Ovulation Calculator

How to estimate your fertile window

1. Enter the first day of your last period.
2. Set your average cycle length.
3. Read your estimated ovulation day and fertile window.
4. See the next few cycles in the table.

Not a method of contraception

This ovulation calculator, a simple fertility calculator and ovulation calendar, estimates when ovulation is likely so that people trying to conceive can time things. It is not birth control. Predicted fertile windows are approximate, ovulation can shift, and pregnancy is possible on days the calendar marks as low chance. Do not rely on it to avoid pregnancy. For contraception, use a proven method and speak to a doctor.

How ovulation timing is estimated

Ovulation usually happens about 14 days before your next period, because the second half of the cycle, the luteal phase, is fairly constant at around 14 days. So the tool takes your cycle length and subtracts 14 to place the ovulation day. A longer cycle moves ovulation later and a shorter one moves it earlier, which is why cycle length matters more than a fixed "day 14" rule.

The fertile window

The fertile window is the span when conception is most likely. It runs from about five days before ovulation to the day of ovulation itself, because sperm can survive several days while the egg lives only about a day. That gives roughly a six-day window, with the two days before ovulation usually the most fertile.

Source: fertile-window timing as described by the American College of Obstetricians and Gynecologists (ACOG) and the UK National Health Service (NHS).

An estimate, not medical advice

These dates are estimates that vary from cycle to cycle and person to person, and irregular cycles are especially hard to predict. This is not medical advice and cannot diagnose fertility. For help conceiving, or for any concern about your cycle, speak to a doctor.

FAQ

When do I ovulate?

Usually about 14 days before your next period. Enter your cycle length and this tool estimates the ovulation day, since a longer or shorter cycle shifts it.

What is the fertile window?

The roughly six days ending on ovulation day: the five days before, when sperm can survive, plus ovulation itself. The two days before ovulation are typically the most fertile.

Can I use this as birth control?

No. It is an estimate, not a contraceptive method, and pregnancy is possible outside the predicted window. Use a proven method of contraception and ask a doctor.

Why does cycle length matter?

Because ovulation is counted back from the next period, not forward from the last one. The luteal phase is fairly fixed, so your cycle length sets when ovulation falls.

Is my information private?

Yes. Everything is calculated in your browser and nothing you enter is uploaded or saved.