Week number and day of week
See the ISO week number, ISO week-year and weekday name for a date.
What the result means
ISO weeks start on Monday. Week 1 is the week that contains the first Thursday of the Gregorian year. Early January can still be week 52 or 53 of the previous ISO year.
How it is calculated
ISO week-year is the year of the Thursday of that week. Week 1 contains 4 January
- Weekday: Sunday through Saturday in English.
The algorithm finds the Thursday of the current week, then counts weeks from the first Thursday of that ISO year.
Assumptions
- ISO-8601. Monday is day 1 of the week for week numbering.
Worked example
1 January 2026
New Year’s Day 2026 is a Thursday.
- Weekday
- Thursday
- ISO week
- Week 1 of 2026
Because Thursday sits in week 1, 1 January 2026 is week 1 — unlike years when 1 January is a Friday–Sunday.
Not a US week-numbering system
Some US systems count Sunday-start weeks and treat 1 January as week 1 always. This page is ISO-8601, common in Australia for business calendars.
Questions
Is week 53 real?
Yes, in some years. ISO years have 52 or 53 weeks.
Related calculators
Sources
ISO
Last reviewed 2026-08-19
Dates & time · Last reviewed 2026-08-19
Estimates only. Not tax, legal or financial advice. Check official sources before relying on a figure.
Printed from OzCalc (ozcalc.com). Last reviewed 2026-08-19. Estimates only — not tax, legal or financial advice.