Week Number Calculator

What week of the year is it?

This tool gives the ISO 8601 week number for any date — the international standard used in business, manufacturing, logistics, and project planning, where schedules are tracked as "week 14" rather than a specific date. It also shows the ISO week-year, the day of the year, and the weekday.

How ISO week numbers work

The ISO standard has two simple rules that make week numbers consistent worldwide:

• Weeks start on Monday and end on Sunday.
• Week 1 is the week containing the year's first Thursday — equivalently, the week containing January 4. This guarantees every week belongs mostly to the year it's numbered in.

Most years have 52 weeks, but some have 53. A year is a 53-week year when it starts on a Thursday (or a Wednesday in a leap year).

Why the "week-year" can differ from the calendar year

Because week 1 is tied to the first Thursday, the first few days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. For example, January 1 might fall in "week 52 of the previous ISO year." That's why this tool reports the ISO week-year separately — it's occasionally one off from the calendar year, and that's correct, not a bug.

FAQ

Is this the same as "week of the month"?

No. This is the week of the year (1–53). Week-of-the-month numbering isn't standardized and isn't what "week number" usually refers to in business.

Do all countries use this?

ISO 8601 is the global standard and is dominant in Europe and in business worldwide. Some regions informally start weeks on Sunday or number week 1 differently, so if a partner uses another scheme, confirm which one before relying on a shared week number.

How is ISO week 1 determined?

Under ISO 8601, week 1 is the week containing the year's first Thursday — equivalently, the week that contains January 4. Weeks run Monday to Sunday, which keeps the numbering consistent worldwide.

Can a year have 53 weeks?

Yes. Most years have 52 weeks, but a year has 53 when it starts on a Thursday, or on a Wednesday in a leap year. The calculator handles these long years automatically.

Why does the week-year sometimes differ from the calendar year?

Because week 1 is tied to the first Thursday, early-January dates can belong to the last week of the previous ISO year, and late-December dates can fall in week 1 of the next. That's why the tool reports the ISO week-year separately — it's occasionally one off from the calendar year, and that's correct.

Is the week number calculator free and private?

Yes to both. It's free with no sign-up, works on mobile, and runs entirely in your browser, so the dates you enter are never sent anywhere.