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) |
Real-World Applications
Agile Sprint Planning
Teams plan 1-4 week sprints. Calculate how many sprints fit between project start and deadline to allocate user stories.
Pregnancy Tracking
Pregnancy is measured in weeks (40 total). Calculate current gestational week from LMP date for milestone tracking.
Academic Terms
Semesters typically run 15-16 weeks. Calculate weeks remaining for assignment scheduling and exam prep.
Training Programs
Fitness programs (Couch to 5K, marathon training) are structured in weeks. Find your completion date from start.
Solved Examples
Example 1: A project runs from February 3 to June 30, 2025. How many complete weeks?
Total days: Feb(25) + Mar(31) + Apr(30) + May(31) + Jun(30) = 147 days
Weeks: 147 / 7 = 21 weeks and 0 days
Answer: Exactly 21 complete weeks.
Example 2: You need to deliver a project in 8 weeks starting from Monday, March 10, 2025. What is the delivery date?
8 weeks = 56 days
March 10 + 56 days = May 5, 2025
Answer: The delivery date is Monday, May 5, 2025.
Example 3: A 2-week sprint starts on ISO Week 14. What ISO week does it end?
Sprint duration: 2 weeks
End week: 14 + 2 = Week 16 (start of next sprint)
Answer: The sprint ends at the start of ISO Week 16. The last working day is in Week 15.
Practice Questions
1. A semester runs from September 2 to December 13. How many weeks?
Answer: 14 weeks and 4 days. Sep(28) + Oct(31) + Nov(30) + Dec(13) = 102 days; 102/7 = 14w 4d.
2. A 12-week fitness program starts January 6, 2025. When does it finish?
Answer: March 31, 2025. 12 weeks = 84 days from January 6.
3. How many weeks are between today and New Year 2026 (January 1, 2026)?
Answer: Use the calculator with today as start and Jan 1, 2026 as end. The result depends on the current date.
Key Takeaways
- One week is always exactly 7 days regardless of month or year.
- ISO weeks start on Monday; Week 1 contains the first Thursday of the year.
- A year has 52 complete weeks plus 1 day (or 2 days in a leap year).
- Adding weeks preserves the day of the week (Monday + 3 weeks = Monday).
- For approximate month conversions: 1 month is about 4.35 weeks.
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.