Week Number Calculator

What this calculator does

The Week Number Calculator tells you which week of the year it is right now and lets you convert freely between calendar dates and week numbers. It uses the ISO 8601 standard — the system used across UK business, payroll, logistics and project planning — in which weeks begin on Monday and week 1 is the week containing the first Thursday of the year (equivalently, the week that contains 4 January). This is the same numbering shown in Microsoft Outlook, most enterprise calendars and HR systems.

It is built for payroll administrators, office and project managers, logistics and supply-chain teams, students and anyone who simply needs to answer “what week is it?”. As a bonus, every result also shows the matching HMRC PAYE tax week, which is numbered separately from the tax year that starts on 6 April. There are three modes: see the current week instantly, look up the week number for any date, or turn a week number and year back into a Monday–Sunday date range. All maths runs in your browser — nothing is stored or sent anywhere.

How it works

• Current week: Tap “Show current week” to get today’s ISO week number, weekday and the HMRC tax week.
• Date → week number: Pick any date and the tool returns its ISO week (1–53), the ISO week-numbering year and the day of the week. Note that early-January and late-December dates can belong to the previous or next ISO year — the calculator handles these boundary cases automatically.
• Week number → dates: Enter a week number and a year to get the exact Monday-to-Sunday range. The tool knows that most years have 52 weeks but some — like 2026, which has 53 ISO weeks because 1 January 2026 falls on a Thursday — have an extra week, and it validates your input accordingly.

Worked example

Suppose you choose 15 June 2026. That date is a Monday, and it falls in ISO week 25 of week-year 2026. Converting week 25 of 2026 back to dates gives the range Monday 15 June 2026 to Sunday 21 June 2026. Because 2026 is a 53-week ISO year, valid week numbers run from 1 to 53; week 1 of 2026 actually starts on Monday 29 December 2025 and ends on Sunday 4 January 2026. For PAYE, 15 June 2026 falls in HMRC tax week 11 (the tax year having begun on 6 April 2026).

Frequently asked questions

What week number is it today in the UK?

Tap “Show current week” above for the live answer. The UK follows the ISO 8601 standard, where weeks start on Monday and week 1 is the week containing the year’s first Thursday (the week that includes 4 January).

How many weeks are there in 2026?

2026 has 53 ISO weeks. This happens because 1 January 2026 is a Thursday; under ISO 8601, any year whose 1 January is a Thursday (or a leap year starting on a Wednesday) is a 53-week year. Most years, including 2025, have 52 weeks.

Why is the ISO week number different from the HMRC tax week?

They count from different start points. ISO weeks are tied to the calendar year and reset around 1 January, while HMRC PAYE tax weeks are tied to the tax year, which starts on 6 April. So tax week 1 covers 6–12 April, and the two numbering systems will not match.

Do weeks start on Monday or Sunday?

Under ISO 8601 — the UK and European standard used here — weeks start on Monday and end on Sunday. Some US systems start the week on Sunday, which can produce a different week number, so always check which standard a tool uses.

Source: ISO 8601 week-date standard (weeks start Monday; week 1 contains the first Thursday). UK tax year and PAYE tax-week numbering per HMRC — the tax year runs 6 April to 5 April: gov.uk self-assessment deadlines.