Weeks Calculator – How Many Weeks Between Dates?

A standard year has 52 weeks and 1 day (365 days ÷ 7 = 52.143 weeks). A leap year has 52 weeks and 2 days. For planning, most calendars treat a year as 52 working weeks.

90 days ÷ 7 = 12 weeks and 6 days (exactly 12.857 weeks). This is close to 3 calendar months but not exactly, since calendar months have 28–31 days. Legal and commercial contracts often specify 90-day periods (a quarter year) rather than months to avoid ambiguity — '90 days' is unambiguous while 'three months' is not when starting on the 31st.

Count the days from the first day of your last menstrual period (LMP) to today, then divide by 7. For example, if your LMP was 70 days ago, you are 10 weeks pregnant.

Six months equals approximately 26 weeks, but the exact count depends on which 6 months. January–June has 181 days = 25 weeks 6 days. June–December has 184 days = 26 weeks 2 days. Specific week counts matter in insurance policies, probationary employment periods, and clinical trial timelines where the exact number of weeks must be calculated from start to end dates.

Beginner marathon plans typically run 16–20 weeks. Intermediate plans are 12–16 weeks. The most popular plans (Hal Higdon, Jack Daniels) are 18 weeks. The first week should start when you can comfortably run 8–10 miles (13–16 km) per week.