How to use the week number calculator
Enter one date, choose a value in Convention, and select Calculate week number. The result gives the week number, the relevant year, and the dates at the start and end of that week. The page also shows the formula and assumptions used for the calculation.
- Enter a valid calendar date in Date.
- Choose ISO 8601, US / Sunday-start, or Simple Monday start in Convention.
- Select Calculate week number.
- Select Reset to clear Date, restore ISO 8601, and hide the result.
How ISO week numbers differ from US and Monday-start weeks
The selected Convention sets the first day of the week and the rule for week 1. The calculator applies that rule to the date you enter.
- ISO 8601 (Monday start, week 1 has the year's first Thursday) runs Monday to Sunday. Week 1 contains the first Thursday, or 4 January. Near New Year's Day, the ISO week-numbering year can differ from the calendar year.
- US / Sunday-start (week 1 contains 1 January) runs Sunday to Saturday. Week 1 contains 1 January, so it may be a partial week. The result uses the calendar year.
- Simple Monday start (week 1 contains 1 January) runs Monday to Sunday. Week 1 may be partial, and the result uses the calendar year.
Date: 2026-01-15
Convention: ISO 8601 (Monday start, week 1 has the year's first Thursday)
Result: Week 3, ISO week-numbering year 2026
Week start date: Monday, 2026-01-12
Week end date: Sunday, 2026-01-18What the week number result includes
Result shows the week number, its year label, the week start date, and the week end date. The assumptions table lists Date entered, Convention selected, the year label, Week number, Week start date, and Week end date.
Formula used gives the rule for the selected Convention. Select Copy result to place the plain-text summary on the clipboard, or select Download result (.txt) to save week-number-calculator-result.txt. If the copy action fails, the page shows Could not copy - select and copy manually.
What dates the calculator accepts
Date must be a real calendar date with a four-digit year from 0100 through 9999. A blank or invalid value shows Enter a valid date.. For example, 2026-02-30 is rejected. If Convention contains an unsupported value, the page shows Choose a valid week-numbering convention.
The calculation runs in the browser from Date and Convention. The script does not submit those values or fetch external calendars. It applies the general rule you select, without a separate jurisdictional or institutional week-numbering rule.
Where this comes up
- A project coordinator checks which ISO week contains a meeting date, then copies the week and date range into a project update.
- A reporting analyst receives a date from a spreadsheet and checks it under the same Sunday-start or Monday-start rule used by the report.
- A developer verifies a calendar screen near New Year's Day, where an ISO week-numbering year can differ from the calendar year.
Week number calculator or another date calculator?
Use this page when you need a numbered calendar week and its date range. The Date and Time Calculators category covers other date and time tasks. Choose the Date Add Calculator to add or subtract calendar units, the Date Difference Calculator to compare two dates, or the Business Days Calculator when the calculation includes workdays and a holiday list.
Frequently asked questions
How do I calculate a week number from a date?
Enter the date in Date, choose a rule in Convention, and select Calculate week number. The result includes the week number, year label, week start date, and week end date.
How do I find an ISO week number?
Select ISO 8601 (Monday start, week 1 has the year's first Thursday) in Convention, then select Calculate week number. The output can show an ISO week-numbering year different from the date's calendar year near the start or end of a year.
What are the US Sunday-start and Monday-start week numbers?
US / Sunday-start counts Sunday to Saturday, with week 1 containing 1 January. Simple Monday start counts Monday to Sunday with the same week-1 rule, and both use the calendar year directly.