The UK tax year: why 6 April runs your British filings

It is the twelve-month period Britain uses for income tax, running from 6 April to 5 April. HMRC's residence guidance uses the same period when it tests where you live.

Everything British hangs off it. Your tax code, your allowances, your P60 and your Self Assessment return all refer to one British tax year. So the 2026 to 2027 year starts on 6 April 2026 and ends on 5 April 2027.

Rates and allowances usually change at the same boundary. That makes 6 April the natural date to check what has moved, before the first payslip of the new year arrives.

Employers, pension providers and banks all report to HMRC on this same basis. So every British document you receive tells the story of one April-to-April year.

The American tax year runs from 1 January to 31 December. So every British year straddles two American ones. Roughly nine months fall in one US year and three in the next, and income has to be split accordingly.

For salary this is mostly arithmetic. You add up the payslips for each calendar year rather than using British annual totals. For one-off events the timing matters more, because a bonus in March and a bonus in April sit in different British years.

Credits follow the same logic. The British tax on a payment needs matching to the American year of that payment. Our guide to avoiding double taxation covers how those credits are claimed.

Americans who arrive or leave mid-year feel this most. Their first and last British years are partial, while the American years around them are full.

HMRC can issue a fresh tax code for the new year, and your employer applies it from the first payslip after 6 April. The code tells your employer how much of your pay to treat as tax-free. So a wrong code can shift tax between years.

Codes can also change during the year when HMRC learns something new. A new benefit, a second job or untaxed income can all prompt a change. GOV.UK explains how tax codes work and how to check yours.

Check the code on your first April payslip every year. A two-minute look catches most problems before they build into a refund claim or a bill.

Three dates matter most. You must register by 5 October if you need to file for the first time. A paper return is due by 31 October. An online return, and the tax you owe, are both due by 31 January.

HMRC's deadlines page gives the current dates. For the year ending 5 April 2026, that means a paper return by 31 October 2026 and an online return by 31 January 2027.

Our guide to registering for Self Assessment covers the first date in detail. Meanwhile the July instalment for advance payments adds a fourth date for many filers.

Because residence is tested one UK tax year at a time. Day counts, home tests and ties all run from 6 April. So a move in October lands in the middle of a year, not at the start of one.

That catches people who expect the clock to start on arrival. The test still looks at the whole year, and split-year treatment then decides how to divide it. Our guide to the statutory residence test explains those stages.

America counts its own presence rules on the calendar year. So one move can affect two different years in two different ways, which is why the dates deserve writing down.

Payments on account also hang off the British tax year. They fall due on 31 January and 31 July, and each one is half of the previous year's bill. So the January payment often covers two years at once.

That first January can feel brutal for new filers. The balancing payment and the first instalment arrive together. Our guide to payments on account walks through the arithmetic.

For Americans the timing matters twice. American estimated payments run on their own calendar, so British and US cash calls can bunch into the same months.

Keep a note of what you paid and when. The American return may need those British payments matched to calendar years, just like salary.

By their date. A bonus, a share vesting or a sale belongs to the British year in which it happens, and to the American year of the same date. So one event can sit early in a British year yet late in an American one.

Share schemes need extra care. Vesting and sale dates can fall in different years on each side, which splits one award across several reporting periods. A dated schedule keeps it straight.

Where you can choose the timing, check both calendars first. A date that suits one country can complicate the other, and a few days either way can move an event into a different year entirely.

This routine keeps both returns consistent. It works for salary, bonuses and most one-off events.

Build the workings once and reuse them. The second year takes a fraction of the effort.

Your US return converts sterling into dollars, usually by calendar year. The IRS publishes yearly average rates for that purpose, and they follow the American year rather than the British one.

So salary split by calendar year converts cleanly at the matching yearly rate. One-off payments can justify a specific-date rate instead. Either way, apply one method consistently and keep the workings.

Mixing the calendars here causes subtle errors. A British-year total converted at an American-year rate belongs to neither year properly, and it rarely survives a close look.

Take an illustrative example: an American analyst in London paid a bonus in March and another in May. The March bonus falls in the British year ending 5 April. The May bonus falls in the next one.

On the American side, both fall in the same calendar year. So her US return shows two bonuses, while each British return shows one. Both returns are correct, because each follows its own calendar.

Her credits then need care. The British tax on each bonus belongs to the American year it was paid in. Getting that match right is what stops the same income being taxed twice.

The first is using a P60 for the US return. A P60 covers the British year, so its totals mix two calendar years. Using it directly puts part of your income in the wrong American year.

The second is assuming deadlines line up. They do not, and the British January date sits months away from the American spring one. Families who plan for one often miss the other.

The third is forgetting the boundary when timing a sale. A disposal on 5 April and one on 6 April sit in different British years, with different allowances. A single day can change the outcome.

A fourth is forgetting that pension contributions follow the British year too. A payment on 5 April counts toward one year's allowances and a payment on 6 April toward the next, which matters for anyone near a limit.

Use one spreadsheet with both calendars on it. Mark 6 April, 5 October, 31 October, 31 January and 31 July for Britain. Then add the American dates beside them.

Keep monthly records rather than annual ones. Monthly figures can be summed either way, while annual totals cannot be unpicked. That one habit solves most of the split-year work.

In our practice we see the same pattern in every tidy file. The families with fewest problems keep dated records, not totals, and they review both calendars in the same sitting.

The start of each British year is the natural moment to plan. Allowances reset, your tax code updates, and the previous year's records are still fresh.

Use it to diarise the year's dates on both sides and to close off last year's paperwork. Then decide early on anything with timing flexibility, such as a planned sale or a pension contribution.

Families who plan at the boundary rarely meet surprises in January. The calendar stops being a trap once it becomes a routine.

It is also the moment to check both sets of estimates. American estimated payments and British payments on account run on different calendars, so a quick look at the year ahead avoids two cash calls landing in the same month.

We prepare both returns from one set of dated records, through our Self Assessment service. Income gets split across the calendars correctly, and credits land in the right American year.

If the two calendars have caused trouble before, send us the outline and we will map your dates and figures at a fixed fee agreed first. HMRC's income tax rates page shows what changes at each boundary. This article is general information, not personal tax advice; take advice on your own facts from a qualified adviser.