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.
Similar calculators
You may find the following calculators on the same topic useful:
- Day of the week calculator. Choose a date to see whether it falls on a Monday, Tuesday or another day of the week.
- Online Piano. Play 24 notes from C3 to B4 on screen, or use the mapped computer keys from C3 to G4.
- Leap Year Calculator. Enter a positive whole-number year to check whether it is leap or common and see its total number of days.
- Poker Odds Calculator. Choose your cards and table size to estimate your share of the pot at showdown.
- Comprehensive Text Analysis Calculator. Get a complete breakdown of your text's characters, words, sentences, paragraphs, and estimated reading and speaking times for thorough editing and analysis.
- Text Paragraph Count Calculator. Efficiently determine the total number of paragraphs in your text to improve its structure and clarity.
- Text Sentence Count Calculator. Quickly find out the total number of sentences in your text to enhance your writing's readability and structure.
- Text Word Count Calculator. Easily determine the total and unique word count in your texts for more effective writing and editing.
- Countdown Timer to Earth Day. Track the exact time remaining in days, hours, minutes, and seconds until the celebration of Earth Day on April 22nd.
- Countdown Timer to International Children's Day. Track the exact time remaining in days, hours, minutes, and seconds until the celebration of International Children's Day on June 1st.
Share on social media
If you liked it, please share the calculator on your social media platforms. It`s easy for you and beneficial for the project`s promotion. Thank you!