Weeks are the natural unit for pregnancies, projects, school terms and training plans. ISO week numbers (1–52 or 53) start on Monday, and week 1 is the week containing the year’s first Thursday – used widely in business and in Europe.
How to use the Week Calculator
- Start date
- End date
- Weeks to add to the start date
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter two dates, and optionally a number of weeks to add to the start date.
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| Start date | A date |
| End date | A date |
| Weeks to add to the start date | A number |
What each result means
- Days between = [End date] – [Start date]
- Full weeks = sign([Days between])×floor(abs([Days between])÷7)
- and days = sign([Days between])×mod(abs([Days between]), 7)
- Decimal weeks = [Days between]÷7
- ISO week of start date = (floor((dayofyear(([Start date]) – mod(weekday([Start date]) + 6, 7) + 3) – 1)÷7) + 1)
- ISO week of end date = (floor((dayofyear(([End date]) – mod(weekday([End date]) + 6, 7) + 3) – 1)÷7) + 1)
- Start date + weeks = [Start date] + 7×[Weeks to add to the start date]
Example values
How the main result changes when Weeks to add to the start date changes and everything else stays at the example values above:
| Weeks to add to the start date | Full weeks |
|---|---|
| 6 | 14 |
| 9 | 14 |
| 12 | 14 |
| 15 | 14 |
| 18 | 14 |
| 24 | 14 |
Formula
Weeks between = days ÷ 7 (full weeks + remaining days)ISO week = week (Monday–Sunday) containing the date's ThursdayDate + n weeks = date + 7n daysWorked example
Inputs — Start date: 2026-10-25; End date: 2027-02-02; Weeks to add to the start date: 12.
Results — Days between = 100; Full weeks = 14; and days = 2; Decimal weeks = 14.29; ISO week of start date = 43; ISO week of end date = 5; Start date + weeks = Sun, January 17, 2027.
Frequently asked questions
How many weeks between two dates?
Divide the days between them by 7.
How many weeks are in a year?
52 weeks and 1 day (2 in a leap year); ISO years have 52 or 53 weeks.
What week number is it?
See "ISO week of start date" (with today as the start date).
What formula does the Week Calculator use?
Weeks between = days ÷ 7 (full weeks + remaining days).