Week Number Calculator

How to use this tool

Choose ISO 8601 (Monday start) or US (Sunday start) week numbering.
Select any date; the week number and corresponding week year update instantly.
Check the selected system’s full Monday–Sunday or Sunday–Saturday date range.

ISO 8601 week numbering

The ISO 8601 standard defines week numbers with these rules:

Weeks start on Monday
Week 1 is the week containing the year's first Thursday
This means Dec 29–31 may fall in week 1 of the following year, and Jan 1–3 may fall in week 52/53 of the previous year

US vs. ISO week numbering

Choose a convention in the calculator above to recompute the number, week year, and date span together. ISO 8601 is selected by default. It starts weeks on Monday and assigns Week 1 to the week containing the first Thursday. The US WEEKNUM(date, 1) convention starts weeks on Sunday and assigns Week 1 to the week containing January 1. Its displayed week year is the date’s calendar year, even when the Sunday–Saturday span crosses into December of the previous year or January of the next.

Because the start day and Week 1 rule both change, a date near New Year can have a different week number, year, and span under the two systems. For example, January 1, 2021 is ISO Week 53 of 2020 (December 28–January 3), while the US system calls it Week 1 of 2021 (December 27–January 2). For schedules, reports, or lot codes, confirm which convention your team or trading partner expects before comparing week numbers.

How to find week number in common tools

Excel: =ISOWEEKNUM(A1) for ISO weeks; =WEEKNUM(A1,1) for US convention (1=Sunday start).
Python: from datetime import date; date.today().isocalendar()[1] returns the ISO week number.
JavaScript: use the date-fns library’s getISOWeek(date), or the upcoming Temporal API’s date.weekOfYear.
SQL (PostgreSQL): EXTRACT(WEEK FROM date_column) returns the ISO week number.

Business applications

Manufacturing: ISO week calendars are standard in production planning; parts are scheduled and tracked by week number, not calendar date.
Retail: many retailers use fiscal week reporting aligned to ISO or a 4-4-5 calendar for year-over-year comparison.
Project management: Agile sprints are often numbered by week; “Sprint W22” is immediately understood across time zones without date conversion.

Note: the ISO week year can differ from the calendar year for dates in late December or early January. Pair the ISO week number with its ISO week year to avoid off-by-one errors at year boundaries. The US convention shown above always uses the selected date’s calendar year.

Long years (53-week years)

Most years have 52 ISO weeks. A year has 53 weeks when January 1 falls on a Thursday (or on a Wednesday in a leap year), because this causes week 1 to start in the previous December, pushing the last week of the year to number 53. Recent and upcoming 53-week years: 2020, 2026, 2032, 2037.

Frequently asked questions

What is ISO 8601 week numbering?

ISO 8601 defines Week 1 as the week containing the first Thursday of the year (equivalently, the week containing January 4). Weeks run Monday through Sunday. This means Week 1 can start in late December.

Can a year have 53 weeks?

Yes. An ISO week-year has 53 weeks when January 1 is Thursday, or when a leap year starts on Wednesday. The US Sunday-start system can also number 53 weeks; its total follows the week containing December 31.

How does US week numbering differ from ISO week numbering?

ISO 8601 weeks start Monday, and Week 1 contains the first Thursday (or January 4). The US WEEKNUM type 1 convention starts weeks Sunday and makes the week containing January 1 Week 1. A date can therefore have different week numbers and boundaries in the two systems.

Why might January 1 fall in ISO week 52 or 53?

If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last ISO week of the previous year, not Week 1 of the current year.