Week Number Calculator
Find the ISO week number for any date.
Week 11
ISO Week 11 of 2026
Day of Week:Saturday
Day of Year:73 / 365
Days Remaining:292
Quarter:Q1
Leap Year:No
ISO Format:2026-W11
How to Do It Manually
The ISO 8601 standard defines week 1 as the week containing the year's first Thursday. Week numbers run Monday–Sunday.
- Find January 4th of the year (it always falls in week 1 by ISO definition).
- Count back to the Monday of that week — that is the start of ISO week 1.
- Count the number of complete weeks from that Monday to your date.
- Add 1 to get the week number.
Frequently Asked Questions
Why does week 1 sometimes start in December?
Under ISO 8601, if the new year starts on a Friday, Saturday, or Sunday, week 1 begins in late December of the previous year.
Are week numbers the same in all countries?
Most countries use ISO 8601 (Monday start), but the US and Canada traditionally start weeks on Sunday, which can shift week numbers by 1.