Ovulation Calculator
Estimate your ovulation day and fertile window.
Average 28 days
Results update as you type.
This calculator provides estimates based on population research. Results are not medical advice. Consult a qualified healthcare provider before making health or nutrition decisions.
How it works
This ovulation calculator estimates the day you are most likely to ovulate and the fertile window around it, using the first day of your last period and your average cycle length. Ovulation, when an ovary releases an egg, tends to happen about 14 days before the start of the next period, so the calculator counts forward by your cycle length minus 14 days. It then marks the fertile window as the five days leading up to ovulation plus the ovulation day itself, the span during which conception is most likely. This works because sperm can survive for up to about five days inside the reproductive tract, so intercourse in the days before ovulation can still result in fertilisation when the egg is released, which remains viable for roughly 24 hours. People use this estimate both to increase their chances of conceiving by timing intercourse and, more cautiously, to understand their cycle, though it is not a reliable method of contraception. The accuracy depends heavily on having regular, predictable cycles; irregular cycles make the estimate less certain. Tracking basal body temperature or using ovulation predictor kits gives more precise timing. This tool is for general information only. Enter your last period date and cycle length to see your estimated fertile days.
ovulation ≈ LMP + (cycle length − 14); fertile window ≈ ovulation − 5 to +1 days
Ovulation typically occurs about 14 days before the next period, so it is estimated as the first day of the last period plus the cycle length minus 14. The fertile window spans roughly the five days before ovulation plus the day of ovulation itself, because sperm can survive up to five days in the reproductive tract while the released egg remains viable for about a day, making intercourse in that window most likely to result in conception.
Source:Standard cycle-based ovulation estimate (ACOG)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter first day of last period.
- 2
Adjust the options
Adjust cycle length if they apply — sensible defaults are already set.
- 3
Read your result
Estimated ovulation day updates instantly as you type — no button to press.
Frequently asked questions
When does ovulation happen?
Ovulation usually occurs about 14 days before the start of your next period, regardless of how long your cycle is. For a typical 28-day cycle that places it around day 14, but for a 32-day cycle it would fall closer to day 18. The calculator estimates your ovulation day by subtracting 14 from your cycle length and counting forward from your last period.
What is the fertile window?
The fertile window is the range of days during a cycle when intercourse is most likely to lead to pregnancy. It spans roughly the five days before ovulation plus the day of ovulation itself. This window exists because sperm can survive several days in the reproductive tract, so they may still fertilise the egg once it is released, when it stays viable for about a day.
How accurate is the ovulation estimate?
The estimate is most reliable for people with regular, predictable cycles. Because it assumes ovulation occurs 14 days before the next period, irregular cycles, stress, illness, and hormonal changes can shift the actual day. For greater accuracy, many people track basal body temperature, monitor cervical mucus, or use ovulation predictor kits that detect the hormone surge preceding ovulation.
Can I use this for birth control?
No. Calendar-based ovulation estimates are not a reliable method of contraception because ovulation timing can vary from cycle to cycle, and sperm can survive for days. Relying on predicted fertile days to avoid pregnancy carries a high failure rate. If you want to prevent pregnancy, use a proven contraceptive method and consult a healthcare professional about the best option.
What cycle length should I enter?
Enter the average number of days from the first day of one period to the first day of the next, which for most people is between 21 and 35 days, with 28 being typical. If your cycle length varies, use an average of several recent cycles. The more accurately your entered cycle length reflects your real pattern, the better the ovulation estimate will be.