What it does
This calculator answers two questions about ISO 8601 week numbering. Give it a date and it tells you which ISO week that date sits in, along with the week-year and the Monday–Sunday range that week covers. Switch to the reverse mode, give it a week-year and a week number, and it hands back the exact dates that week spans. Everything runs in your browser; no dates are sent anywhere.
How ISO 8601 weeks work
ISO 8601 — the international standard used by UK and EU business systems, ICS/iCal calendars, and most enterprise software — defines a week that runs from Monday to Sunday. Week 1 of any year is the week that contains the year's first Thursday, which is equivalently the week that contains 4 January. That rule has two useful consequences.
The first is the idea of a week-year, which can differ from the calendar year for a few days either side of 1 January. If 1 January falls on a Friday, Saturday or Sunday, its week's Thursday lies in the previous year, so those dates belong to week 52 or 53 of the year before. Conversely, if late December's dates share a week whose Thursday has already tipped into January, they belong to week 1 of the new year.
The second is that most years have 52 ISO weeks, but some have 53. A year has 53 ISO weeks when 1 January falls on a Thursday, or when 1 January falls on a Wednesday and that year is a leap year. Every other year has 52.
Worked example
Take the date 2024-06-15. Its ISO week is week 24 of 2024, which runs from Monday 2024-06-10 to Sunday 2024-06-16. The calculator finds this by locating the Thursday of that Monday–Sunday week, taking the calendar year of that Thursday as the week-year, and counting how many whole weeks separate it from the Thursday of week 1.
Why 53-week years happen
Fifty-two weeks is 364 days, so an ISO week-year is always one or two days shorter than the calendar year. Those spare days accumulate: whenever they add up to a full extra week under the "first-Thursday" rule, that year gets a week 53. This is why 2015, 2020, 2026 and 2032 have 53 ISO weeks — a small quirk that catches out annual reports, payroll schedules and calendars that only allow for 52.
Common uses
- Project and sprint planning: teams that plan by "week 32" or "wk 15" need a reliable mapping to actual dates.
- Payroll and shift rosters: weekly pay runs and rota systems typically use ISO week numbering.
- Calendar apps and ICS files: ISO week numbers are the default in Outlook, Google Calendar (in the UK/EU) and Apple Calendar.
- Financial and manufacturing reporting: annual reports often reference results by ISO week, especially in Europe.
- Cross-checking dates that fall near year-end: confirming whether 30 December belongs to week 1 of the new year, or 1 January to week 52 of the old one.
Frequently asked
Why does 1 January sometimes belong to the previous year?
Because ISO week-years are defined by where the Thursday of the week sits, not by the calendar. When 1 January is a Friday, Saturday or Sunday, that week's Thursday is still in December, so the whole Monday–Sunday block — including that 1 January — is counted as week 52 or 53 of the previous year.
What's the difference between the ISO week-year and the calendar year?
They usually match, but not always. The calendar year runs from 1 January to 31 December. The ISO week-year runs in whole Monday–Sunday weeks, so it can start a few days early or late. For most of the year the two labels agree; they only diverge for the handful of days at each year boundary.
Are week numbers the same in every country?
No. ISO 8601 (Monday-start, first-Thursday rule) is the standard used across the UK, EU and most of the rest of the world. The United States, parts of the Middle East and a few other regions commonly use different conventions — for example, weeks starting on Sunday, or week 1 being the week that contains 1 January. If you're sharing week numbers with a US audience it's worth stating which system you're using.
Which years have 53 weeks?
A year has 53 ISO weeks when 1 January is a Thursday, or when it's a leap year whose 1 January is a Wednesday. Recent and upcoming examples: 2004, 2009, 2015, 2020, 2026, 2032.
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