Projects, pregnancies, courses, pay cycles and fitness plans are all counted in weeks. This weeks calculator tells you how many weeks and days lie between two dates, gives the total as a decimal, counts the working weekdays and shows the ISO week number of each date, so you can match the answer to a schedule that uses numbered weeks.
How to use the weeks calculator
- Enter the start date and the end date. The order does not matter; the calculator always measures the gap.
- Read the weeks and days (for example “14 weeks 2 days”), the decimal weeks, the total days and the weekday count.
- Check the ISO week numbers if your plan refers to “week 12” or similar.
Weeks calculator formula
Weeks = ⌊ days ÷ 7 ⌋ · extra days = days mod 7 · decimal weeks = days ÷ 7
Dates are converted to day numbers so months and leap years need no special handling. The ISO week of a date is found by moving to the Thursday of its Monday-based week and counting how many weeks into that Thursday’s year it falls.
Worked example: October 15, 2026 to February 14, 2027
- Days left in October: 16; November: 30; December: 31; January: 31; February up to the 14th: 14. Total 122 days.
- 122 ÷ 7 = 17 remainder 3, so 17 weeks 3 days (17.43 decimal weeks).
- Weekdays in that period: 87.
Weeks in each year
A year is never an exact number of weeks. ISO years have either 52 or 53 weeks; the table shows the last ISO week number of each year (December 28 always falls in a year’s final ISO week).
| Year | Days | Weeks and days | ISO weeks in the year |
|---|---|---|---|
| 2024 | 366 | 52 weeks 2 days | 52 |
| 2025 | 365 | 52 weeks 1 day | 52 |
| 2026 | 365 | 52 weeks 1 day | 53 |
| 2027 | 365 | 52 weeks 1 day | 52 |
| 2028 | 366 | 52 weeks 2 days | 52 |
| 2029 | 365 | 52 weeks 1 day | 52 |
| 2030 | 365 | 52 weeks 1 day | 52 |
| 2031 | 365 | 52 weeks 1 day | 52 |
| 2032 | 366 | 52 weeks 2 days | 53 |
| 2033 | 365 | 52 weeks 1 day | 52 |
Week numbering systems
- ISO 8601 — weeks start Monday, week 1 contains the first Thursday. Used across Europe and in business reporting.
- US convention — weeks often start Sunday and week 1 is the one containing January 1, so numbers can differ by one from the ISO value.
- Pregnancy weeks — counted from the first day of the last menstrual period, so a 40-week pregnancy is 280 days.
To find the date a number of weeks away, use Weeks From Date Calculator. To count days only, try the Date Calculator or Days Until Calculator, and for a visual month view see the Calendar.
Frequently asked questions
How many weeks are there between two dates?
Count the days between the dates and divide by 7. The whole-number part is the weeks, and the remainder is the extra days: 100 days is 14 weeks and 2 days.
How many weeks are in a year?
A common year of 365 days is 52 weeks and 1 day; a leap year of 366 days is 52 weeks and 2 days. Some years have 53 ISO weeks, such as 2026.
What is an ISO week number?
ISO 8601 numbers weeks from Monday, and week 1 is the week that contains the first Thursday of the year. This means that early-January dates can belong to week 52 or 53 of the previous year, and late-December dates to week 1 of the next.
Which weeks does the weekday count include?
Monday-to-Friday days from the start date up to, but not including, the end date. Public holidays are not subtracted.
Do the start and end dates count?
The start date is counted and the end date is not, which matches the usual “days between” convention. Add 1 day if you need both ends included.