Week Number Calculator
Find the ISO-8601 week number, week-year, and weekday for any date.
Tue, 30 Jun 2026 — ISO week 27 of week-year 2026. Tuesday is ISO weekday 2 (weeks run Monday to Sunday).
To find the ISO-8601 week number, count weeks that start on Monday, where week 1 is the week containing the year’s first Thursday. For example, 1 January 2024 falls in ISO week 1, while 1 January 2023 belongs to ISO week 52 of 2022 — its week-year is the earlier year.
What an ISO week number is
ISO-8601 splits the year into numbered weeks so that any date maps to a single week label such as 2026-W27. Two rules define it: every week starts on Monday and ends on Sunday, and week 1 is the week that contains the year’s first Thursday (equivalently, the week holding 4 January). Because whole weeks cannot be split across years, a date in early January or late December can belong to a week-year that differs from its calendar year. Payroll systems, manufacturing schedules, and European calendars use this scheme.
Thursday = the date shifted by (4 − ISO weekday); ISO weekday runs Monday = 1 to Sunday = 7.
- 1 Convert the weekday to an ISO number. Monday = 1, Tuesday = 2, … Sunday = 7. For 30 June 2026, a Tuesday, the ISO weekday is 2.
- 2 Jump to the Thursday of that week. Add (4 − ISO weekday) days. From Tuesday 30 June 2026 that is +2 days, landing on Thursday 2 July 2026.
- 3 Read the week-year off that Thursday. The Thursday sits in 2026, so the week-year is 2026 — the same as the calendar year here, but not always.
- 4 Count weeks from 1 January. ⌈((2 July − 1 January) ÷ 1 day + 1) ÷ 7⌉ = ⌈183 ÷ 7⌉ = 27, so 30 June 2026 is ISO week 27.
ISO week rule vs the US convention
The two systems can assign different week numbers to the same date, so always state which one you mean.
| Property | ISO-8601 (Europe / business) | US / other common convention |
|---|---|---|
| Week starts on | Monday | Sunday |
| Week 1 defined by | The week containing the first Thursday (holds 4 January) | The week containing 1 January |
| Weeks per year | 52 or 53 | 52 or 53 (partial first week) |
| 1 Jan can be in | Week 52 or 53 of the previous year | Always week 1 |
| Week-year | May differ from the calendar year at the edges | Matches the calendar year |
Week-years, 53-week years, and edge cases
The week-year can differ from the calendar year. Because ISO weeks are never split, the last few days of December sometimes fall in week 1 of the next year, and the first few days of January can fall in week 52 or 53 of the previous year. That is why 1 January 2023 reads as week 52 of 2022 — always pair a week number with its week-year (for example 2022-W52), never with the plain calendar year.
Some years have 53 weeks. A year gets a 53rd ISO week when 1 January is a Thursday, or a Wednesday in a leap year — for instance 2020, 2026, and 2032. Every other year has 52 weeks. This is why a naive “day-of-year ÷ 7” shortcut fails near the year boundary.
Watch the convention mismatch. A spreadsheet function or US calendar that starts weeks on Sunday and puts 1 January in week 1 will often report a number one higher than ISO for the same date. If you exchange week numbers with a system in another region, confirm both sides use ISO-8601 before trusting them.