ISO Week Number Calculator

The week number calculator finds the ISO 8601 week number for any date you enter, along with the ISO year and the Monday-to-Sunday date range for that week. Use it for project planning, sprint numbering, manufacturing week codes, and any system that tracks time in weeks rather than months.

How to use this calculator

1. Enter the date you want the week number for.
2. The result shows the ISO week number, the ISO year, and the Monday–Sunday date range that the date falls within.
3. Use the week range to see exactly which days share that week number, which is useful when scheduling across teams.

Worked example

July 7, 2026 is a Tuesday. It falls in ISO week 28 of 2026, and that week runs from Monday July 6 to Sunday July 12, 2026. If a sprint is labeled "Sprint 28", this is the calendar week it covers — so the sprint starts Monday July 6 and the review lands on Friday July 10.

Frequently asked questions

What is an ISO week number?

ISO 8601 defines week numbers where Week 1 is the week containing the first Thursday of January. Weeks always start on Monday and end on Sunday. This system is standard in Europe and commonly used in project management.

Why can Week 1 start in December?

If January 1 falls on a Friday, Saturday, or Sunday, the first Thursday of January is in the following week — so the days before Thursday belong to the last week of the previous year (Week 52 or 53).

Can a year have 53 weeks?

Yes. A year has 53 ISO weeks when January 1 falls on a Thursday (or Wednesday in a leap year). Most years have 52 weeks.

How is the ISO year different from the calendar year?

The ISO year (also called the week date year) is the year that the majority of the week's days belong to. For example, December 31, 2025 might be in ISO week year 2026 if that week's Thursday falls in January 2026.