Week Number Calculator
The ISO 8601 week number for any date — the numbering payroll, retail and manufacturing calendars run on.
ISO 8601 defines week 1 as the week containing the first Thursday of the year, and every week runs Monday to Sunday.
How the week number calculator works
ISO 8601 defines week 1 as the week containing the first Thursday of the year, and every week runs Monday to Sunday. That has two consequences people trip over.
The first: early January can belong to the previous year's week 52 or 53. The 1st of January 2027 is a Friday, so it sits in week 53 of 2026, not week 1 of 2027.
The second: most years have 52 weeks, but some have 53 — whenever the year starts on a Thursday, or is a leap year starting on a Wednesday. Retail and payroll calendars built on 52 weeks need an explicit rule for those years.
Note that the US convention differs: many American calendars start weeks on Sunday and count week 1 as the one containing 1 January, which can give a different number for the same day.
Formula: week 1 contains the first Thursday of the year; weeks run Monday to Sunday
Worked examples
| Inputs | ISO week | Note |
|---|---|---|
| A midsummer date | Week 27 of 2026 | week 27 of 2026 |
| New Year in the previous week-year | Week 53 of 2026 | week 53 of 2026, not week 1 of 2027 |
| The 4th of January is always in week 1 | Week 1 of 2026 | week 1 |
FAQFrequently asked questions
What week number is it?
Enter today's date above. ISO weeks run Monday to Sunday and week 1 is the week containing the first Thursday of the year.
Why is 1 January sometimes in week 52 or 53?
Because ISO weeks belong to whichever year holds most of their days. If 1 January falls Friday to Sunday, that week is the last week of the previous year.
How many weeks are in a year?
52 in most years, 53 when the year starts on a Thursday or is a leap year starting on a Wednesday.
Does the US use the same week numbers?
Often not. Many US calendars start the week on Sunday and put week 1 wherever 1 January falls, which can differ from ISO by one.
What is the ISO week-numbering year?
The year the week belongs to, which is not always the calendar year of the date — 1 January 2027 sits in ISO year 2026.
Where these figures come from
- ISO 8601 — Date and time — Representations for information interchange — the yyyy-mm-dd format and the week-numbering rules used here
- US Naval Observatory — Calendars — the Gregorian leap-year rule
- GOV.UK — bank holidays — bank holidays by nation (not applied here)
Last checked: September 2026. The Gregorian calendar rules do not change; working-day counts exclude Saturdays and Sundays only.