Week Number Calculator
Enter any date to find its ISO 8601 week number — the standardized week numbering (1 through 52 or 53) commonly used in business planning, payroll, and project scheduling.
How the Week Number formula works
ISO 8601 defines week 1 as the week containing the year's first Thursday (equivalently, the week containing January 4th), with weeks running Monday through Sunday.
Step-by-step calculation
- Find the Thursday of the week containing the given date — ISO weeks are assigned based on where that Thursday falls.
- Count how many Monday-to-Sunday weeks have elapsed since the first Thursday of that Thursday's year.
- The result, plus 1, is the ISO week number.
Worked example
January 1, 2026 falls on a Thursday, so it's automatically in week 1 of 2026. A date like July 27, 2026 falls in week 31.
Frequently asked questions
Why does ISO week numbering sometimes assign early-January dates to the previous year's last week?
Because ISO weeks are defined by where each week's Thursday falls, not by January 1st itself — if January 1st falls on a Friday, Saturday, or Sunday, that partial first week actually belongs to week 52 or 53 of the previous year.
Is this the same as simply counting 7-day chunks from January 1st?
No — that naive approach doesn't align with the ISO 8601 standard used in most business and international contexts, which anchors week 1 to the first Thursday of the year rather than a fixed start date.