Ovulation Calculator
Estimate your fertile window and ovulation day from the first day of your last period and your cycle length.
About this calculator
This free ovulation calculator estimates the days you are most likely to conceive. Enter the first day of your last period and your average cycle length to see your fertile window, your most fertile day, and when your next period is expected.
How it works
In a typical cycle, ovulation occurs about 14 days before the next period starts. Because sperm can live for up to five days, the fertile window spans the five days before ovulation plus ovulation day itself.
Tips for accuracy
- Track a few cycles to find your true average length
- Combine with ovulation test kits for a tighter estimate
- Note that stress and illness can shift ovulation
Already expecting? Switch to the due date calculator to find out when your baby is likely to arrive.
Frequently asked questions
Ovulation usually happens about 14 days before your next period. The calculator finds your next period from your cycle length, then counts back 14 days to estimate ovulation.
It is the roughly six-day span ending on ovulation day — the five days before plus ovulation day itself — when intercourse is most likely to lead to pregnancy, because sperm can survive several days.
It is most reliable for regular cycles. If your cycle length varies a lot, the fertile window is only a rough guide; ovulation predictor kits or tracking basal body temperature give better accuracy.
It should not be relied on for contraception. Calendar estimates are not precise enough to prevent pregnancy on their own — use a proven contraceptive method instead.
Yes. Everything is calculated in your browser and nothing you enter is uploaded or saved.
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