Ovulation Calculator & Fertile Window
Estimate when you are most likely to ovulate. Enter the first day of your last period and your usual cycle length to see your fertile window and next periods for the coming three months.
Next fertile window
| Cycle | Fertile window | Ovulation | Next period |
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Estimates only, based on average cycles. Not a method of contraception and not medical advice. Cycles vary; ovulation tests or tracking basal body temperature give more personal information.
How to use the ovulation calculator
- Enter the first day of your most recent period.
- Enter your average cycle length: the number of days from the first day of one period to the first day of the next. 28 is average, but 21–35 is normal.
- If you know your luteal phase length (from tracking), enter it; otherwise leave it at 14.
How the fertile window is estimated
Ovulation usually happens about 14 days before the next period starts, a span called the luteal phase. So with a 28-day cycle ovulation is around day 14, and with a 32-day cycle around day 18. The calculator subtracts your luteal phase length from your cycle length to estimate the ovulation day.
The fertile window is the six days ending on ovulation day: the five days before (because sperm can survive up to about five days) plus the day of ovulation. The day after is shown as possible because the egg lives about 12–24 hours. Chances of conception are highest in the two or three days before ovulation.
Calendar estimates work best for regular cycles. Stress, illness, travel and breastfeeding can shift ovulation. Ovulation predictor kits (which detect the LH surge), cervical mucus changes and basal body temperature charts give more personal confirmation. If you have been trying to conceive for a year (six months if over 35), talk to a healthcare provider.
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Frequently asked questions
When am I most fertile?
In the two or three days before ovulation and on ovulation day itself.
Can I use this to avoid pregnancy?
No. Calendar estimates are not reliable enough to prevent pregnancy. Talk to a healthcare provider about contraception.
What if my cycles are irregular?
Estimates will be less accurate. Ovulation tests or tracking temperature and cervical mucus can pinpoint ovulation better.
Is my data saved?
No. Everything is calculated in your browser and nothing is stored or sent anywhere.