Week Number Calculator (ISO 8601)
ISO week number of a date, and the dates in any week.
Dates in a week
Use via API
ISO 8601 week number, ISO week date, day of year and quarter for a date, or the Monday–Sunday dates of a given ISO week. Send JSON, get JSON back.
POST https://pithlab.com/api/v1/tools/week-number-calculator
curl https://pithlab.com/api/v1/tools/week-number-calculator \
-H "Authorization: Bearer $PITHLAB_KEY" \
-H "Content-Type: application/json" \
-d '{"date":"2026-10-20"}'About the Week Number Calculator
Find the ISO 8601 week number of any date, plus day of year, days left, quarter and leap year, or look up the Monday-to-Sunday dates of any week.
How to use the Week Number Calculator
- Pick a date. It starts on today's date.
- Read its ISO week number and week-year, day of the year, days remaining, quarter and whether the year is a leap year.
- To go the other way, enter a year and week number to see the Monday and Sunday that start and end that week.
Examples
Input
2026-10-19
Output
Week 43 of 2026 (2026-W43-1), day 292 of 365, Q4
Input
2021-01-01
Output
Week 53 of 2020 (2020-W53-5), day 1 of 365, Q1
Input
2024-12-30
Output
Week 1 of 2025 (2025-W01-1), day 365 of 366, Q4
Frequently asked questions
How are ISO week numbers defined?
ISO 8601 weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing 4 January). Most of Europe and many businesses use this system.
Why does 1 January sometimes fall in week 52 or 53?
If 1 January is a Friday, Saturday or Sunday, its week belongs mostly to the previous year, so it takes that year's last week number. Likewise 29–31 December can be week 1 of the next year. That's why the week-year can differ from the calendar year.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday, such as 2020 and 2026. All other years have 52 ISO weeks.
Is this the same as US week numbering?
Not always. US calendars often start weeks on Sunday and call the week containing 1 January week 1, so numbers can differ by one. This tool uses the ISO 8601 standard.