HEALTH · OVULATION
Ovulation Calculator
Estimate your ovulation date and fertile window from your last menstrual period and cycle length.
Estimated Ovulation
May 24, 2026
21 days ago this cycle
Fertile window startMay 19, 2026
Fertile window endMay 25, 2026
Not for contraception or fertility treatment — consult a healthcare provider.
About This Calculator
For a typical 28-day cycle, ovulation occurs around day 14. The fertile window spans five days before ovulation to one day after — when conception is most likely. This calculator estimates that window from your last period and your average cycle length.
How It Works
Enter the first day of your last menstrual period and your cycle length. The calculator estimates ovulation as cycle_length − 14 days after your LMP, then sets the fertile window as the 7-day span ending one day post-ovulation.
The Formula
Ovulation ≈ LMP + (cycle_length − 14)
- LMP
- first day of last menstrual period
- cycle_length
- typical cycle length in days
Frequently Asked Questions
- Can I use this to avoid pregnancy?
- No. Ovulation prediction based on cycle averages is not a reliable method of contraception. Actual ovulation varies cycle-to-cycle. Consult a healthcare provider for contraceptive guidance.
- Does this work for irregular cycles?
- Less reliably. The formula assumes ovulation occurs a fixed number of days before the next period. For cycles shorter than 21 or longer than 45 days, or conditions like PCOS or perimenopause, prediction accuracy is significantly reduced.
- What is the fertile window exactly?
- Sperm can survive in the reproductive tract for up to five days; an egg is viable for about 24 hours post-ovulation. The fertile window spans roughly five days before ovulation to one day after.