The method behind the estimate
A menstrual cycle has two phases. The follicular phase runs from the first day of bleeding until ovulation and can vary in length from person to person and month to month. The luteal phase runs from ovulation to the next period and is far more stable, lasting about 14 days in most people.
Because the luteal phase is the steady part, the most reliable way to pinpoint ovulation is to count backwards: take the start of your next expected period (last period plus cycle length) and subtract 14 days. The fertile window is then built around that day.
How to use the fertile window
The window shown spans from roughly five days before ovulation to one day after. Sperm can survive for several days while they wait for the egg, so intercourse in the days leading up to ovulation is just as likely to result in conception as on the day itself.
After ovulation the egg survives only about a day, so the chance of conceiving drops sharply once the window closes. If you are timing for or against pregnancy, plan around the whole window rather than a single day.
Improving accuracy and common mistakes
The biggest source of error is assuming a textbook 28-day cycle when your own is different. A few practical checks help:
- Enter your real average cycle length, measured over several months.
- Remember that cycle length is measured from the first day of one period to the first day of the next, not from when bleeding stops.
- Confirm the prediction with ovulation kits, cervical mucus changes, or basal body temperature.
- Treat predictions for irregular cycles as rough — the date can shift by several days.
Medical disclaimer
This tool offers general estimates for educational purposes and is not medical advice. It should not be used as a reliable form of contraception or to diagnose fertility problems. Consult a doctor or qualified healthcare provider for personalised advice about ovulation, fertility, or family planning.
Formula
nextPeriod = lastPeriod + cycleLength; ovulation = nextPeriod − 14Frequently asked questions
- How is ovulation estimated?
- By subtracting 14 days from the start of your next expected period, since the luteal phase is roughly constant.