Ovulation Calculator

Your likely fertile window and ovulation day, from your last period.

Most likely ovulation day
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Fertile window
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Best two days to try
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Next period due
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Earliest useful test
Next three cycles
CyclePeriod startsFertile windowOvulation
An estimate based on an average cycle, not a medical result. Ovulation shifts with stress, illness and hormonal changes, and cycles vary from month to month. Do not rely on this to avoid pregnancy — talk to a doctor about contraception, or about difficulty conceiving.
How this works
Ovulation = period start + cycle length − luteal phase Fertile window = 5 days before ovulation, through the day after Next period = period start + cycle length
FactDetail
Typical cycle21 to 35 days; 28 is only the average
Luteal phase12 to 16 days, and fairly fixed for each person
Egg survives12 to 24 hours after release
Sperm surviveup to 5 days in fertile cervical mucus
Chance per cycleroughly 25% at peak fertility, for a healthy couple
  • The fertile window is wide because sperm outlive the egg — the days before ovulation matter most.
  • Day one of a cycle is the first day of full flow, not spotting.
  • The luteal phase is the stable part of a cycle. If your cycle is long, ovulation is late — the luteal phase rarely stretches.
  • Ovulation predictor kits, basal temperature and cervical mucus give a far better signal than a calendar alone.
When am I most fertile?

The two days before ovulation and the day itself. Sperm survive up to five days, so intercourse in the days leading up to ovulation is more likely to result in conception than the day after.

What if my cycles are irregular?

A calendar estimate is unreliable with irregular cycles. Ovulation predictor kits, basal body temperature tracking or cervical mucus changes are far more accurate — and a doctor can help if cycles vary widely.

Can I use this as contraception?

No. Calendar methods fail often because ovulation moves with stress, illness and travel. Use a proper contraceptive method and speak to a doctor.

Keep going
Pregnancy Due Date Calculator Due date and how far along you are, from your last period. Date Difference Calculator Days between two dates, with or without weekends — plus add or subtract days. Age Calculator Your exact age in years, months, days — plus your next birthday countdown.