Week Number Calculator

Find the ISO 8601 week number of any date, the date range of that week, and the total number of weeks in the year.

How ISO week numbers work

The ISO 8601 standard numbers weeks Monday to Sunday, and defines week 1 as the week containing the year's first Thursday — which is the same as the week containing 4 January. Every date belongs to exactly one ISO week, and every week belongs to exactly one ISO year: the year its Thursday falls in. That single rule settles all the awkward year-boundary cases.

Why 1 January can be week 52 or 53 of the previous year

If a year starts on a Friday, Saturday or Sunday, the Thursday of that first partial week is still in late December — so 1–3 January are numbered as the last week (52 or 53) of the old ISO year. The mirror case also happens: when a year ends on a Monday, Tuesday or Wednesday, its final days already belong to week 1 of the next ISO year. For example, 1 January 2027 is a Friday, so it sits in week 53 of ISO year 2026; week 1 of 2027 only starts on Monday 4 January 2027.

Worked example

Take 19 July 2026, a Sunday. Its week runs Monday 13 July to Sunday 19 July, and the Thursday of that week is 16 July 2026. Counting whole weeks from the Thursday of week 1 (1 January 2026 — 2026 starts on a Thursday) gives week 29 of 2026. Because 2026 starts on a Thursday, it is one of the long years with 53 weeks.

Where week numbers are used

Frequently asked questions

What week is it right now?
The calculator defaults to today’s date, so the answer is on screen the moment the page loads — the hero shows the current ISO week number and the Monday-to-Sunday range it covers.
What is an ISO week number?
ISO 8601 numbers the weeks of the year with weeks running Monday to Sunday. Week 1 is the week containing the year’s first Thursday (equivalently, the week containing 4 January), and it is the standard used across Europe and in most business calendars.
Why can 1 January be in week 52 or 53 of the previous year?
A week belongs to the year that owns its Thursday. If 1 January falls on a Friday, Saturday or Sunday, its week’s Thursday is still in December, so those first days count as week 52 or 53 of the old year.
How many weeks are in a year?
ISO years have 52 or 53 weeks. A year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday — 2026 has 53 weeks, for example. The calculator shows the count for the year of your date.
Why does my spreadsheet show a different week number?
Some systems (like Excel’s default WEEKNUM) call the week containing 1 January "week 1" and start weeks on Sunday. That US-style numbering can differ from ISO by one. Use ISOWEEKNUM in Excel to match this calculator.

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