Why is ISO week 1 based on Thursday?

Thursday is the middle of the week (day 4 of 7). By anchoring week 1 to the week containing the year’s first Thursday, at least 4 days of week 1 always fall in the new year, making "the week belongs to that year" intuitive. Alternative rules (like "week with January 1") can put week 1 mostly in the previous year.

How can a year have 53 weeks?

52 weeks × 7 days = 364 days, one day short of a regular year. The excess accumulates and periodically a year needs 53 weeks to catch up. Rule: if January 1 is a Thursday (or Wednesday in a leap year), the year has 53 weeks. Examples: 2015, 2020, 2026, 2032.

Why does Excel sometimes give the "wrong" week number?

`WEEKNUM()` defaults to the American system (week 1 = week containing January 1, Sunday-starting). For ISO weeks use `ISOWEEKNUM()` or `WEEKNUM(date, 21)`. Google Sheets also has `ISOWEEKNUM()`. LibreOffice Calc uses `ISOWEEKNUM()` or `WEEKNUM(date, 21)`.

Do different industries mean different things by "week"?

Yes. Retail often uses "week 1 = the week starting the first Monday after January 1". Broadcast/TV uses a "broadcast calendar" where week 1 always starts January 1. Accounting and invoicing usually use ISO. Always confirm which system a partner or vendor uses before quoting week numbers.

What week is it? ISO Week Number & Calendar Dates

How it works

Norway uses ISO 8601 week numbers: week 1 is the week containing the year's first Thursday, and weeks start on Monday. Some years have 53 weeks.

About this tool

Week number tells you which week of the year a date belongs to. Norway and the rest of Europe use ISO 8601: the week runs Monday–Sunday, and week 1 is the week containing the year’s first Thursday. This means a year can have 52 or 53 weeks. The tool converts between date and week number, shows all dates in a given week, and works for any date between 1900 and 2100. The US and some other countries use a different system (week 1 = week containing January 1, Sunday-starting), which the calculator can also show. All calculations happen locally in your browser.

How to use it

Pick a date with the date picker or type it in YYYY-MM-DD format.
The tool immediately shows the ISO week number and year (e.g. "2024-W15").
Click "show all days in the week" to see Monday through Sunday.
Switch to "US standard" if you need American week numbering.
Everything runs locally, your date choice isn’t stored or sent.

Examples

January 4

Input2024-01-04 (Thursday)

OutputISO week 1 of 2024 (1–7 January)

January 4 is always in ISO week 1, useful as a mnemonic.

Year-boundary trap

Input2023-01-01 (Sunday)

OutputISO week 52 of 2022, not week 1 of 2023!

Because the Thursday of that week (December 29, 2022) belongs to 2022.

Year with 53 weeks

Input2020

Output2020 has 53 ISO weeks (week 53 = 28 Dec 2020 – 3 Jan 2021)

A 53-week year occurs roughly every 5–6 years. Next is 2026.

Common use cases

Project management and pay periods organised by week (e.g. "starts in week 15").
Time and absence tracking in HR systems (Visma, Xledger, Tripletex).
School and daycare plans that follow European weeks.
Seasonal retail analysis ("sales in week 40 were 15 % above budget").
Planning meetings and trips with colleagues abroad, check if their week matches.
Automatic reports and invoicing based on weekly cycles.

Frequently asked questions

Why is ISO week 1 based on Thursday?: Thursday is the middle of the week (day 4 of 7). By anchoring week 1 to the week containing the year’s first Thursday, at least 4 days of week 1 always fall in the new year, making "the week belongs to that year" intuitive. Alternative rules (like "week with January 1") can put week 1 mostly in the previous year.
How can a year have 53 weeks?: 52 weeks × 7 days = 364 days, one day short of a regular year. The excess accumulates and periodically a year needs 53 weeks to catch up. Rule: if January 1 is a Thursday (or Wednesday in a leap year), the year has 53 weeks. Examples: 2015, 2020, 2026, 2032.
Why does Excel sometimes give the "wrong" week number?: `WEEKNUM()` defaults to the American system (week 1 = week containing January 1, Sunday-starting). For ISO weeks use `ISOWEEKNUM()` or `WEEKNUM(date, 21)`. Google Sheets also has `ISOWEEKNUM()`. LibreOffice Calc uses `ISOWEEKNUM()` or `WEEKNUM(date, 21)`.
Do different industries mean different things by "week"?: Yes. Retail often uses "week 1 = the week starting the first Monday after January 1". Broadcast/TV uses a "broadcast calendar" where week 1 always starts January 1. Accounting and invoicing usually use ISO. Always confirm which system a partner or vendor uses before quoting week numbers.

Technical background

ISO 8601 defines week-based dates as `YYYY-Www-D` where `w` is the week number (1–53) and `D` is the weekday (1 = Monday, 7 = Sunday). The rule: the first week is the one containing the year’s first Thursday, equivalent to "the week with at least 4 days in the new year". To compute ISO week: (1) find the Thursday of the week; (2) the year of that Thursday is the ISO week year; (3) the week is (day-of-year-of-Thursday − 1) / 7 + 1. JavaScript has no built-in support, but `Intl.DateTimeFormat` with `weekday` and polyfills like `date-fns` (`getISOWeek()`), `luxon` (`.weekNumber`) or `dayjs` (via plugin) handle it. The browser has `<input type="week">` with format `YYYY-Www`.