Ovulation calculator
Ovulation usually happens about 14 days before your next period. Enter the first day of your last period and your average cycle length to estimate your ovulation day and 6-day fertile window.
This calculator is an estimate based on averages. It is not reliable as contraception and cannot confirm ovulation.
🔒 Calculated locally — your dates never leave your device
The calendar marks your estimated fertile window (the 5 days before ovulation plus ovulation day and the day after). Sperm can survive up to about 5 days, while an egg lives roughly 12–24 hours — that is why the window opens before ovulation.
Calendar estimates work best with regular cycles. Stress, illness, travel and weight changes can shift ovulation, so treat the result as a guide, not a guarantee. For higher precision, combine it with ovulation tests (LH strips) or basal body temperature.
Frequently asked questions
How accurate is an ovulation calculator?
It gives a statistical estimate. Real ovulation can shift by several days even in regular cycles, so use it as a starting point and confirm with LH tests or body signs (cervical mucus, basal temperature) if timing matters.
When am I most fertile?
The two days before ovulation and ovulation day itself carry the highest chance of conception. The full fertile window spans about 6 days.
Can I use this if my cycles are irregular?
Calendar methods become unreliable with irregular cycles. If your cycle varies by more than a few days, ovulation tests or a consultation with a doctor will serve you better.
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Related reading
References
- ACOG – Fertility Awareness-Based Methods of Family Planning (acog.org)
- NHS – How can I tell when I am ovulating? (nhs.uk)
- Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995.
This page will be reviewed by a named OB-GYN before launch. Educational content — not medical advice.
Medical disclaimer: This tool and content are for educational purposes only and are not a substitute for professional medical advice. If something feels wrong, see a doctor.