How to use
- Choose when your week starts. Sunday is the default (as on most US calendars); switch to Monday or ISO 8601 if your workplace or country uses it. Your choice is remembered.
- Date → week shows today's week of the year, week of the month and the dates of the current week as soon as the page opens. Pick another date or tap a day in the calendar below.
- Week → dates takes a year and a week number and tells you which days that week covers — handy when a schedule just says "week 42".
- Year calendar lists every week of the year with its dates, and Since a date tells you which week you are in counting from a start date such as the first day of term.
Good to know
There are three common ways to number weeks. Sunday start (the US system) treats Sunday–Saturday as a week and counts the week containing January 1 as week 1 — the same as Excel WEEKNUM. Monday start also makes the week with January 1 week 1, but runs Monday–Sunday (WEEKNUM(date,2)).
ISO 8601, the international standard used across Europe and in many business systems, starts weeks on Monday and makes week 1 the week that contains the first Thursday of the year. As a result, January 1–3 can belong to week 52 or 53 of the previous year, and December 29–31 can be week 1 of the next. Excel's ISOWEEKNUM follows this rule.
The week of the month follows the calendar rows in Sunday and Monday mode (the row with the 1st is week 1). In ISO mode it follows the month that contains the week's Thursday, so October 1 can show as "week 5 of September".
FAQ
What week number is it today?
Open this page and the Date → week tab shows today's week number right away. Around New Year the answer can differ by one depending on whether you use Sunday start, Monday start or ISO 8601.
How many weeks are in a year?
A year has 52 weeks plus one day (two in a leap year), so the last week is numbered 52 or 53. In ISO 8601, years that start on a Thursday — or leap years starting on a Wednesday — have 53 weeks; 2026 is one of them.
How do I get the week number in Excel or Google Sheets?
Use =WEEKNUM(A1) for Sunday start, =WEEKNUM(A1,2) for Monday start, or =ISOWEEKNUM(A1) for ISO 8601. They match the three settings of this calculator.
Why is a date in late December shown as week 1?
That is the ISO 8601 rule: if a week's Thursday falls in the new year, the whole week belongs to week 1 of that year. Switch to Sunday or Monday start to count by calendar year instead.
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