Contents
How the ISO week number calculator works
Choose a day, month and year. The calculator returns the date’s week number according to ISO 8601.
Dates that expose the year boundary
The clearest checks sit near the first Monday or the final days of December.
- 1 January 2021: week 53 of ISO week-year 2020. The Friday still belongs to the week that began in December.
- 4 January 2021: week 1 of 2021. This Monday starts the first complete ISO week of that year.
- 31 December 2024: week 1 of 2025. The ISO week-year changes before the calendar page does.
The rule behind week 1
ISO weeks run from Monday to Sunday. Week 1 is the week containing 4 January, which is the same as the week containing the year’s first Thursday.
N is the ordinal day of the year and d is the ISO weekday from 1 for Monday to 7 for Sunday. A preliminary value below 1 moves to the previous ISO year; a value beyond its final week becomes week 1 of the next ISO year.
ISO weeks are precise, but not the only convention
This calculator follows ISO 8601 for Gregorian dates from year 1 to 9999. Some local calendars, payroll systems and spreadsheet settings number weeks differently, for example by starting on Sunday or treating 1 January as part of week 1 in every case.
An ISO year has 52 or 53 weeks. It has 53 when the year begins on Thursday, or when a leap year begins on Wednesday.
Questions about ISO calendar weeks
Most surprises come from the week-year boundary or from comparing two different numbering systems.
Does every ISO week begin on Monday?
Yes. Monday is day 1 and Sunday is day 7 in the ISO week-date system.
Why can 1 January belong to the previous year?
The date belongs to the ISO week containing it. If that week has most of its days in December, its week-year can be the previous calendar year.
Can December belong to week 1 of the next year?
Yes. The last Monday, Tuesday or Wednesday of December can fall in the first ISO week of the following year.
Why do some apps show a different number?
They may use a Sunday-based or locale-specific rule instead of ISO 8601. Check the week settings before comparing results.
Do I need a time or timezone?
No. A plain civil date is enough. A timezone matters only when a timestamp is converted across midnight.
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