How to use the Week Number Calculator
- Enter the date you need the week number for.
- Read the ISO week, shown in the standard W-prefixed form.
- Check the ISO week year — at the turn of the year it can differ from the calendar year.
- Use the day of year for Julian-style date references.
- Use the ISO weekday, where Monday is 1 and Sunday is 7.
How the calculation works
ISO 8601 defines week 1 as the week containing the first Thursday of January, equivalently the week containing 4 January. Weeks always start on Monday. The algorithm finds the Thursday of the date's own week, finds the Thursday of week 1 in that Thursday's year, and counts the whole weeks between them. Anchoring on Thursday is what makes the rule self-consistent across year boundaries.
The consequence is that a date can belong to a week year different from its calendar year. 31 December 2026 falls in ISO week 53 of 2026, but 1 January 2027 falls in that same week, so its ISO week year is 2026. Reporting systems that pair an ISO week with the plain calendar year produce a phantom week 53 in the wrong year — a classic and quietly damaging reporting bug.
Not every country uses the ISO rule. North American practice commonly starts weeks on Sunday and treats the week containing 1 January as week 1, which can differ from the ISO number by one. Manufacturing date codes, retail 4-4-5 calendars and payroll periods all layer their own conventions on top, so always state which system a week number belongs to.
Week 1 contains the first Thursday of January; week = 1 + round((Thursday of date's week − Thursday of week 1) / 7 days)Source: ISO 8601-1:2019, Clause 5.2.3 (week dates).
Worked example
A production batch is coded for 1 January 2027 and needs an ISO week stamp.
- 1 January 2027 is a Friday.
- The Thursday of that week is 31 December 2026.
- That Thursday's year is 2026, so the ISO week year is 2026.
- Counting from the Thursday of week 1 in 2026 gives week 53.
2026-W53 — stamping it as 2027-W53 would be wrong, since 2027 has only 52 weeks.
Frequently asked questions
Can a year have 53 weeks?+
Yes. Years starting on a Thursday, and leap years starting on a Wednesday, contain 53 ISO weeks. Most years have 52.
Why does the week year differ from the calendar year sometimes?+
Because a week is never split across two week years. The first few days of January can belong to the last week of the previous year, and the reverse also happens.
Does the week start on Sunday or Monday?+
Monday under ISO 8601. Sunday-start numbering is a separate North American convention that can give a different number.
What is the day of year used for?+
Ordinal dating, common in aviation, astronomy and manufacturing date codes — 2026-060 is the sixtieth day of 2026.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.