ISO Week Number Calculator
Find the ISO 8601 week number, week-year, and day-of-year for any date — runs 100% in your browser with no upload.
2026-W31
31
2026
212
🔒 Calculated in your browser — nothing is uploaded.
What this tool does
Pick any date and this tool instantly shows its ISO 8601 week number, the matching ISO week-year, and the day of the year. Under the ISO 8601 standard, weeks always start on Monday and week 1 is the week that contains the year's first Thursday — equivalently, the week that contains January 4. Because of that rule, the last few days of December or the first few days of January can belong to a neighbouring week-year: for example January 1 2023 falls in ISO week 52 of 2022, while December 31 2018 falls in ISO week 1 of 2019. The result is shown both as separate values and as the compact ISO string such as 2026-W01.
Why ISO week numbers matter
ISO week numbers give teams a single, unambiguous way to label weeks that works the same in every country and calendar system, which is why they are widely used in payroll, manufacturing schedules, project planning, and financial reporting. The ISO week-year can differ from the ordinary calendar year at the turn of January, so pairing the week number with its week-year avoids the off-by-one confusion that plagues homemade spreadsheets. Everything here is computed locally in your browser, so you can look up as many dates as you like without limits.
Frequently asked questions
Why does January 1 sometimes show a week number in the previous year?
ISO 8601 assigns week 1 to the week containing the first Thursday of the year. If January 1 falls on a Friday, Saturday, or Sunday, that week's Thursday belongs to the previous year, so those early January days are counted as the last week (52 or 53) of the previous ISO week-year.
How can a year have 53 weeks?
A year has 53 ISO weeks when January 1 is a Thursday, or when it is a Wednesday in a leap year. In those years an extra Thursday is squeezed in, so week 53 appears. For example, December 31 2020 falls in ISO week 53 of 2020.
Can I go the other way — from a week number to its dates?
Yes. Switch to 'Week → dates' mode, enter an ISO week-year and week number, and the tool shows that week's Monday-to-Sunday date range. For example, week 1 of 2026 runs from 2025-12-29 to 2026-01-04. It rejects a week number the year doesn't have, such as week 53 in a 52-week year.
Is my date sent to a server?
No. The date you pick never leaves your device. The week number, week-year, and day-of-year are all calculated entirely in your browser with JavaScript — nothing is uploaded, logged, or stored.