Ovulation Calculator

Count backwards, not forwards

The common rule "ovulation is day 14" only holds for a textbook 28-day cycle. What is actually stable is the luteal phase — the stretch from ovulation to the next period — which runs 12 to 16 days in most people and varies little from cycle to cycle. The follicular phase before ovulation is what stretches or shortens. So ovulation is estimated as next period minus luteal length: on a 35-day cycle with a 14-day luteal phase, that is day 21, not day 14.

The fertile window is six days, and it ends at ovulation

Sperm survive up to five days in fertile cervical mucus; an egg survives 12 to 24 hours. That makes the fertile window the five days before ovulation plus the day itself — and intercourse after ovulation has passed is very unlikely to result in conception. The two days immediately before ovulation carry the highest probability.

This is an estimate, not a test

Cycle-based prediction assumes your cycles are regular and your luteal phase is typical. Stress, illness, travel and many conditions shift ovulation, sometimes by a week. Ovulation predictor kits detect the LH surge 24–36 hours before release, and basal body temperature confirms ovulation after it happens. Use the calendar to know when to start testing, not as a substitute for testing.

Why this is unreliable as contraception

Calendar-based avoidance has a high real-world failure rate precisely because ovulation timing moves. An unusually short follicular phase can bring ovulation forward into days assumed safe, and sperm survival extends the risk window backwards. Formal fertility-awareness methods that track temperature and mucus daily perform far better than a calendar alone, and even they demand strict adherence. If preventing pregnancy is the goal, this page should not be the method.

Frequently Asked Questions