Ovulation is when an ovary releases an egg. It’s the one point in the cycle when pregnancy is possible without a gap, which is why people trying to conceive, and people trying to avoid it, want to know when it falls.
It works from your last period and your usual cycle length. It’s an estimate and not a medical test.
How to use the Ovulation Calculator#
- Enter the first day of your last period. Use the day bleeding began, not the day it ended.
- Enter your cycle length. Count from the first day of one period to the first day of the next. Use an average if it varies.
- Read your dates. The result shows your estimated ovulation day, your fertile window and your next period.
How the date is worked out#
Take a first day of 3 March and a 28-day cycle. The luteal phase is about 14 days, so ovulation falls on day 14, which is 17 March. The fertile window runs from five days before ovulation to one day after, so 12 to 18 March. The next period would start on 31 March.
The fertile window#
Sperm can survive for up to five days in the reproductive tract, and the egg lives for about a day after it’s released. That makes the fertile window around six days long, and the days just before ovulation are the most fertile. A longer or shorter cycle moves the whole window, which is why cycle length matters.
Why it’s only an estimate#
Stress, illness, travel, breastfeeding, and conditions such as PCOS can all shift ovulation, and cycles that vary a lot make predictions unreliable. Ovulation predictor kits, tracking basal body temperature and watching cervical mucus can give a better read. The result here shouldn’t be used as contraception. If you have been trying to conceive for a year, or six months if you’re over 35, talk to a doctor. See also the conception calculator.
Cycle length changes the date#
The textbook ovulation day of 14 holds only for a 28-day cycle. Ovulation actually happens about 14 days before the next period, so what varies is the first half of the cycle. With a 35-day cycle, ovulation is around day 21. With a 24-day cycle, around day 10. The calculator subtracts the luteal phase, which you can change if you know yours, from the cycle length.
| Cycle length | Estimated ovulation (day) | Fertile window (days) |
|---|---|---|
| 24 | 10 | 5 to 11 |
| 26 | 12 | 7 to 13 |
| 28 | 14 | 9 to 15 |
| 30 | 16 | 11 to 17 |
| 35 | 21 | 16 to 22 |
Calendar estimates and other signs#
A calendar method assumes your cycles are regular, and even healthy cycles vary by a few days from month to month. Other signs add information. Cervical mucus becomes clear and stretchy in the days before ovulation. Basal body temperature rises slightly, about 0.2 to 0.5 °C, after ovulation, which confirms it afterwards but does not predict it. Urine tests detect the luteinising hormone surge that comes 24 to 36 hours before the egg is released. Combining the calculator with one of these makes the estimate sharper. Stress, illness, travel and breastfeeding can all shift ovulation.
When to speak to a doctor#
If your cycles are shorter than 21 days or longer than 35, or they vary by more than a week from month to month, a calendar estimate will not be reliable, and it is worth asking a doctor, as irregular cycles can have a treatable cause. Couples who have not conceived after a year of regular unprotected sex, or six months if the woman is over 35, should see a doctor. Do not rely on this calculator as contraception. For dates after conception, the conception calculator works out the due date. This page is information only and does not replace medical advice.
Frequently asked questions#
Usually around day 14, counting the first day of your period as day 1.
About six days: the five days before ovulation and the day of ovulation or just after.
No. It’s an estimate, and cycles vary. It isn’t a method of contraception.
The estimate will be less reliable. Consider tracking temperature or using ovulation tests, and speak to a doctor if you’re worried.
It is unlikely but possible, because cycles vary and sperm survive for up to five days. The days just before ovulation carry the highest chance. Estimates from a calendar are not exact, so it is not a reliable way to prevent pregnancy.