Week Number Calculator: What Week Is It?
To find the week number of a date, use the ISO 8601 rule: weeks start on Monday and week 1 is the week with the year's first Thursday. December 31, 2026 is in ISO week 53 of 2026 (2026-W53), because 2026 has 53 ISO weeks. Counted the US way (Excel's WEEKNUM), with Sunday as the first day and week 1 holding January 1, the same day is week 53 of 2026, a partial week of 5 days: Dec 27, 2026 to Dec 31, 2026 is week 53 of 2026, while Jan 1, 2027 to Jan 2, 2027 is week 1 of 2027. Pick any date below for both numbers, or turn a week number back into the dates it covers.
Updated Runs in your browser: nothing you enter is uploadedHow this tool works
Week number of a date
Result
ISO week 53 of 2026
Thursday, December 31, 2026 is in ISO week 53 of 2026 (2026-W53) and US week 53 of 2026.
- ISO 8601 week
- Week 53 of 2026 (2026-W53); 53 weeks in 2026
- That ISO week runs
- Monday, December 28, 2026 to Sunday, January 3, 2027
- US week
- Week 53 of 2026; 53 weeks in 2026
- That US week runs
- Sunday, December 27, 2026 to Thursday, December 31, 2026 (partial week: 5 days)
- Day of the year
- 365 of 365
- Days left in the year
- 0 days
- Quarter
- Q4 (Oct–Dec)
- The US system cuts weeks at the year boundary, so US week 53 of 2026 has only 5 days; the rest of that calendar week is counted in the neighbouring year.
Dates of a week number
Result
Dec 28, 2026 to Jan 3, 2027
ISO 8601 week 53 of 2026 runs from Monday, December 28, 2026 to Sunday, January 3, 2027.
- First day
- Monday, December 28, 2026
- Last day
- Sunday, January 3, 2027
- Week code
- 2026-W53
- Weeks in that year
- 2026 has 53 ISO 8601 weeks
- This week crosses a year boundary: it starts in 2026 and ends in 2027.
This week
ISO weeks in each year, 2025 to 2030
| Year | ISO weeks | ISO week 1 starts (Monday) | US weeks | US week 1 covers | January 1 is |
|---|---|---|---|---|---|
| 2025 | 52 | Dec 30, 2024 | 53 | Jan 1, 2025 to Jan 4, 2025 | Wednesday |
| 2026 | 53 | Dec 29, 2025 | 53 | Jan 1, 2026 to Jan 3, 2026 | Thursday |
| 2027 | 52 | Jan 4, 2027 | 53 | Jan 1, 2027 to Jan 2, 2027 | Friday |
| 2028 | 52 | Jan 3, 2028 | 54 | Jan 1, 2028 to Jan 1, 2028 | Saturday |
| 2029 | 52 | Jan 1, 2029 | 53 | Jan 1, 2029 to Jan 6, 2029 | Monday |
| 2030 | 52 | Dec 31, 2029 | 53 | Jan 1, 2030 to Jan 5, 2030 | Tuesday |
How to find the ISO week number of a date by hand
- Find the Thursday of the date's week. Move to the Monday of the week the date is in, then add 3 days. ISO weeks belong to the year their Thursday falls in.
- Note the year of that Thursday. This is the week-based year. For January 3, 2021 the Thursday is December 31, 2020, so the week belongs to 2020, not 2021.
- Count the days since January 1 of that year. Take the Thursday's day of the year (January 1 is day 1) and subtract 1.
- Divide by 7 and add 1. Round the division down. The result is the ISO week number: 1 to 52, or 53 in a long year.
Frequently asked questions
- What week number is it today?
- This page reads the date on your device and shows the current ISO and US week at the bottom of the calculator. Weeks change at midnight on Monday (ISO) or Sunday (US), in your local time.
- How are ISO weeks different from US weeks?
- ISO 8601 weeks start on Monday and week 1 is the week with the first Thursday of the year, so it always has at least four days in the new year. The US system used by Excel's WEEKNUM (type 1) starts weeks on Sunday, makes week 1 the week that contains January 1 and counts on to December 31 within the same calendar year. The first and last weeks are therefore often partial: in 2026, Dec 27 to Dec 31 is week 53 and Jan 1 to Jan 2, 2027 is week 1 of 2027. The two numbers often differ by one, and near New Year the ISO week can belong to another year while the US week never does.
- Which year has 53 weeks?
- An ISO year has 53 weeks when January 1 is a Thursday, or a Wednesday in a leap year. Among 2025 to 2030, 2026 is 53-week year; the next one is 2032. A US-system year always has 53 weeks or, in a leap year that starts on a Saturday (2000, 2028), 54, because its partial first and last weeks both count.
- Why is January 1 sometimes in week 52 or 53?
- Under ISO 8601 a week belongs to the year of its Thursday. If January 1 falls on a Friday, Saturday or Sunday, its week has its Thursday in the old year, so January 1 is in week 52 or 53 of that year. For example January 3, 2021 is in 2020-W53.
- Why is December 30 sometimes week 1 of the next year?
- The same rule works at the other end. When a week's Thursday falls in the new year, the whole week counts as week 1 of that year, even if its Monday is in December. December 30, 2024 is in 2025-W01.
- What is the week-based year?
- It is the year an ISO week belongs to, which is what ISO 8601 writes in a date like 2026-W53. It matches the calendar year for most of the year but can be one year earlier or later during the first and last days. Use it with the week number, never the calendar year, when you write a week date.
- What are the dates of a given week number?
- Choose the system, type the year and week in the second box and read the first and last day. For example ISO week 53 of 2026 runs Dec 28, 2026 to Jan 3, 2027.
- Which countries use ISO week numbers?
- ISO week numbers are common in Europe and much of the world, especially in business, logistics and schedules. In the United States calendars and spreadsheets more often start the week on Sunday. When a document does not say which system it uses, ask or check a known date.
How it works
Each date is turned into a day number (days since January 1, 1970) with integer arithmetic on the Gregorian calendar, so the visitor's time zone and daylight saving time can never move a date into another week. ISO 8601 weeks start on Monday; the week belongs to the year of its Thursday, and the week number is the number of whole weeks from January 1 of that year to the Thursday, plus one. That is the same as saying week 1 holds January 4. US weeks start on Sunday and week 1 is the week with January 1; the number is the whole weeks from the Sunday on or before January 1 to the date, plus one, and always stays in the date's calendar year, as in Excel's WEEKNUM(date, 1). The first and last US weeks are cut at the year boundary, so they can have fewer than seven days, and a year has 53 weeks (54 in a leap year that starts on a Saturday). The day of the year counts January 1 as day 1, and the days left are the year's length minus that. Quarters are three calendar months each. Years from 1583 to 9998 are supported. "This week" uses the date on your device and is added after the page loads. Everything runs in your browser.
Sources
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