Guides · Tax year and deadlines
Tax year and filing deadlines
United States — tax year structure
The federal tax year is the calendar year. For most individual taxpayers (W-2 employees, single-member LLCs, sole proprietors), tax year 2026 runs from 1 January 2026 through 31 December 2026, and the liability is settled by 15 April 2027.
Business entities can elect a different fiscal year, but most small businesses stick with the calendar year for simplicity. Personal federal income tax is always calendar-year.
Quarterly estimated payments (Form 1040-ES) divide the expected full-year tax into four roughly equal installments:
- Q1 — 15 April (covering 1 January-31 March)
- Q2 — 15 June (covering 1 April-31 May)
- Q3 — 15 September (covering 1 June-31 August)
- Q4 — 15 January of the following year (covering 1 September-31 December)
Note the Q4 timing — it falls on the 15th of January of the year AFTER the tax year ends. The cumulative effect is that the Q4 payment is due just a month after the tax year closes, leaving very little buffer.
United States — key dates
| Date | Event | Notes |
|---|---|---|
| 31 December | Tax year ends | Calendar year — tax year 2026 ends 31 December 2026. |
| 31 January | W-2 and 1099 issued | Employers and payers must issue by this date for the prior year. |
| 15 April | Filing + balance due | Form 1040 due; balance due to avoid penalties and interest. |
| 15 April | First estimated payment (Q1) | Form 1040-ES for self-employment and other non-W-2 income. |
| 15 June | Second estimated payment (Q2) | Second Form 1040-ES installment. |
| 15 September | Third estimated payment (Q3) | Third Form 1040-ES installment. |
| 15 October | Filing extension deadline | Form 4868 extends filing (but not payment) to 15 October. |
| 15 January | Fourth estimated payment (Q4) | Final Form 1040-ES installment for the tax year. |
The IRS also publishes each autumn a Revenue Procedure with the updated tax year parameters (brackets, standard deduction, Social Security wage base, etc.). For tax year 2026, the relevant update is Rev. Proc. 2025-32 (IR-2025-103, 9 October 2025). The next update (Rev. Proc. 2026-XX) lands in October 2026 and sets tax year 2027 parameters.
Extensions
Filing an extension (Form 4868) gives you until 15 October to file. It does not extend the payment deadline — you still owe any balance by 15 April or face interest and late-payment penalties. Estimate and pay with the extension.
United Kingdom — tax year structure
The UK personal tax year runs 6 April to 5 April. This is a quirk of history — when the UK adopted the Gregorian calendar in 1752, it skipped 11 days. To preserve the tax year's relative position, Parliament shifted the start of the tax year by 11 days, from 25 March to 6 April. The two dates have not lined up since.
The tax year is named after the year in which it ends. The year running 6 April 2026 through 5 April 2027 is tax year 2026/27 (often written 2026-27). Statements issued for that year are typically dated 2027, even though most of the year is in 2026.
This naming convention trips people up — including people who have lived in the UK for years. If you see "tax year 2026" in a document and the dates look like 2025/26, the document is probably referring to 2025/26 (which started in April 2025 and ended April 2026) and the document was issued in early 2026.
United Kingdom — key dates
| Date | Event | Notes |
|---|---|---|
| 6 April | Tax year starts | UK personal tax year runs 6 April to 5 April. |
| 31 May | P60 issued | Employers must issue P60 to all employees by this date. |
| 5 April | Tax year ends | Last day for ISA subscriptions, pension contributions, and gift-aid for the year. |
| 31 October | Paper filing deadline | Self Assessment paper returns; HMRC calculates the bill. |
| 31 January | Online filing deadline | Self Assessment online return; balancing payment; first payment on account. |
| 31 July | Second payment on account | Second half of prior-year estimated tax payment. |
Two things to notice about the UK timeline:
- The tax year ends in April, but the filing deadline is the following January — about 9 months of breathing room for the online return, vs. 3.5 months for the US.
- Two payment dates — 31 January and 31 July — split the prior year's liability into roughly equal halves. The 31 January payment is also the deadline for the new tax year's first estimated payment.
Self Assessment
The UK's Self Assessment regime applies to:
- Self-employed individuals.
- Company directors.
- Anyone with untaxed income (rental, dividends above the dividend allowance, capital gains above the annual exempt amount).
- Higher earners with complex tax affairs.
- Anyone who has to pay tax on a pension or benefit.
Most employees whose only income is via PAYE are not in Self Assessment — their tax is settled automatically. They receive a P60 at year-end and a P45 if they leave their employer, but do not file an annual return.
US vs UK at a glance
| Item | United States | United Kingdom |
|---|---|---|
| Tax year | Calendar year (1 Jan - 31 Dec) | Personal year (6 Apr - 5 Apr) |
| Year naming | Year it starts (2026 = Jan-Dec 2026) | Year it ends (2026/27 = Apr 2026 - Apr 2027) |
| Year-end statement | W-2 (issued 31 Jan) | P60 (issued 31 May) |
| Standard filing deadline | 15 April | 31 January (online) / 31 October (paper) |
| Extension | Form 4868 (until 15 Oct, payment still due) | No formal extension; late filing incurs penalties |
| Estimated payments | Quarterly (15 Apr / 15 Jun / 15 Sep / 15 Jan) | Twice-yearly (31 Jan + 31 Jul) |
| Payment at filing | Balance due with return | Balancing payment + first payment on account at 31 January |
| Direct deposit / direct debit | Yes (default) | Yes (BACS / Faster Payments) |
Penalties for late filing or late payment
Both countries penalize late filing and late payment, but the structures differ.
United States
The IRS failure-to-file penalty is 5% of unpaid tax per month (or part month), capped at 25%. The failure-to-pay penalty is 0.5% of unpaid tax per month, also capped at 25%. They stack — if you both file late and pay late, the combined penalty tops out at 5%/month for the first five months.
On top of the penalties, the IRS charges interest on unpaid tax. The interest rate is the federal short-term rate plus 3% and is reset quarterly. For 2026 it has been running around 8%.
On the estimated-payment side, the IRS uses Form 2210 to calculate an underpayment penalty if you fell short of the safe-harbor thresholds (90% of current year, or 100%/110% of prior year). The penalty is interest-only — no separate failure to pay.
United Kingdom
HMRC's late-filing penalty for Self Assessment is a flat £100 if you are up to three months late, £10/day after that (up to £900), then 5% of the tax due if more than six months late, then a further 5% if more than twelve months late. The penalty applies even if you owe no tax.
Late-payment interest on tax owed is the Bank of England base rate plus 2.5%, reset quarterly. For 2026 it has been around 6.75%.
Payment on account shortfalls also attract interest from the due date. There is no separate underpayment penalty, but the interest compounds quickly if the shortfall is large.