Week Number Calculator

How it works

The calculator follows the ISO 8601 week date system, used for week numbers in most of Europe and much of Asia and in many business calendars. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the year.

Because of that rule, 29–31 December can belong to week 1 of the next year and 1–3 January can belong to week 52 or 53 of the previous year. The result shows the week-numbering year next to the week, together with all seven dates of that week.

The ISO 8601 rules

• Weeks run from Monday (day 1) to Sunday (day 7).
• Week 1 is the week with the year’s first Thursday in it; equivalently, the week that contains 4 January.
• A year has 52 weeks, or 53 when it starts on a Thursday, or is a leap year starting on a Wednesday.

week = ⌊(day of year − weekday + 10) ÷ 7⌋

Other week numbering systems

Not every country counts weeks the ISO way. In the United States and Canada calendars often start weeks on Sunday and make the week containing 1 January week 1, and parts of the Middle East start on Saturday. Around New Year these systems can differ from ISO by one week, so say “ISO week” when the exact number matters.

Tips

• Write ISO weeks as 2026-W43, or 2026-W43-2 for a single day (Tuesday of week 43).
• Spreadsheets have a function for this: ISOWEEKNUM in Excel and Google Sheets.
• About every five to six years there is a week 53, which matters for weekly payroll and reporting.

Standards and sources

• ISO 8601 Date and time format · ISO
• ISO 8601-1:2019 Date and time — Representations for information interchange · ISO

Last reviewed October 20, 2026