Estimate the day you ovulate and the fertile window around it. Enter the first day of your last period and your usual cycle length to see your most fertile days and your next predicted period.
The number of days from the start of one period to the start of the next.
Enter your last period date to see the result.
This tool gives an estimate based on a regular cycle and is not medical advice. It is not a method of contraception. If you are trying to conceive or to avoid pregnancy, speak to a healthcare provider for guidance suited to you.
This is an estimate based on a regular cycle. It is a guide, not a substitute for medical advice or a method of contraception.
About this calculator
Ovulation is the moment an ovary releases an egg. It usually happens about 14 days before the next period starts, so for a regular cycle the timing can be estimated by counting back from the next expected period. This calculator does that for you. It takes the first day of your last period and your usual cycle length, places ovulation 14 days before the next period would be due, and marks the fertile window: the five days before ovulation plus ovulation day itself. Conception is most likely from intercourse during that window, because sperm can survive for several days while the egg stays viable for about a day.
The estimate is only as regular as the cycle behind it. Cycle length varies from person to person and from month to month, and stress, illness and other factors can shift ovulation. For a cycle longer than about 35 days, or one that changes a lot, the calculator flags the result as less reliable. It is a planning guide, not a precise forecast, and it should never be used as contraception. The American College of Obstetricians and Gynecologists, the Mayo Clinic and the NHS all describe how fertility awareness works and where its limits lie.
Frequently asked questions
When do you ovulate?
Ovulation usually happens about 14 days before the next period is due. For a regular 28-day cycle that places it around day 14, counting day one as the first day of the last period. For a longer or shorter cycle the day shifts, because the gap between ovulation and the next period stays close to 14 days while the first half of the cycle varies. This calculator uses that rule. Tracking signs such as basal body temperature or using ovulation tests can pinpoint the day more precisely.
What is the fertile window?
The fertile window is the stretch of days in a cycle when intercourse can lead to pregnancy. It runs from about five days before ovulation to the day of ovulation itself, roughly a six-day window. The reason it starts early is that sperm can survive in the body for several days, so intercourse before ovulation can still result in conception. The egg itself is viable for only about a day. The day before ovulation and ovulation day are usually the most fertile.
How accurate is an ovulation calculator?
An ovulation calculator gives a useful estimate for someone with a fairly regular cycle, but it is a prediction, not a measurement. It assumes ovulation falls 14 days before the next period and that cycles are consistent. In reality cycle length varies, and stress, illness, travel and other factors can shift ovulation. For a cycle that changes a lot from month to month the estimate is less reliable. Ovulation tests and body-temperature tracking give a closer reading of an individual cycle.
Can I use this calculator as contraception?
No. This calculator is a planning tool for people hoping to conceive, not a method of birth control. It estimates the fertile window from an average cycle, and that estimate can be wrong if a cycle is irregular or simply varies, as cycles often do. Sperm can also survive several days, which widens the real window. If you want to avoid pregnancy, use a reliable contraceptive method and speak to a healthcare provider about the options.
What does cycle length mean?
Cycle length is the number of days from the first day of one period to the first day of the next. It is not the length of the period itself. A common figure is 28 days, but anything from about 21 to 35 days is considered typical, and it can differ between people and between months. This calculator asks for your usual cycle length so it can place ovulation correctly, because the day of ovulation depends on it.