Date & Time · Dates
Formula verified
Week Number Calculator
Enter a date to find its ISO 8601 week number — the standard used in business planning, payroll and shipping schedules worldwide.
Date
ISO week
Week 38
2026
Step-by-step proofCheck by hand
Ad slot · 970×250 billboard
Ad slot · 300×250
How ISO week numbering works
ISO 8601 weeks run Monday to Sunday. Week 1 of a year is the week that contains that year's first Thursday — equivalently, the week containing 4 January. Because of that rule, a date right at the start or end of a calendar year can belong to a week numbered in the other year.
The rule
week 1 = the week containing 4 January · weeks run Monday–Sunday
Worked example
31 December 2020 falls in week 53 of 2020 — 2020's final ISO week — even though 1 January 2021 also falls in that same week 53, not week 1 of 2021.
Frequently asked questions
What is an ISO week number?
ISO 8601 defines weeks as running Monday to Sunday, with week 1 of a year being the week that contains that year's first Thursday (equivalently, the week containing 4 January). Weeks are numbered 1 through 52 or 53 depending on the year.
Why can a date near January 1st belong to a different year's week?
Because week 1 is defined by which week contains the first Thursday, not by which week contains January 1st. A late-December date can fall in week 1 of the following year (if that week's Thursday lands in January), and an early-January date can fall in the last week (52 or 53) of the previous year (if that week's Thursday is still in December).
Why do some years have 53 weeks?
A year has 53 ISO weeks when 1 January falls on a Thursday, or on a Wednesday in a leap year — both cases push an extra partial week into the year's count. Every other year has 52.
Sources and method
- ISO 8601 — Date and time format, week numbering.