Week Number Calculator
Find the ISO 8601 week number for any date.
Inputs
ISO weeks start on Monday. Week 1 is the week containing the first Thursday of the year.
Result
ISO week
W20
of 2026 · Thursday
Visual breakdown
Week
20
ISO year
2026
Weekday
Thursday
Format
2026-W20
Formula
ISO 8601: week 1 is the week containing the first Thursday of the year. Days from Mon–Sun belong to the same week.
Example
2026-01-01 is in week 1 of 2026. 2024-12-30 is already week 1 of 2025.
Frequently asked
Why does Dec 30 sometimes show next year?
ISO weeks span Mon–Sun. Late-December days that fall in a week with most days in January belong to the next ISO year.
Does the US use the same numbering?
Some US calendars start weeks on Sunday and number differently. We use the ISO standard (Mon–Sun).
Why ISO week format?
It is unambiguous and used worldwide in payroll, scheduling and software systems.