Ovulation Calculator
Estimate your ovulation date, fertile window, and next period from your last menstrual period, instantly in your browser.
How to use the Ovulation Calculator
- Enter the first day of your last menstrual period.
- Enter your average cycle length in days (28 is typical, but anything from 21 to 35 is common).
- Click Run to see your estimated ovulation date, fertile window and next expected period.
How the Ovulation Calculator works
Ovulation usually happens about 14 days before the next period starts. The calculator therefore estimates the ovulation date as the first day of your last period plus your cycle length minus 14 days, and the next period as the last period plus one cycle length.
Sperm can survive for up to five days and an egg for about a day, so the fertile window runs from five days before ovulation to one day after. Cycles vary month to month, so this is an estimate; ovulation tests or tracking basal body temperature give more precise timing. It is not a reliable method of contraception.
Frequently asked questions
- Is this ovulation calculator free?
- Yes. Estimate your fertile window at no cost, with no account required.
- How is ovulation estimated?
- Ovulation typically occurs about 14 days before the next period, so ovulation date = last period date + (cycle length - 14) days.
- What is the fertile window?
- The fertile window is the days when conception is most likely, typically from 5 days before ovulation through 1 day after.