Week Calculator
Calculate the number of weeks between two dates, or find the ISO week number of any date.
About the Week Calculator
This dual-purpose calculator either measures how many weeks fall between two dates, or tells you which numbered week of the year a given date falls in — both common needs in project planning, payroll cycles, and scheduling that follows a standard week-numbering system.
How It Works
In "weeks between" mode, the calculator finds the total number of days between two dates and divides by seven. In "week number" mode, it applies the ISO week-numbering convention, which anchors week 1 to the week containing the year's first Thursday.
Weeks Between = Days Between Dates / 7. ISO Week Number follows the standard ISO 8601 week-numbering rule.
Example
Scenario: Finding either the number of weeks between two dates, or the ISO week number of a specific date.
Result: A decimal week count for the 'between' mode, or a whole week number (1–53) for the 'week number' mode.
Assumptions & Limitations
- ISO week numbering starts weeks on Monday and assigns week 1 to the week containing the first Thursday of the year, which occasionally places the final days of December into week 1 of the following year, or vice versa.
- Weeks between two dates is reported as a decimal, not rounded to a whole number.
- Some countries and industries use non-ISO week numbering conventions that start on Sunday or use a different week 1 definition — this calculator follows the ISO 8601 standard specifically.