What week number is it right now?

Today is read from your device’s local calendar. While Today is active, the date refreshes each visible second and when you return to the page. A selected date stays fixed. Today returns to the live date; Previous week and Next week move a selected date by seven calendar days.

What is the difference between the ISO and US week numbers?

Two things: which day starts the week, and which week gets number 1. ISO 8601 weeks run Monday to Sunday and week 1 is the week containing 4 January, which is the same as saying the week containing the year's first Thursday. The US convention runs Sunday to Saturday and week 1 is the week containing 1 January, however few days of it fall in the new year. How far apart the two numbers run depends on the weekday 1 January falls on. In a year opening Sunday through Thursday, which includes 2026, they agree on every day except Sundays, where the US number is one higher because a US week has just begun while the ISO week is still finishing. In a year opening on a Friday or a Saturday the US number is one higher all week and two higher on Sundays, because the stub of days before that year's first Sunday takes a whole number to itself. Both are shown here so you never have to work out which one was meant.

Why do some years have 53 weeks?

A year is 52 weeks plus one day, or plus two days in a leap year, so the extra days accumulate and every five or six years the ISO calendar needs a 53rd week to absorb them. Precisely: an ISO year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 has 53; 2025 has 52. The US convention can go further still and produce a 54th week, but only in a leap year that opens on a Saturday, and that week is always exactly one day long: 31 December. 2000 was one, and 2028 is the next.

Which week are the first days of January in?

Under ISO 8601 they often belong to the year before. A week belongs to the year that holds its Thursday, so 1 January 2027, a Friday, is in week 53 of 2026, and 30 December 2024, a Monday, is already in week 1 of 2025. This is not an edge case that only a standard cares about: it is why an ISO week number always comes with its own year, written 2026-W36. The US convention never does this, because it restarts numbering on 1 January and keeps the calendar year, at the cost of first and last weeks that are usually cut short.

Why does Excel give a different week number?

Because WEEKNUM defaults to the US convention. WEEKNUM(date) and WEEKNUM(date, 1) start weeks on Sunday and put 1 January in week 1, which is the US column here. For the ISO number use ISOWEEKNUM(date), or WEEKNUM(date, 21). Google Sheets follows the same rules. If a spreadsheet and a European colleague disagree about which week a date is in, this is almost always why.

Do time zones or daylight saving change the answer?

Selected dates are calendar fields, so daylight saving does not alter their week numbers. Today follows your device clock and local time zone. Calculations cover Gregorian dates from 0001 through 9999. Enter a calendar date in the date field; this does not convert a timestamp to another time zone.

How do I convert a week number to dates?

Choose Week to date, enter the week-numbering year and week, then select ISO or US. ISO week 1 of 2025 runs from Monday 30 December 2024 through Sunday 5 January 2025. US week 1 of 2025 covers 1–4 January because US numbering restarts on 1 January. A week that does not exist in the chosen year is refused.

How is the ISO week number calculated?

Find the Monday of the date’s week. Its Thursday determines the week year. Count full seven-day weeks from the Monday of the week containing 4 January, then add one. Dividing the calendar day of year by seven misses ISO year boundaries.

Can I view a calendar with week numbers for another year?

Enter any date in that year, or enter its year and week in Week to date. The table lists every numbered week with start and end dates and marks the selected week. This is a range table; it does not provide monthly day grids, printable calendar templates or calendar-app settings.

Does a work week number mean a fiscal or employer week?

Here it means the ordinary ISO or US calendar week. Fiscal years, employer-specific shifts, GPS or epoch weeks, NFL seasons and weeks within a month use other rules and are not calculated.

How many weeks are between two dates, or how many hours are in a week?

Use Date Calculator’s Days between mode for whole seven-day weeks plus remaining days and Monday-to-Friday counts; weekday counts include both dates and omit no holidays. Unit Converter’s Time category converts one fixed week to 168 hours, 10,080 minutes or 604,800 seconds. A fixed unit week is different from the elapsed clock hours of a local calendar week crossing daylight saving.

Week Number

See the current week number, or the week any date falls in, under both rules that are actually in use. Under ISO 8601, weeks start on Monday and week 1 holds 4 January. Under the US convention, weeks start on Sunday and week 1 holds 1 January. Both numbers are on the page at once, alongside the exact days the week covers, the day of the year, and a table of every week of the year with its start and end dates. Choose Week to date to look up a numbered week directly.

ISO week numberReadyPick a date to see the week it falls in.

Lookup

Date

Convention

ISO 8601: weeks run Monday to Sunday, and week 1 is the week holding 4 January. Standard across Europe and in most software. A week belongs to the year that holds its Thursday, so the first days of January can fall in the last week of the year before.

Following today on your device. Refreshes while visible and when you return to the page.

ISO and US conventions. ISO 8601 is the international standard and the one meant by "week 36" in most of Europe and in most software. The US convention is what a spreadsheet returns from WEEKNUM unless you ask it for the ISO rule. Away from year boundaries, how far apart they run depends on the weekday 1 January falls on: in a year opening Sunday through Thursday they agree every day except Sundays, when the US number is one higher, and in a year opening on a Friday or a Saturday the US number is one higher all week and two higher on Sundays. At a year boundary, compare the week year too. This page uses Gregorian calendar dates from 0001 through 9999; it does not model fiscal, GPS, month-relative or employer schedules. Date lookup shows both week numbers; reverse lookup uses the selected rule.

Common questions

What week number is it right now?: Today is read from your device’s local calendar. While Today is active, the date refreshes each visible second and when you return to the page. A selected date stays fixed. Today returns to the live date; Previous week and Next week move a selected date by seven calendar days.
What is the difference between the ISO and US week numbers?: Two things: which day starts the week, and which week gets number 1. ISO 8601 weeks run Monday to Sunday and week 1 is the week containing 4 January, which is the same as saying the week containing the year's first Thursday. The US convention runs Sunday to Saturday and week 1 is the week containing 1 January, however few days of it fall in the new year. How far apart the two numbers run depends on the weekday 1 January falls on. In a year opening Sunday through Thursday, which includes 2026, they agree on every day except Sundays, where the US number is one higher because a US week has just begun while the ISO week is still finishing. In a year opening on a Friday or a Saturday the US number is one higher all week and two higher on Sundays, because the stub of days before that year's first Sunday takes a whole number to itself. Both are shown here so you never have to work out which one was meant.
Why do some years have 53 weeks?: A year is 52 weeks plus one day, or plus two days in a leap year, so the extra days accumulate and every five or six years the ISO calendar needs a 53rd week to absorb them. Precisely: an ISO year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 has 53; 2025 has 52. The US convention can go further still and produce a 54th week, but only in a leap year that opens on a Saturday, and that week is always exactly one day long: 31 December. 2000 was one, and 2028 is the next.
Which week are the first days of January in?: Under ISO 8601 they often belong to the year before. A week belongs to the year that holds its Thursday, so 1 January 2027, a Friday, is in week 53 of 2026, and 30 December 2024, a Monday, is already in week 1 of 2025. This is not an edge case that only a standard cares about: it is why an ISO week number always comes with its own year, written 2026-W36. The US convention never does this, because it restarts numbering on 1 January and keeps the calendar year, at the cost of first and last weeks that are usually cut short.
Why does Excel give a different week number?: Because WEEKNUM defaults to the US convention. WEEKNUM(date) and WEEKNUM(date, 1) start weeks on Sunday and put 1 January in week 1, which is the US column here. For the ISO number use ISOWEEKNUM(date), or WEEKNUM(date, 21). Google Sheets follows the same rules. If a spreadsheet and a European colleague disagree about which week a date is in, this is almost always why.
Do time zones or daylight saving change the answer?: Selected dates are calendar fields, so daylight saving does not alter their week numbers. Today follows your device clock and local time zone. Calculations cover Gregorian dates from 0001 through 9999. Enter a calendar date in the date field; this does not convert a timestamp to another time zone.
How do I convert a week number to dates?: Choose Week to date, enter the week-numbering year and week, then select ISO or US. ISO week 1 of 2025 runs from Monday 30 December 2024 through Sunday 5 January 2025. US week 1 of 2025 covers 1–4 January because US numbering restarts on 1 January. A week that does not exist in the chosen year is refused.
How is the ISO week number calculated?: Find the Monday of the date’s week. Its Thursday determines the week year. Count full seven-day weeks from the Monday of the week containing 4 January, then add one. Dividing the calendar day of year by seven misses ISO year boundaries.
Can I view a calendar with week numbers for another year?: Enter any date in that year, or enter its year and week in Week to date. The table lists every numbered week with start and end dates and marks the selected week. This is a range table; it does not provide monthly day grids, printable calendar templates or calendar-app settings.
Does a work week number mean a fiscal or employer week?: Here it means the ordinary ISO or US calendar week. Fiscal years, employer-specific shifts, GPS or epoch weeks, NFL seasons and weeks within a month use other rules and are not calculated.
How many weeks are between two dates, or how many hours are in a week?: Use Date Calculator’s Days between mode for whole seven-day weeks plus remaining days and Monday-to-Friday counts; weekday counts include both dates and omit no holidays. Unit Converter’s Time category converts one fixed week to 168 hours, 10,080 minutes or 604,800 seconds. A fixed unit week is different from the elapsed clock hours of a local calendar week crossing daylight saving.

Exact by the ISO 8601 rule: weeks start on Monday and week 1 is the week with 4 January. The US convention (weeks start Sunday, week 1 holds 1 January) is offered separately, and the two can differ by one or two, or fall in different years, so both numbers are always shown.