How to calculate a week number
Use the calculator to turn a Date and a selected Convention into a week number. Enter a valid four-digit date from 0100 through 9999, choose the convention your organization or software expects, then select Calculate week number.
- Enter the date in Date.
- Select a week-numbering rule in Convention.
- Select Calculate week number.
- To start over, select Reset. This clears the date, restores ISO 8601, hides errors, and hides the result.
Choose the week-numbering convention
- ISO 8601 (Monday start, week 1 has the year's first Thursday): weeks run Monday to Sunday. Week 1 contains the first Thursday, or 4 January. The ISO week-numbering year can differ from the calendar year near the start or end of a year.
- US / Sunday-start (week 1 contains 1 January): weeks run Sunday to Saturday. Week 1 contains 1 January and may be a partial week. The calendar year is used directly.
- Simple Monday start (week 1 contains 1 January): weeks run Monday to Sunday. Week 1 contains 1 January and may be a partial week. The calendar year is used directly.
The tool applies only these general conventions. It does not fetch or apply jurisdiction-specific or institutional week-numbering rules.
Read, copy, or download the result
After calculation, the result shows the week number, applicable year, week start date, and week end date. The assumptions table lists the entered date, selected convention, applicable year, week number, and week start and end dates.
The displayed formula follows the selected convention's week-1 rule. Select Copy result to copy the generated plain-text summary, including the formula, calculation details, and disclaimer. Select Download result (.txt) to create week-number-calculator-result.txt.
If clipboard access fails, the tool shows Could not copy - select and copy manually.
Validation and limitations
A missing or invalid date shows Enter a valid date. An invalid convention shows Choose a valid week-numbering convention. The calculation is deterministic arithmetic based on the entered date and selected convention.
The tool does not use external calendars or specific jurisdictional or institutional rules. Its result is not official, legal, payroll, or compliance advice.
Privacy and local processing
The calculator script performs the calculation in your browser and does not submit the entered date or convention. The page also loads analytics services and records page views through the shared site components.
Frequently Asked Questions
What is the ISO 8601 week-numbering convention?
Under ISO 8601, weeks run Monday to Sunday, and week 1 of a year is the week that contains the year's first Thursday (equivalently, the week that contains 4 January). Because of this, the first few days of January can belong to the last week of the previous year, and the last few days of December can belong to week 1 of the following year - the calculator reports this as a separate "ISO week-numbering year" when it happens.
How is the US / Sunday-start convention different?
In the US / Sunday-start convention, weeks run Sunday to Saturday, and week 1 is simply the week containing 1 January, even if that week only has a few days in it. There is no separate week-numbering year - the calendar year is used directly.
What is the "simple Monday-start" convention?
This convention uses Monday-to-Sunday weeks like ISO 8601, but numbers week 1 as the week containing 1 January rather than the week containing the first Thursday. Week 1 and the final week of the year can be short (partial) weeks, and the calendar year is used directly with no separate week-numbering year.
Which convention should I use?
This tool does not choose a convention for you. ISO 8601 is common in Europe and in many software systems; the US Sunday-start convention is common in US calendars and spreadsheets. Check what your organization, software, or region expects, then select the matching convention explicitly.
Is this official guidance for legal, payroll, or compliance purposes?
No. This is a deterministic arithmetic calculation based only on the date and convention you select. It does not apply any specific jurisdiction's or institution's official week-numbering rule beyond the general convention you choose, and is not legal, payroll, or compliance advice.