Ovulation Calculator
Estimate your ovulation day and most fertile window from your last period and cycle length, plus when your next period is due.
Enter the first day of your last period to estimate your fertile window.
How to use this calculator
Enter the first day of your last period and your average cycle length. The calculator estimates your ovulation date, your fertile window, and your next period.
How ovulation timing works
After ovulation, the luteal phase is fairly consistent at about 14 days, so ovulation is estimated by counting back 14 days from your next expected period. Because sperm can live up to five days, the days leading up to ovulation are also fertile.
Improving accuracy
Track several cycles to find your average length, and combine this estimate with ovulation predictor kits, basal body temperature, or cervical mucus changes for a clearer picture.
Estimates only, not medical advice or a contraception method. Consult a healthcare provider for personalized guidance.
Frequently asked questions
When do I ovulate?
Ovulation typically happens about 14 days before your next period starts. For a 28-day cycle that's around day 14, but it shifts earlier or later with shorter or longer cycles.
What is the fertile window?
The fertile window is roughly the five days before ovulation plus the day of ovulation, because sperm can survive several days. Having intercourse during this window gives the best chance of conception.
How accurate is this calculator?
It's an estimate based on average cycle timing. Actual ovulation varies between cycles and people. Ovulation predictor kits, basal body temperature, and cervical mucus tracking give more personalized signals.
Can I use this to avoid pregnancy?
No. Calendar estimates are not a reliable form of contraception because ovulation timing varies. Talk to a healthcare provider about effective birth control options.