Ovulation Calculator
Estimated ovulation day and fertile window for your next three cycles, using ACOG's rules: ovulation about 14 days before the next period, fertile from 5 days before to 1 day after.
Next estimated ovulation
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| Cycle | Fertile window | Ovulation | Next period |
|---|
An estimate, not contraception. Cervical mucus and basal body temperature, or urine LH tests, tell you more about a given cycle.
Common questions
When do I ovulate?
ACOG: about 14 days before your next period starts — the time from ovulation to the next period is the most consistent part of the cycle. In a 28-day cycle that's around day 14; in a 32-day cycle, around day 18.
Which days can I get pregnant?
ACOG: from 5 days before ovulation until 1 day after. Sperm can survive about 3 (sometimes 5) days; an egg about 24 hours. In Wilcox 1995 (625 cycles), conception happened only from intercourse in the 6 days ending on ovulation day.
Can I use this as birth control?
Not on its own. Calendar estimates are the least precise fertility-awareness method; ACOG reports 12–24 pregnancies per 100 women in the first year of typical use of these methods, and advises learning them from a qualified teacher.
What is the Standard Days method?
For cycles between 26 and 32 days, ACOG says it treats days 8–19 as the fertile days. The calculator shows that window too when your cycle length fits.
How it's computed
Next period = last start + cycle length. Ovulation = next period − 14 days. Fertile window = ovulation − 5 days to ovulation + 1 day (ACOG). Standard Days window = cycle days 8–19, shown only for 26–32-day cycles. Cycle day 1 is the first day of bleeding. Shares its date engine and your on-device log with the period calculator and cycle tracker.
Sources
- ACOG. Fertility Awareness-Based Methods of Family Planning (FAQ024, reviewed Feb 2025). www.acog.org
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995;333(23):1517-1521. pubmed.ncbi.nlm.nih.gov