30 June. Japan uses 1 April to 31 March. The UK is unusual but the calendar is deeply entrenched in payroll, accounting software, and HMRC infrastructure — change has been discussed but never enacted.
Tax weeks and tax months for 2025/26
HMRC numbers each week (W1 to W52/W53) and each month (M1 to M12) of the tax year for PAYE purposes. This matters for:
- Cumulative PAYE — tax codes apply year-to-date based on the current tax week/month.
- Emergency tax codes (W1, M1, X) — non-cumulative basis applies allowance per week/month.
- Statutory benefits — SSP, SMP, Statutory Adoption Pay use tax weeks for entitlement counts.
- Pension contributions — annual allowance uses tax-year quarters.
Tax week 1 = 6-12 April. Tax month 1 = 6 April – 5 May. Subsequent weeks/months follow. A "53rd tax week" occurs in years where 5 April falls on a different day, requiring an extra payroll period.
Key 2025/26 deadlines:
- 5 October 2026 — Register for self-assessment if you became liable in 2025/26.
- 31 October 2026 — Paper self-assessment return deadline.
- 31 January 2027 — Online self-assessment return deadline AND payment of any tax due AND payment of first payment on account for 2026/27.
- 31 July 2027 — Second payment on account for 2026/27.
Three worked examples (UK 2025/26)
Example 1: 1 May 2025 — early in 2025/26
1 May 2025 falls in the UK 2025/26 tax year (after 6 April 2025). Tax week 4. Tax month 1.
Example 2: 1 March 2026 — late in 2025/26
1 March 2026 is still in the 2025/26 tax year (before 6 April 2026). Tax week 48. Tax month 12.
Example 3: 10 April 2026 — start of 2026/27
10 April 2026 falls after 5 April 2026, so it's in the new 2026/27 tax year. Tax week 1, tax month 1.
Common mistakes to avoid
- Confusing the personal tax year (6 Apr – 5 Apr) with the financial year for companies (1 Apr – 31 Mar) — they overlap by 5 days at year-end.
- Submitting paper returns after 31 October — must use online or face a £100 penalty.
- Missing 31 January for online returns and tax payment — automatic £100 penalty + interest on unpaid tax.
- Treating 6 April as a Friday tax week start — actual day depends on year (6 April 2025 = Sunday; 6 April 2026 = Monday).
- Using calendar months for SSP/SMP eligibility — must use tax weeks.
- Believing employed contributions to ISA roll over after 5 April — they do not, allowance lost.
When to use this calculator
Use this calculator any time you need to know which tax year a date falls in: backdated claims, payment timing, ISA subscription dates, capital gains realisation. Critical when planning year-end transactions — selling shares before 5 April vs after determines the year of CGT, and timing dividend declarations affects the year of income tax.
Regional differences (Scotland, Wales, Northern Ireland)
The UK tax year is uniform across England, Scotland, Wales, and Northern Ireland — all run from 6 April to 5 April. Devolution does not affect the dates. The Scottish, Welsh, and Northern Irish income tax bands operate within the same year. Crown Dependencies (Isle of Man, Jersey, Guernsey) use 1 January to 31 December calendar years for individuals.