Ovulation & Fertility Calculator
Find your most fertile days and next period from your last cycle and length.
This ovulation calculator estimates the days you are most likely to be fertile based on the first day of your last period and the length of your cycle. It works out when ovulation is likely to occur, marks the fertile window around it, and shows when your next period is due, so you can plan around the days that matter most.
The estimate uses the length of your luteal phase — the time between ovulation and your next period, which is typically around 14 days and fairly stable — to count backwards from your expected period. Because cycles vary from month to month and from person to person, treat the fertile window as a guide rather than a guarantee, and combine it with other signs such as basal body temperature or ovulation tests for a clearer picture.
How to use it
- 1Pick the first day of your last period, or tap Today to use the current date.
- 2Enter your average cycle length in days (28 is typical).
- 3Adjust the luteal phase if you know it; otherwise leave it at 14 days.
- 4Read your estimated ovulation day, fertile window and next period date.
Frequently asked questions
How is the ovulation date calculated?+
The calculator subtracts your luteal phase length from your cycle length and counts that many days from the first day of your last period. With a 28-day cycle and a 14-day luteal phase, ovulation is estimated around day 14 of your cycle.
What is the fertile window?+
The fertile window spans the days when conception is most likely — roughly the five days before ovulation plus the day after. Sperm can survive several days in the reproductive tract, so intercourse in the days leading up to ovulation can still result in pregnancy.
How accurate is an ovulation estimate?+
It is an estimate based on averages, not a precise prediction. Cycle length and ovulation timing can shift from month to month due to stress, illness and other factors. For a more reliable read, track basal body temperature, cervical mucus or use ovulation predictor kits.
Related calculators
Calculate your Body Mass Index in metric or imperial units and see your weight category.
Estimate your daily calorie needs (BMR and TDEE) for maintaining, losing or gaining weight.
Estimate your body fat percentage using the US Navy tape-measurement method.
Calculate the calories your body burns at rest using the Mifflin-St Jeor equation.