Ovulation & Fertility Calculator
Estimate your ovulation day and fertile window from your last period.
Results update as you type.
About this calculator
An ovulation calculator estimates your most fertile days from the first day of your last period and your average cycle length. Ovulation typically occurs about 14 days before your next period, and the fertile window spans the five days before ovulation plus the day itself, because sperm can survive several days. Cycle-based estimates are a guide, not a guarantee.
Frequently asked questions
When do I ovulate?
Ovulation usually happens about 14 days before your next period starts, so on a 28-day cycle that is roughly day 14. This tool counts forward from your last period using your cycle length.
What is the fertile window?
It is the six-day span ending on ovulation day — the five days before plus the day itself — because sperm can survive up to five days and the egg lasts about a day. Intercourse in this window gives the best chance of conception.
How reliable is a cycle-based estimate?
It is only an estimate and assumes regular cycles. Ovulation predictor kits or basal body temperature tracking pinpoint the day more precisely, especially for irregular cycles.
Results are estimates for general guidance only, not financial, medical or tax advice.