Ovulation Calculator
Track your next period, ovulation, and fertile window
Assumes a 14-day luteal phase (next period - 14 days). Fertile window spans ovulation -5 to +1 days. Learn more
For reference only. Don't use this as the sole basis for contraception or trying to conceive.
How is it calculated?
The ovulation date is estimated from the observation that the luteal phase averages 14 days. Subtracting 14 days from the expected start of the next period gives ovulation day for that cycle.
Formula: ovulation = expected next period − 14 days. Fertile window = ovulation −5 to +1 days (7 days total).
The window reflects sperm survival in cervical mucus (up to 5 days) and egg viability (about 12-24 hours) — the practical conception window (Wilcox et al., NEJM 1995).
Limitations of this estimate
This is a reference estimate, not a clinical measurement:
- The luteal phase averages 14 days but ranges 10-16 days between individuals.
- If your cycles are irregular, the predicted next period — and thus ovulation — is itself uncertain.
- Stress, weight changes, hormonal medications, and conditions like PCOS shift the ovulation timing.
- Reliability is low while breastfeeding and before the first postpartum period.
The most accurate methods are LH urine tests (ovulation predictor kits) or basal body temperature (BBT) tracking. For active conception attempts, an OB-GYN can run hormone panels for a more precise picture.
Sources
Frequently asked
What if my cycle isn't 28 days?
The 14-day luteal phase assumption holds fairly well regardless of cycle length (with 10-16 days of individual variation). So a 30-day cycle puts ovulation around day 16, a 35-day cycle around day 21.
If your luteal phase is shorter than 14 days the 'next period − 14' estimate drifts — confirm with LH test strips or BBT tracking.
Do we need intercourse every day during the fertile window?
No — every 1-2 days during the 6-day fertile window is enough. Sperm can survive up to 5 days in cervical mucus, and the 2-3 days leading up to ovulation give the highest conception probability.
Daily intercourse can temporarily lower sperm count, so ASRM (American Society for Reproductive Medicine) does not recommend it.