What is an ISO week number?

ISO 8601 defines weeks as Monday through Sunday, with week 1 being the week that contains the year's first Thursday (equivalently, the week containing January 4th). This is the standard used by most businesses, calendars, and project-planning tools worldwide.

Why can a year have 53 weeks instead of 52?

52 weeks is only 364 days, one or two days short of a full year, so the extra day(s) eventually accumulate into a 53rd week. This happens in years where January 1st falls on a Thursday (or on a Wednesday in a leap year).

Why might the last days of December show as week 1 of the next year, or vice versa?

Because ISO weeks don't reset exactly at January 1st — a week is assigned to whichever year contains its Thursday. So late-December dates can belong to "week 1" of the following year, and early-January dates can belong to the last week of the previous year.

Is my date sent anywhere?

No. The calculation runs locally in your browser with JavaScript — nothing is transmitted or stored.

WEEK NUMBER CALCULATOR

Find the ISO week number and date range for any day. Nothing leaves your browser.

Date

ISO week numberWeek 41of 53 weeks in 2026

Week runs Oct 5, 2026 through Oct 11, 2026 (Monday–Sunday).

How to use the week number calculator

1
Pick a date, or click "Today" to use the current date.
2
Read the ISO week number for that date.
3
See the exact Monday–Sunday date range that week covers.
4
Check how many total weeks are in that year (52 or 53).

Questions

What is an ISO week number?: ISO 8601 defines weeks as Monday through Sunday, with week 1 being the week that contains the year's first Thursday (equivalently, the week containing January 4th). This is the standard used by most businesses, calendars, and project-planning tools worldwide.
Why can a year have 53 weeks instead of 52?: 52 weeks is only 364 days, one or two days short of a full year, so the extra day(s) eventually accumulate into a 53rd week. This happens in years where January 1st falls on a Thursday (or on a Wednesday in a leap year).
Why might the last days of December show as week 1 of the next year, or vice versa?: Because ISO weeks don't reset exactly at January 1st — a week is assigned to whichever year contains its Thursday. So late-December dates can belong to "week 1" of the following year, and early-January dates can belong to the last week of the previous year.
Is my date sent anywhere?: No. The calculation runs locally in your browser with JavaScript — nothing is transmitted or stored.