Week Number Calculator

1. Pick any date with the date picker.
2. Read its ISO 8601 week number.
3. See the ISO week-year, which can differ from the calendar year.
4. Check the weekday the date falls on.

Here is what each result means:

An ISO week number labels each week of the year with a number from 1 up to 52 or 53. It comes from the international standard ISO 8601, which is widely used in business, industry and software to schedule and report by week. Instead of saying "the first week of July", you can refer precisely to "week 27".

The standard has two firm rules. Weeks always start on Monday and end on Sunday, and week 1 is defined as the week that contains the year's first Thursday, which is the same as the week containing 4 January. This avoids the awkwardness of a first week that is only a day or two long.

A consequence of these rules is the ISO week-year, which can differ from the ordinary calendar year. A date in late December can belong to week 1 of the next year, and a date in early January can belong to the last week of the previous year. The calculator shows this week-year so the week number is never ambiguous.

It applies the ISO 8601 rules using the nearest Thursday to the date.

1. Find the Thursday of the same week as your date.
2. The year of that Thursday is the ISO week-year.
3. Count weeks from that year's first Thursday to get the week number.

For other calendar tools, see the day of the week calculator and the date calculator.

Take 1 January 2026.

Result: that date is a Thursday, so it falls in ISO week 1 of the week-year 2026. By contrast, 31 December 2027 falls in the final week of its ISO year, showing how the week-year can differ from the calendar year at the edges.

Near the new year, the ISO week-year and the calendar year can disagree.

This is why the calculator reports the ISO week-year next to the week number. Quoting "week 1" without the year can be ambiguous for a date sitting right on the boundary between two years.

Most years have 52 ISO weeks, but some have 53.

A 53-week year happens roughly every five to six years. These long years matter in payroll and finance, where an extra pay week or reporting period has to be planned for. The calculator handles both cases automatically.

The Starting Monday

Because ISO weeks start on Monday, the same calendar date can shift between weeks compared with a Sunday-start system.

The First Thursday

Week 1 is anchored to the first Thursday, so the exact start of the year's numbering depends on which weekday 1 January is.

Leap Years

Leap years affect which years gain a 53rd week, by changing the weekday the year starts on.

Business and Payroll

Report and schedule by ISO week, as many finance and HR systems do.

Project Planning

Refer to deadlines by week number rather than exact dates.

Software and Data

Match the week numbering used by spreadsheets and programming libraries.

1. Ignoring the Week-year

Near the new year, always quote the ISO week-year, or "week 1" can mean either of two years.

2. Starting the Week on Sunday

ISO weeks start on Monday. A Sunday-start system can give a different week number.

3. Assuming 52 Weeks Always

Some years have 53 ISO weeks, which can surprise payroll and reporting.

4. Using a Simple Day Count

Dividing the day of the year by seven does not give the ISO week number, because of the Thursday rule.

5. Mixing Standards

Some tools use their own week numbering. Check that yours follows ISO 8601 before comparing.

The week number follows ISO 8601 exactly for any date entered.

A week number labels each week of the year from 1 to 52 or 53. Under ISO 8601, weeks start on Monday and week 1 contains the year's first Thursday.

Find the Thursday of the date's week, take that Thursday's year as the week-year, then count weeks from that year's first Thursday to your date.

Because week 1 is anchored to the first Thursday. A date in early January can fall in the last week of the previous ISO year, and late December can fall in week 1 of the next.

Yes. Years that start on a Thursday, and leap years that start on a Wednesday, have 53 ISO weeks. Most years have 52.

Monday. ISO 8601 always starts the week on Monday and ends it on Sunday, unlike some calendars that start on Sunday.

It depends on the weekday. If 1 January is Monday to Thursday it is in week 1; if it is Friday to Sunday it falls in the last week of the previous ISO year.

Most spreadsheet and programming libraries offer an ISO week option that matches this. Some default to a different, non-ISO numbering, so check the setting.

No. A date is treated as a whole calendar day, so the time and time zone do not change the week number.

No. The calculation runs in your browser and nothing you enter is stored or sent anywhere, unless you choose Save, which keeps the result only on this device.

• ISO week date (Wikipedia).
• ISO 8601 (Wikipedia).
• Week (Wikipedia).