Ovulation Calculator — Find Your Fertile Window Free

Understanding your cycle is the first step in family planning, and ovulation is its key event. This ovulation calculator estimates when you're likely to ovulate and marks your fertile window — the days when conception is most likely. It works from three simple inputs: the first day of your last period, your average cycle length, and how many days your period lasts.

Ovulation usually occurs about 14 days before your next period begins, so the calculator projects your next period from your cycle length and counts back to find your estimated ovulation date. Around it, it highlights the fertile window — roughly the five days before ovulation plus ovulation day — which reflects the fact that sperm can survive for several days while the egg is viable for about a day. A three-month calendar shows it all visually, colour-coding your period days, fertile days and ovulation day so you can plan ahead at a glance.

The estimate is most reliable when your cycles are fairly regular. In reality, ovulation can shift from month to month due to stress, illness, travel, sleep and other factors, so think of the dates as a helpful guide rather than a guarantee. To pinpoint ovulation more precisely, many people combine the calculator with body signals — a rise in basal body temperature, changes in cervical mucus — or with ovulation predictor kits.

One important caution: this tool is for awareness and planning only and must never be used as a method of contraception, because the fertile window can move unpredictably. For birth control, rely on a proven method and your doctor's advice. Everything is calculated privately in your browser, so your cycle data never leaves your device. Enter your details above, and use the pregnancy calculator once you have a positive test.