Weeks Calculator — Calculate Weeks Between Dates
Calculate the number of weeks between two dates or add/subtract weeks from a date. See also Months Calculator and Date Calculator.
How the Weeks Calculator Works
The Weeks Calculator determines the number of complete weeks (and remaining days) between any two dates, or calculates a future/past date by adding or subtracting a specified number of weeks. It uses calendar-accurate arithmetic, accounting for varying month lengths and leap years. One week always equals exactly 7 days.
Week Calculation Formula
Total Days = End Date - Start Date (in days)
Complete Weeks = floor(Total Days / 7)
Remaining Days = Total Days mod 7
Add Weeks: Result Date = Start Date + (Weeks x 7) days
Example
Start: January 1, 2025 | End: March 26, 2025
Total Days = 84
Complete Weeks = 84 / 7 = 12 weeks, 0 remaining days
ISO Week Numbering
The ISO 8601 standard defines week numbering where Week 1 is the week containing the first Thursday of the year (or equivalently, the week containing January 4). Weeks start on Monday. A year has either 52 or 53 ISO weeks. This system is widely used in business, project management, and international commerce for consistent week references across different calendar systems.
Weeks Reference Table
| Weeks | Days | Months (approx) | Common Use |
|---|---|---|---|
| 1 | 7 | ~0.23 | One week |
| 2 | 14 | ~0.46 | Two-week sprint |
| 4 | 28 | ~0.92 | Lunar month |
| 6 | 42 | ~1.38 | Six-week cycle |
| 12 | 84 | ~2.76 | Quarter (approx) |
| 13 | 91 | ~3.0 | Fiscal quarter |
| 26 | 182 | ~6.0 | Half year |
| 40 | 280 | ~9.2 | Pregnancy term |
| 52 | 364 | ~12.0 | One year (approx) |
Frequently Asked Questions
How many weeks are in a year?
A regular year has 52 weeks and 1 day (365 / 7 = 52.14). A leap year has 52 weeks and 2 days. In ISO week numbering, some years have 53 weeks (called "long years"), which occurs roughly every 5-6 years.
What is the difference between calendar weeks and ISO weeks?
Calendar weeks can start on Sunday (US convention) or Monday (ISO/European convention). ISO weeks are strictly defined: they start on Monday, and Week 1 contains the year's first Thursday. This can mean the first few days of January belong to the previous year's last week.
How do I calculate weeks for project planning?
Enter your project start date and deadline to find the total weeks available. For sprint planning, divide by your sprint length (typically 1-4 weeks). Remember to account for holidays and non-working days in your actual schedule.
How many weeks is a pregnancy?
A full-term pregnancy is approximately 40 weeks (280 days) from the first day of the last menstrual period. This is divided into three trimesters: weeks 1-12, weeks 13-26, and weeks 27-40.
Does the calculator include the start and end dates?
The calculator counts the days between the start and end dates (exclusive of the start date, inclusive of the end date). If you need to include both dates, add 1 day to the result.
How do I convert weeks to months?
Since months vary in length (28-31 days), there is no exact conversion. On average, 1 month equals approximately 4.345 weeks. For precise results, use specific dates rather than approximate conversions.