Weeks Calculator – How Many Weeks Between Dates?

How Weeks Between Dates Are Calculated

The number of weeks between two dates is found by dividing the total number of days by 7:

1. Calculate the total calendar days between start and end dates
2. Full weeks = floor(Total days ÷ 7)
3. Remaining days = Total days mod 7

Example: From January 1 to March 31, 2026 = 89 days = 89 ÷ 7 = 12 weeks and 5 days.

To find a future date from a number of weeks: Target date = Start date + (Weeks × 7) days. For example, 16 weeks from March 1 = March 1 + 112 days = June 21.

The Gregorian calendar has a predictable relationship with weeks: a standard year has 52 weeks and 1 day (365 = 52 × 7 + 1). A leap year has 52 weeks and 2 days (366 = 52 × 7 + 2). This is why your birthday shifts one day of the week each year (or two after a leap year) — it can never stay on the same day without a 5- or 6-year gap.

ISO 8601 defines a standardized week numbering system where Week 1 is the week containing the first Thursday of the year. This means January 1 can fall in the last week of the previous year (if Jan 1 is Friday, Saturday, or Sunday). Most European countries and business calendars use ISO week numbering.

Common Duration Reference Table (in Weeks)

Running training plans in weeks:

Common Use Cases

• Pregnancy tracking: Pregnancy is measured in weeks from the first day of the last menstrual period (LMP). Full term = 40 weeks. Preterm = before 37 weeks. Key milestones: first heartbeat (~6 weeks), gender scan (~20 weeks), viability (~24 weeks), full term (~39 weeks). This calculator helps count weeks from LMP to any date.
• Training plan scheduling: When registering for a race, calculate weeks between today and race day to choose the right training plan. A runner with 14 weeks until a half marathon should start a 12-week plan in 2 weeks (allowing for taper). Use the marathon pace calculator to set target race paces once you begin training.
• Project management: Break project timelines into weekly sprints. A 13-week quarter has 13 sprint cycles. Knowing exact weeks helps with resource allocation, deadline setting, and milestone tracking. Many Agile teams work in 2-week sprints, making a quarter exactly 6.5 sprints.
• Financial planning: Weekly budgeting is increasingly popular. Knowing that a year has 52.14 weeks helps calculate weekly budget from annual income: $60,000 annual = $1,150/week. Some people find weekly budgets more manageable than monthly because the periods are shorter and more tangible.
• Academic scheduling: College semesters are typically 15–16 weeks. Summer terms are 6–8 weeks. Knowing week numbers helps plan study schedules, assignment deadlines, and countdown to finals.

Step-by-Step Examples

Example 1: Marathon Training Countdown

Today is March 15, 2026. Your marathon is October 11, 2026. How many weeks of training do you have?

1. March 15 to October 11 = 210 days
2. 210 ÷ 7 = 30 weeks exactly
3. An 18-week training plan would start July 13 (12 weeks from now)
4. You have 12 weeks of base building before starting the formal plan — ideal for building mileage from 30 to 50+ km/week gradually
5. Use the pace calculator to set training paces once you do a baseline time trial

Example 2: Due Date Calculation

Last menstrual period (LMP) was January 5, 2026. When is the due date (40 weeks)?

1. 40 weeks = 280 days from January 5
2. January: 26 remaining days. February: 28. March: 31. April: 30. May: 31. June: 30. July: 31. August: 31. September: 30. October: 12.
3. 26 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 12 = 280
4. Due date: October 12, 2026
5. Today (March 15) is day 69 = 9 weeks and 6 days pregnant

Example 3: Fiscal Year in Weeks

A company's fiscal year runs October 1 to September 30. How many working weeks?

1. October 1 to September 30 = 365 days (or 366 in leap year)
2. 365 ÷ 7 = 52 weeks and 1 day
3. Working weeks (excluding holidays): 52 − ~2 (holidays) = ~50 working weeks
4. At 5 workdays/week: 250 working days (~260 weekdays minus ~10 holidays)
5. With 4 weeks PTO: 46 productive weeks = 230 working days

Tips and Common Mistakes

• Inclusive vs exclusive counting: "How many weeks between Jan 1 and Jan 8?" is either 1 week (exclusive — 7 days between them) or 8 days/1 week and 1 day (inclusive of both dates). Be clear about whether start and end dates are included. This calculator uses the most common approach: counting the gap between dates.
• Months are NOT 4 weeks: A common mistake is assuming 1 month = 4 weeks. A month averages 4.35 weeks (30.44 days). Over 9 months, this 0.35 weeks per month adds up to 3.15 extra weeks — significant for pregnancy tracking and project planning.
• Leap year awareness: February has 29 days in leap years (divisible by 4, except centuries not divisible by 400). 2024 and 2028 are leap years; 2100 is not. This affects any calculation spanning February and can shift your week count by one day.
• ISO week vs calendar week: In ISO 8601 (used in Europe and business), weeks start on Monday and Week 1 contains January 4. In the US, weeks commonly start on Sunday. This means the "week number" can differ by 1 depending on the system. Check which standard your tools use.
• Don't mix week-counting with month-counting: "12 weeks" is not "3 months." 12 weeks = 84 days. 3 months ≈ 91 days (7 days more). For precision, always convert to days first, then to weeks. This matters especially in medical contexts (pregnancy, recovery timelines) and legal deadlines.
• Time zones affect date boundaries: A calculation between "today" in different time zones may differ by a day. Use the time zone converter to clarify which "today" you mean when working across time zones.

Weeks Calculator vs Days Calculator: When to Use Each

Use weeks when the context naturally thinks in weeks (training plans, pregnancy, work sprints). Use days when exact day counts matter (legal deadlines, age calculations, countdowns). The date difference calculator provides both formats, while the days until calculator focuses on countdown-style calculations. For precise age calculations, use years, months, and days together.