How to use the ovulation calculator
- Last period, enter the first day of your most recent period using the date picker.
- Cycle length, enter your average cycle length in days (most people fall between 20 and 40, with 28 a common average).
- Read your ovulation date, fertile window, next period and estimated due date, they update instantly as you type.
Nothing is submitted or stored: your dates never leave your device, so you can check your cycle privately.
How ovulation timing is estimated
Ovulation is the point in your cycle when an ovary releases an egg. In a typical cycle it happens about14 days before your next period begins, regardless of how long your overall cycle is. This calculator first adds your cycle length to the first day of your last period to estimate when your next period is due, then counts back 14 days to pinpoint your likely ovulation date.
From there it works out your fertile window, the days when conception is most likely. The fertile window is the 5 days before ovulation plus the day of ovulation itself. That range exists because sperm can survive for several days inside the body, so intercourse in the days leading up to ovulation can still result in pregnancy. The estimated due date is calculated as 280 days (around 40 weeks) from the first day of your last period, which is the standard method used to date a pregnancy.
Remember, these are estimates
Every body is different and real cycles rarely run like clockwork. Stress, illness, travel, sleep and many other factors can shift ovulation earlier or later from one month to the next. Treat the dates here as a helpful guide, not a guarantee. If you want greater accuracy, combine this estimate with ovulation predictor tests, basal body temperature tracking or changes in cervical mucus.
Important: This calculator is for general information only and is not medical or contraceptive advice. It cannot diagnose fertility and should never be used as a method of birth control. Always consult a qualified doctor or healthcare professional about fertility, pregnancy or contraception.
Frequently asked questions
How is the ovulation date estimated?
Ovulation usually happens about 14 days before your next period starts. The calculator adds your cycle length to the first day of your last period to find your next period, then counts back 14 days to estimate ovulation.
What is the fertile window?
The fertile window covers the 5 days before ovulation plus the day of ovulation itself, because sperm can survive for several days inside the body.
How accurate is an ovulation calculator?
It gives an estimate based on average cycle timing. Real cycles vary from month to month, so actual ovulation can fall earlier or later. Ovulation tests, basal body temperature and cervical mucus tracking add precision.
Can I use this for birth control?
No. This calculator is for general information only and is not a contraceptive method. Do not rely on it to avoid pregnancy, speak to a healthcare professional about reliable birth control.
How is the due date worked out?
The estimated due date is 280 days (about 40 weeks) from the first day of your last period, the standard way pregnancy dates are estimated.