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ISO Week Number Calculator

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Manufacturing, retail and payroll all run on week numbers, and the ISO definition has edges that catch people out every January. Look up the week for a date, or the seven dates in a week — with the week-year shown separately, because it is not always the calendar year.

Start from
Try:

Result

of 2026

Week 34

Sunday, 2026-08-23 · 2026-W34-7

ISO week
34

of 53 that year

Week-year
2026

Same as the calendar year

Day of the week
7 — Sunday

Monday is 1

Day of the year
235
Week runs
2026-08-17 to 2026-08-23

Monday to Sunday

Weeks in the year
53

A long year

2026 is a 53-week year. Those come round every five or six years and break anything that assumed 52 — a year-on-year comparison by week number is comparing different amounts of time across one.

ISO week date
2026-W34-7
Week starts (Monday)
2026-08-17
Week ends (Sunday)
2026-08-23

The whole system follows from one rule: week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January, equivalently the first week with four of its days in the new year. That is why 1 January is often not in week 1, why 31 December is often in week 1 of the next year, and why a year has 52 or 53 weeks rather than a fixed number. Everything here is computed in UTC, so the answer does not change with the viewer's time zone.

How to use the iso week number calculator

  1. 1Enter a date to get its ISO week, or switch to look up a week number instead.
  2. 2Read the week-year alongside the week — around New Year they differ.
  3. 3Check the Monday-to-Sunday range the week actually covers.
  4. 4Note whether the year has 52 or 53 weeks before comparing one year to another.

Examples

New Year's Day landing in the previous year

Input
1 January 2027
Result
Week 53 of 2026, written 2026-W53-5. The week runs from Monday 28 December 2026 to Sunday 3 January 2027.

Recording this as 'week 53 of 2027' would be wrong — the week-year is 2026.

Looking up a week

Input
Week 35 of 2026
Result
Monday 24 August to Sunday 30 August 2026, in a year that has 53 ISO weeks rather than the usual 52.

About the iso week number calculator

One rule, three consequences

ISO 8601 fixes week 1 as the week holding the first Thursday of January, and everything odd about week numbering follows from it. Because Thursday is the middle of a Monday-start week, that rule is the same as requiring four of the week's days to fall in the new year — the majority wins the week.

So 1 January is often not in week 1, because the year may start late in a week that mostly belongs to December. 31 December is often in week 1 of the following year, for the same reason in reverse. And a year holds 52 or 53 weeks depending on how the days line up, because 365 days is 52 weeks plus a remainder that accumulates.

Where week numbering goes wrong in practice

The 53-week year is the failure that reaches production. A report comparing week 40 this year against week 40 last year works fine for years at a stretch, then crosses a 53-week year and starts comparing periods a week apart, with no error and no obvious symptom beyond figures that look slightly odd.

The other one is formatting. Date libraries use different tokens for the calendar year and the week-year — YYYY against GGGG in one common convention, Y against o in another — and they agree for all but a few days a year. A report labelled with the wrong one is correct for 360 days and wrong over the New Year period, which is precisely when someone is looking closely at it.

Frequently asked questions

How is week 1 defined?
As the week containing the first Thursday of January. Three other statements say the same thing: it is the week containing 4 January, it is the first week with the majority of its days in the new year, and it is the week containing the year's first Monday-to-Sunday span that leans into January. Everything else about ISO weeks follows from that one rule.
Why does the week-year differ from the calendar year?
Because a week cannot be split between two years, so the days at each end of December and January have to belong to one or the other. When a year starts on a Friday, Saturday or Sunday, its first days fall in the final week of the previous year. That is why the standard has a separate week-year field, and why a date can sit in January 2027 while belonging to week 53 of 2026.
Which years have 53 weeks?
Those starting on a Thursday, and leap years starting on a Wednesday. Between 2000 and 2040 that means 2004, 2009, 2015, 2020, 2026, 2032 and 2037 — roughly every five or six years. Any year-on-year comparison keyed on week number crosses one of these sooner or later and quietly compares 53 weeks of trading against 52.
Why does my spreadsheet give a different week number?
Because spreadsheet defaults are usually not ISO. The common default starts weeks on Sunday and calls the week containing 1 January week 1 regardless of how few days it has, which disagrees with ISO by one for much of the year. Most spreadsheets have a separate ISO week function or a mode argument; a bare week-number function is the one to distrust.
Does my time zone affect the answer?
Not here. Every calculation runs in UTC, so a given date always returns the same week regardless of where you load the page from. The subtlety only arises when a timestamp rather than a date is converted: an instant late on a Sunday evening in one zone is already Monday in another, and that lands in a different week.

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