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.
Worked example
Scenario: Finding the ISO week number of Friday, January 1, 2027.
- ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday.
- The Thursday of the week containing January 1, 2027 is December 31, 2026.
- So January 1, 2027 belongs to the last week of 2026: 2026-W53.
Result: ISO week 2026-W53.
Tips and common mistakes
- Near New Year the ISO week-year can differ from the calendar year — always record both (e.g. 2026-W53).
- Years have 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year.
- In 'Weeks between two dates' mode, the result is decimal: 10 days is 1.43 weeks.
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.
Understanding week calculations
A week is a fixed seven-day duration, but week numbers and the first day of a week can vary by convention. ISO week dates use Monday as the first day and have specific rules for week 1 of a year.
Practical example
Seven days after a Monday is the following Monday. Converting a date to a numbered week may differ between systems, so use the calculator result according to the convention shown.