Ovulation Calculator
Estimate your most fertile days and likely ovulation date from the first day of your last period and your average cycle length. You'll also see your estimated next period. Updates instantly.
A general estimate for understanding your cycle — not medical advice and not a reliable form of contraception. For trying to conceive or avoid pregnancy, speak to a healthcare professional.
How it works
ovulation ≈ last period + (cycle length − 14 days)
- Ovulation is estimated about 14 days before the next period — i.e. cycle length minus 14, from the first day of your last period.
- Fertile window is the 5 days before ovulation plus ovulation day (sperm last ~5 days; the egg ~24 hours).
- Next period is estimated as last period + cycle length.
- It's a calendar estimate — most reliable with regular cycles.
FAQ
How is ovulation calculated?
Ovulation usually happens about 14 days before your next period starts — not 14 days after your last one. So for a 28-day cycle that's around day 14; for a 30-day cycle, around day 16. This calculator estimates ovulation as cycle length minus 14 days from the first day of your last period.
What is the fertile window?
The roughly six days when conception is possible: the five days before ovulation plus ovulation day. Sperm survive up to ~5 days and the egg ~24 hours, so the days leading up to ovulation are most fertile.
How accurate is a calendar ovulation calculator?
It's an estimate, most reliable with regular cycles. Ovulation timing shifts with stress, illness and natural variation, so treat it as a guide. Many people combine it with ovulation predictor kits or tracking physical signs.
Can I use this calculator to avoid pregnancy?
No. It's a general estimate, not reliable contraception — calendar methods have a meaningful failure rate. If you're avoiding pregnancy, use a proven method and speak to a healthcare professional.
What if my cycle is irregular?
A single estimate is less reliable if your cycle varies. Use your average length for a rough guide, treat the window as approximate, and consider predictor kits. Persistently irregular cycles are worth discussing with a doctor.
Trying to conceive? See our due date calculator for when a pregnancy would be due, or browse all free tools.