Find the ISO week for a date
Week numbers give a shared label to a seven-day planning period. They are useful in production schedules, reporting, delivery plans, and teams that organise work by week rather than by individual dates. This calculator uses ISO 8601 rules, so the result includes both a week number and the ISO year that owns that week.
Select a calendar date
Choose any day inside the week you want to identify.
Calculate the week number
The main result gives the ISO week. Every date from Monday through Sunday in that week shares the same number.
Read the week-year
Check the year shown underneath, especially for dates near the end of December or the beginning of January.
The rule for week one
ISO weeks start on Monday. Week one is the week containing the year’s first Thursday, which is also the week containing 4 January. A week belongs to the year containing its Thursday. Those rules keep each numbered week intact instead of splitting a single Monday-to-Sunday period between two years.
| Calendar date | ISO week | ISO week-year |
|---|---|---|
| 1 January 2021 | 53 | 2020 |
| 4 January 2021 | 1 | 2021 |
| 31 December 2025 | 1 | 2026 |
| 1 January 2027 | 53 | 2026 |
The date 1 January 2021 still belongs to ISO week 53 of 2020. That is not an error: the week began in December and its Thursday was in 2020. Conversely, 31 December 2025 belongs to week one of 2026. Store or share both the week number and week-year to preserve that distinction.
Why some years have week 53
Fifty-two complete weeks account for 364 days. The remaining calendar days shift the weekday pattern from year to year. Under ISO rules, years that start on Thursday, and leap years that start on Wednesday, have fifty-three weeks. Other ISO week-years have fifty-two. Do not assume that the week after week 52 is always week one.
Comparing with another calendar or spreadsheet
Not every week-number system follows ISO. Some calendars start weeks on Sunday or call the week containing 1 January “week one” regardless of its length. Spreadsheet functions can also offer several numbering options. When two results disagree, check the selected system before treating either result as incorrect.
Using week labels in a schedule
Write a label such as 2026-W01 rather than W01 alone. For people unfamiliar with week numbers, add the date range as well. An ISO week may cross a month boundary, so a monthly report needs a separate decision about how to allocate that week’s work. The calculator identifies the week; it does not set accounting or reporting policy.
For the number of complete seven-day periods between two dates, use date difference instead. For a deeper explanation of boundary cases, read ISO week numbers explained and the counting weeks guide.