Ovulation Calculator Online — Fertile Window Calculator

Calculate your estimated ovulation date, fertile window, and next expected period based on your menstrual cycle. View a calendar with fertile days highlighted. No upload, no sign-up — runs entirely in your browser.

First Day of Last Period

Average Cycle Length (days)

Standard is 28 days. Adjust if your cycle is different.

Free Ovulation Calculator — Find Your Fertile Window

OptiDrop's Ovulation Calculator helps you estimate your ovulation date and fertile window based on your menstrual cycle. Ovulation typically occurs 14 days before the start of your next period. By entering the first day of your last period and your average cycle length, the calculator determines your most fertile days and displays them on an easy-to-read calendar.

Understanding Your Fertile Window

Your fertile window spans 6 days: the 5 days before ovulation and the day of ovulation itself. Sperm can survive in the female reproductive tract for up to 5 days, while the egg survives for only 12-24 hours after being released. Having intercourse during the 2-3 days leading up to ovulation and on ovulation day provides the highest chance of conception.

Calendar View

The interactive calendar highlights your period days in red, fertile days in green, and ovulation day in blue. You can navigate between months to see predictions for future cycles. This visual representation makes it easy to plan and track your fertility. For the most accurate results, track your cycles over several months to confirm your average cycle length.

Frequently Asked Questions

Ovulation typically occurs 14 days before the start of your next period. To calculate it, subtract 14 from your cycle length. For example, with a 28-day cycle, ovulation occurs around day 14. With a 32-day cycle, ovulation occurs around day 18. This calculator uses your last period date and cycle length to estimate your ovulation day accurately.

The fertile window is the 6-day period during which conception is possible. It includes the 5 days before ovulation and the day of ovulation itself. Sperm can survive in the female reproductive tract for up to 5 days, while the egg survives for about 12-24 hours after ovulation. Having intercourse during this window maximizes the chances of conception.

You are fertile for about 5 days before ovulation and on the day of ovulation itself. This is because sperm can survive in the reproductive tract for up to 5 days. The most fertile days are the 2-3 days leading up to ovulation and the day of ovulation. The probability of conception is highest when intercourse occurs on the day before or day of ovulation.

To use an ovulation calculator, enter the first day of your last menstrual period and your average cycle length. The calculator will estimate your ovulation date, fertile window, and next expected period. For the most accurate results, track your cycles for several months to determine your average cycle length. You can also combine this with other methods like basal body temperature tracking or ovulation predictor kits.

Last updated: June 2026