How this week number calculator works
A week number tells you how far through the year a given week is. The international standard, ISO 8601, defines weeks precisely: each week starts on Monday and ends on Sunday, and week 1 is the first week that contains a Thursday — equivalently, the week containing 4 January. This calculator uses that standard to label the week of any date as a year plus a week number, such as 2026-W23.
The classic way to compute it is the Thursday method: take the date, move to the Thursday of its week, and read that Thursday's year — that is the ISO year. The week number is then the count of weeks from the first Thursday of that ISO year. Anchoring on Thursday is what makes early-January and late-December dates land in the correct year and week, which is why 1 January can sometimes belong to week 52 or 53 of the previous year.
Reference note — ISO vs US conventions: ISO 8601 weeks start on Monday and week 1 is the week with the first Thursday. The common US/North American convention starts weeks on Sunday and counts the week containing 1 January as week 1. Because both the start day and the week-1 rule differ, the same date can carry two different week numbers, so this tool shows the ISO number and the US number side by side. Calculations use the Gregorian calendar and your device's local date.
Frequently asked questions
- What week of the year is it?
- By default this calculator shows the current ISO 8601 week number for today's local date, written as year and week — for example 2026-W23. Enter any other date to see the week it falls in.
- How are week numbers calculated (ISO 8601)?
- Under ISO 8601, weeks start on Monday and week 1 is the week containing the first Thursday of the year (the week that includes 4 January). The standard method finds the Thursday of the current week, takes that Thursday's year, and counts the weeks since the year's first Thursday.
- Does the week start on Monday or Sunday?
- ISO 8601 weeks start on Monday and end on Sunday — the standard across most of Europe and in business and software. The common US convention starts the week on Sunday, which can give a different number for the same date, so this tool shows both.
- How many weeks are in a year?
- An ISO year has 52 or 53 weeks. Most have 52; a year has 53 ISO weeks when it starts on a Thursday, or starts on a Wednesday and is a leap year. The calculator shows the exact count for the year you enter.
- What is week 1?
- ISO week 1 is the first week of the year that contains a Thursday — the week including 4 January. So the first days of January can fall in week 52 or 53 of the previous year, and the last days of December can fall in week 1 of the next year.
- Why do US and ISO week numbers differ?
- They use different rules. ISO 8601 starts weeks on Monday with week 1 as the week of the first Thursday; the common US convention starts weeks on Sunday and treats the week containing 1 January as week 1. Different start day and week-1 rule means the same date can have two week numbers.