What Is the UK Tax Year?

Almost every country organises its tax around a yearly cycle, but the UK picks a famously odd starting point: not 1 January, not even the start of a month, but 6 April. If you have ever wondered why allowances suddenly reset in spring, why accountants get busy in late January, or what HMRC means by "the 2023 to 2024 tax year", the answer lies in understanding this single span of dates. This guide explains what the UK tax year is, where its strange dates come from, and why getting it straight can save you money. This is general information, not financial advice.

What the UK tax year is

The UK tax year is the twelve-month period that runs from 6 April in one calendar year to 5 April in the next, and it is the timeframe HMRC uses to assess personal taxes such as income tax. It is sometimes called the fiscal year.

Because it straddles two calendar years, it is always written as a span. The year that began on 6 April 2023 and ended on 5 April 2024 is "the 2023 to 2024 tax year" (often shortened to 2023/24). Everything you earn, the tax you pay, and the allowances you are entitled to are measured against this window rather than the calendar year.

This is the period that sits behind the figures on your payslip and the deductions made through PAYE. It also governs when annual reliefs become available and when they expire, which is why the dates are worth remembering.

The tax year is simply a 12-month measuring stick. The only surprising thing about it is where the stick starts: 6 April.

Why it starts on 6 April

The date is a genuine historical accident. For centuries the English financial and tax year began on Lady Day, 25 March, one of the traditional quarter days when rents and accounts were settled.

The complication arrived in 1752, when Britain switched from the Julian calendar to the more accurate Gregorian calendar used across much of Europe. To realign the calendar, eleven days were removed from September 1752. The Treasury, unwilling to lose eleven days of tax revenue, effectively pushed the year-end forward by the same number of days. A further adjustment in 1800 (a leap-year discrepancy between the two calendars) nudged it again. The net effect moved the start of the tax year to 6 April, and there it has stubbornly remained ever since.

It is, in short, a date frozen in place by accounting convenience more than two centuries ago. There is no practical reason it could not align with the calendar year today, but changing it would be an enormous administrative undertaking, so the quirk endures.

Tax year versus financial year

The two terms are easy to muddle, but they are not the same:

So when a politician talks about a "financial year" in a Budget, they usually mean 1 April to 31 March, while your own income tax is measured 6 April to 5 April. Limited companies, meanwhile, set their own accounting period. Keeping these straight avoids confusion when deadlines and rate changes are announced.

Why the dates matter

The boundaries of the tax year are not just trivia; they have real financial consequences, because most annual allowances reset on 6 April and do not roll over.

• ISA allowance. You get a tax-free ISA limit each tax year, and any unused portion is lost when the year ends. If you have not used it, the days before 5 April are your last chance. Our guide to what an ISA is explains how the allowance works.
• Personal Allowance. The amount you can earn before paying income tax is an annual figure tied to the tax year, and it resets along with everything else on 6 April.
• Personal Savings Allowance. The tax-free band for savings interest, covered in what the Personal Savings Allowance is, also applies per tax year.
• Pension annual allowance and capital gains exemptions likewise run on the tax-year clock.

Because these reset rather than accumulate, the end of the tax year creates a genuine "use it or lose it" moment for savers and investors. The weeks before 5 April are often when people top up ISAs or pensions to make sure no allowance is wasted.

Key deadlines tied to the tax year

The tax year also anchors a set of fixed deadlines, especially for anyone who completes a self assessment tax return:

• 5 April — the tax year ends.
• 6 April — the new tax year begins, and new rates and allowances take effect.
• 5 October — the deadline to register for self assessment if you have new income to declare for the year just ended.
• 31 October — the deadline for filing a paper self assessment return.
• 31 January — the deadline for filing the online return and paying any tax owed for the previous tax year.

That late-January date is why accountants and HMRC's helplines are so stretched after Christmas. If you need to file, our self assessment tax return guide walks through the process and the records you need. Missing these dates can trigger automatic penalties and interest, so they are worth diarising.

Practical points to remember

A few habits make the tax-year calendar work for you rather than against you:

• Treat early April as a reset. Check whether you have used your ISA, pension and other allowances before 5 April.
• Note the figures that change on 6 April. Tax bands, thresholds and the National Insurance picture can all shift at the start of a new year.
• Keep records by tax year, not calendar year. Filing payslips, interest statements and receipts in 6-April-to-5-April folders makes any future return far easier.
• Diarise 31 January if you are in self assessment, and aim to file well before it.

For free, impartial guidance on allowances and deadlines, MoneyHelper and the official gov.uk pages are reliable starting points.

The bottom line

The UK tax year runs from 6 April to 5 April, a date set not by logic but by a centuries-old calendar change and the Treasury's reluctance to lose revenue. It matters because it defines when your allowances reset and when your tax deadlines fall. Knowing that ISAs, the Personal Allowance and savings allowances expire at midnight on 5 April, and that self assessment is due by the following 31 January, turns an awkward pair of dates into a useful planning tool.