This calculator assumes ovulation about 14 days before the next predicted period. It adds cycle length minus 14 days to the first day of your last period. The displayed date is an estimate; actual timing can vary even when your cycles are regular. A calendar calculation cannot confirm that ovulation occurred.

What is the fertile window?

This tool uses six days: the five days before estimated ovulation and the estimated ovulation day. This follows ASRM's counselling definition. The predicted date can be wrong, and pregnancy can be possible outside the displayed range. The range is not a set of safe days for avoiding pregnancy.

How accurate is an ovulation calculator?

It is a calendar estimate, not a measurement or confirmation of ovulation. It assumes ovulation about 14 days before the next period and uses your usual cycle length. Actual timing varies, particularly when cycles vary. Speak to a healthcare provider if you need individual guidance.

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.

Ovulation Calculator

Estimate ovulation timing and a six-day fertile window from the first day of your last period and your usual cycle length. This calendar method assumes regular cycles; it cannot confirm ovulation.

First day of last period

Average cycle length (days)

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.

Related pregnancy tools: Due date · Pregnancy weeks · Trimester dates · IVF due date · Ovulation estimate · Conception estimate

Calendar estimate only. Actual ovulation varies, even with regular cycles. Do not use this calculator as 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. Cycle length alone cannot tell whether your cycles are regular. Estimates are less reliable when cycles vary or the usual length is outside the common range. 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.

Sources checked 9 October 2026. These methods were checked against published guidance; this tool has not been clinically reviewed.

The six-day range follows the ASRM fertile-window convention. The predicted day is not a measured day: pregnancy may still be possible outside this displayed range.

Read the guide: Understanding calendar ovulation estimates.

Frequently asked questions

When do you ovulate?: This calculator assumes ovulation about 14 days before the next predicted period. It adds cycle length minus 14 days to the first day of your last period. The displayed date is an estimate; actual timing can vary even when your cycles are regular. A calendar calculation cannot confirm that ovulation occurred.
What is the fertile window?: This tool uses six days: the five days before estimated ovulation and the estimated ovulation day. This follows ASRM's counselling definition. The predicted date can be wrong, and pregnancy can be possible outside the displayed range. The range is not a set of safe days for avoiding pregnancy.
How accurate is an ovulation calculator?: It is a calendar estimate, not a measurement or confirmation of ovulation. It assumes ovulation about 14 days before the next period and uses your usual cycle length. Actual timing varies, particularly when cycles vary. Speak to a healthcare provider if you need individual guidance.
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.

Last updated 2026-10-17

Ovulation Calculator

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