Ovulation & Fertile Window Calculator
Ovulation usually happens about 14 days before your next period starts. In a 28-day cycle that is cycle day 14, and your most fertile days are the five days before it plus ovulation day itself.
Cycle length is from the first day of one period to the day before the next. If unsure, leave the luteal phase at 14.
Pick the first day of your last period to see your fertile window.
Predictions only, based on a regular cycle. This is not a method of birth control, and pregnancy can happen on any day of the cycle. Ovulation tests or a doctor can give a more reliable answer.
How the ovulation date is predicted
The calculator counts back from your next period. The stretch between ovulation and the start of the next period, called the luteal phase, is fairly steady at about 14 days for most people, while the first half of the cycle is what varies. So ovulation day = cycle length − luteal phase. In a 28-day cycle that is day 14, in a 32-day cycle it is day 18.
Cycle day 1 is the first day of your period. The calculator converts the ovulation day into a calendar date, and continues into your next three cycles so you can plan ahead. If your last period was a while ago, it projects forward to the cycle you are in now.
Your fertile window
You can become pregnant only during a short window each cycle. Sperm can survive for up to about five days in the reproductive tract, while an egg survives for around 12 to 24 hours after it is released. That makes the five days before ovulation and ovulation day the fertile window, and the two or three days just before ovulation the most fertile.
If you are trying to conceive, having sex every day or two during this window gives the best chance. A pregnancy test is most reliable from the day your period is due.
Limits of a calendar prediction
This is an estimate based on a regular cycle. Stress, illness, travel, breastfeeding, coming off hormonal contraception and conditions such as PCOS can all move ovulation. If your cycles vary by more than a few days, the prediction becomes much less reliable.
For more certainty, combine it with ovulation predictor kits, which detect the hormone surge before ovulation, and with tracking basal body temperature or cervical mucus. This tool must not be used as birth control, since pregnancy can happen outside the predicted window.
Frequently asked questions
- When do you ovulate in a 28-day cycle?
- Usually around day 14, counting the first day of your period as day 1. The luteal phase of about 14 days before your next period is what sets the timing.
- How long is the fertile window?
- About six days: the five days before ovulation and the day of ovulation itself. The egg lives only 12 to 24 hours, but sperm can survive several days.
- What if my cycle is irregular?
- The prediction is less reliable because ovulation moves from month to month. Ovulation predictor kits or advice from a doctor will help more than a calendar.
- Can I use this to avoid pregnancy?
- No. Calendar predictions are not a reliable method of contraception. Speak to a doctor or nurse about birth control options.
- When should I take a pregnancy test?
- Most tests are reliable from the first day of a missed period. Testing earlier can give a false negative because hormone levels are still low.