Week Number Calculator
Find the ISO-8601 week number for any date, along with its ISO week-numbering year.
Formula last reviewed 15 September 2026 · How we verify our calculators
ISO week number
- ISO week-numbering year
- 2,024
Updates live as you type
Frequently asked questions
Under ISO-8601, weeks run Monday to Sunday and week 1 is the week containing the year’s first Thursday. Week numbers therefore range from 1 to 52 or 53.
Early-January days can belong to the last ISO week of the previous year, and late-December days to week 1 of the next. The ISO week-numbering year reflects this.
It depends on the weekday. For 2024, 1 January is a Monday, so it falls in ISO week 1. In other years it can land in week 52 or 53 of the prior year.
They are common in business planning, manufacturing, payroll and logistics across Europe, where schedules are often expressed as “week 14” rather than a date.
Which week is 1 January in?
1 January 2024 falls in ISO week 1 of ISO year 2024 — but that is not automatic, and in many years the first of January belongs to the final week of the *previous* year.
The ISO 8601 week-numbering system is precise and counter-intuitive, and it governs planning calendars, manufacturing schedules, retail reporting and a great deal of European business practice.
The rules
ISO 8601 defines them tightly:
Weeks run Monday to Sunday. Monday is day 1; Sunday is day 7.
Week 1 is the week containing the first Thursday of January. Equivalently, it is the week containing 4 January, and equivalently again, it is the first week with the majority of its days in the new year.
A year has 52 or 53 weeks. A year has 53 when it starts on a Thursday, or when it is a leap year starting on a Wednesday.
The Thursday rule is what produces the surprising results. 1 January 2024 was a Monday, so the week beginning that day contained 4 January and was therefore week 1 — the straightforward case.
But 1 January 2021 was a Friday. That week's Thursday was 31 December 2020, so the week belonged to 2020 and 1 January 2021 fell in week 53 of ISO year 2020. A date in January can genuinely belong to the previous ISO year.
The reverse happens too: 31 December 2019 fell in week 1 of ISO year 2020.
ISO year versus calendar year
This is why the calculator reports an ISO week-numbering year separately. For a few days at each year boundary the two disagree, and any system storing only a week number without its ISO year will mis-sort those dates.
The convention for writing them is `2024-W01` or `2024-W01-1` for a specific day.
Other week-numbering systems
ISO is not universal, which is a frequent source of cross-system disagreement:
US convention — weeks run Sunday to Saturday, and week 1 is simply the week containing 1 January. This produces a partial first week and different numbers from ISO for much of the year.
Broadcast calendar, used by US media, always starts weeks on Monday with months of exactly four or five weeks.
4-4-5 retail calendars divide quarters into 4, 4 and 5-week months so each quarter has 13 weeks and comparisons are like-for-like. Widely used in retail, and unrelated to ISO numbering.
Spreadsheet functions default to different systems too — Excel's `WEEKNUM` uses the US convention unless told otherwise, while `ISOWEEKNUM` follows ISO. A mismatch between two teams' spreadsheets is very often just this.
Where week numbers are used
Manufacturing and supply chain — production slots, delivery windows and lead times are routinely quoted in week numbers, especially with European suppliers.
Project scheduling — Gantt charts and sprint planning aligned to weeks rather than dates.
Retail and finance reporting, for like-for-like period comparison.
Payroll in weekly-paid systems.
Date codes on manufactured goods, where a week and year are stamped rather than a full date.
Common mistakes
Assuming 1 January is always in week 1. It depends on the day of the week it falls on.
Storing a week number without its ISO year. The boundary days become ambiguous.
Mixing ISO and US numbering. They disagree for most of the year.
Forgetting 53-week years. Systems assuming exactly 52 weeks break in those years.
Assuming weeks start on Sunday. ISO starts on Monday, and most of the world's business calendars follow it.