Week Number Calculator
Use this week number calculator to see which week a date falls in. You get the ISO 8601 week (Monday start, used in labels like 2026-W42) and the US week number (Sunday start, the Excel WEEKNUM default), with the start and end dates of each. You can also go the other way: enter a year and a week number to get its Monday–Sunday dates.
Updated
Results
- ISO week dateLast digit = weekday (1 = Monday); date mode only
- ISO week-yearCan differ from the calendar year near New Year
- Week starts (Monday)
- Week ends (Sunday)
- US week numberSunday start, week 1 contains January 1 (Excel WEEKNUM); date mode only
- US week starts (Sunday)Date mode only
- US week ends (Saturday)Date mode only
- Day of the yearJanuary 1 = day 1; date mode only
- ISO weeks in this year52 or 53
| Week | Monday | Sunday |
|---|
Results are estimates. See the methodology below.
How to use the calculator
- Date → week number (the default): pick a date. The form opens on today's date.
- Week → dates: enter an ISO week-year and a week number from 1 to 53 to see the Monday and Sunday of that week.
In date mode you also get the ISO week date (such as 2026-W42-1, where 1 = Monday), the US week number with its Sunday–Saturday range, and the day of the year. In both modes you see how many ISO weeks the year has, and the table lists every ISO week with its dates.
How the two systems work
ISO 8601 week
- Weeks run Monday to Sunday.
- Week 1 is the week that contains the year's first Thursday. That is the same as the week containing January 4.
- A week belongs to the year its Thursday falls in. So December 29–31 can be week 1 of the next year, and January 1–3 can be week 52 or 53 of the previous one. The calculator shows that year separately as the ISO week-year.
- A year has 53 ISO weeks when December 28 falls in week 53. Otherwise it has 52.
US week number
- Weeks run Sunday to Saturday.
- Week 1 is the week that contains January 1, even if it has only one day of the new year.
- Formula: week = floor((day of year − 1 + weekday of January 1) ÷ 7) + 1, with Sunday = 0. This matches Excel
WEEKNUM(date)with return type 1. - Because a partial first week counts, the last days of December can be week 53, and in rare years week 54.
All dates are plain calendar dates, so time zones don't affect the result.
Worked examples
1. Monday, October 12, 2026
ISO week 2026-W42, week date 2026-W42-1, running Monday, October 12 to Sunday, October 18. The US week number is also 42, but that week runs Sunday, October 11 to Saturday, October 17. It is day 285 of the year.
2. Friday, January 1, 2027
2026 starts on a Thursday, so it has 53 ISO weeks. New Year's Day 2027 is still in ISO week 2026-W53 (Monday, December 28, 2026 to Sunday, January 3, 2027). In the US system it is week 1 of 2027 (Sunday, December 27 to Saturday, January 2). On Monday, January 4, 2027, ISO week 2027-W01 begins, while the US count is already at week 2.
3. Dates where the systems split
| Date | ISO week | US week |
|---|---|---|
| Tue, Dec 31, 2024 | 2025-W01 | 53 |
| Mon, Dec 29, 2025 | 2026-W01 | 53 |
| Sun, Jan 1, 2023 | 2022-W52 | 1 |
| Sun, Dec 31, 2028 | 2028-W52 | 54 |
4. Week → dates
ISO 2026, week 1 runs from Monday, December 29, 2025 to Sunday, January 4, 2026. Week 53 of 2026 runs from December 28, 2026 to January 3, 2027. Asking for week 53 of 2027 returns an error, because 2027 has only 52 ISO weeks.
Which week number should you use?
Away from New Year the two numbers are usually equal or one apart; on Sundays in some years they are two apart, because a US week starts the day before an ISO week. They split most around New Year, where a date can even belong to a different year in ISO terms. When a schedule, invoice or calendar app quotes a week number, check which rule it uses before you plan around it. Writing the full ISO label, such as 2026-W53, removes the doubt about the year.
Frequently asked questions
What week number is it today?
The calculator opens on today's date in your browser, so the result shows the current week straight away. For example, Monday, October 12, 2026 is ISO week 42 and US week 42.
Why does Excel give me a different week number?
WEEKNUM(date) with no second argument uses the US rule: Sunday start, and week 1 contains January 1. For ISO weeks use ISOWEEKNUM(date) or WEEKNUM(date, 21). Microsoft documents return type 21 as the ISO 8601 system.
Which years have 53 weeks?
An ISO year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. Between 2020 and 2032 that happens in 2020, 2026 and 2032. All other years in that range have 52.
Is week 1 always the week of January 1?
In the US system, yes. In ISO 8601, no: week 1 is the week with the first Thursday. If January 1 falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous ISO year.
Can a week number be 54?
Only in the US system. In a leap year that starts on a Saturday, December 31 is a Sunday and opens week 54. Sunday, December 31, 2028 is an example. ISO weeks never go past 53.
Does the calculator handle fiscal or retail weeks?
No. Fiscal years, broadcast calendars, 4-4-5 retail calendars and Saturday-start weeks follow their own rules. The US week number shown here is a common spreadsheet convention, not an official federal standard.
Sources
Results follow the ISO 8601 week rule and the Excel WEEKNUM type 1 convention described above. Fiscal, payroll or company calendars may number weeks differently.