Ovulation Calculator

The Ovulation Calculator helps estimate a woman’s fertile window—the days when conception is most likely to occur. Predictions are based on the start date of the last menstrual period (LMP). This tool is only meant as a guide and should not be relied upon as a method of birth control.

Ovulation and Fertility

Ovulation is the release of a mature egg from the ovary, which then travels down the fallopian tube in preparation for possible fertilization. Typically, ovulation happens about 14 days before the start of the next period, though the exact timing can vary by 4 days on either side.

The most fertile phase usually falls between day 10 and day 18 of the menstrual cycle. During this time:

The egg remains viable for 12–24 hours after release.
Sperm can survive inside the female reproductive tract for up to 5 days.
H
aving intercourse in the five days leading up to ovulation and on the day of ovulation offers the best chance of conception.

If fertilization occurs, the embryo typically implants in the uterus 6–12 days later. If not, the uterine lining sheds during menstruation.

How to Predict Ovulation

Many women track ovulation to improve their chances of pregnancy. Here are the most common methods:

1. Tracking Menstrual Cycles

This calculator estimates ovulation based on cycle length. For women with a regular 28-day cycle, ovulation is usually around day 14, and fertility is highest between days 10 and 18. If your cycles are irregular, keeping a menstrual calendar over several months can help identify patterns.

2. Monitoring Basal Body Temperature (BBT)

BBT is your body’s lowest resting temperature, measured first thing in the morning. Just before ovulation, BBT is at its lowest, then rises about 0.5°F (0.3°C) during ovulation and remains higher until the next period. Tracking BBT over time can reveal your fertile days, though illness, poor sleep, or other factors may interfere with accuracy.

3. Using Ovulation Test Kits

Over-the-counter ovulation predictor kits (OPKs) detect a surge in luteinizing hormone (LH), which peaks 24–48 hours before ovulation. These tests are about 99% accurate at detecting LH but cannot pinpoint the exact day ovulation occurs. Some advanced kits also measure estrogen levels in saliva or electrolytes in sweat, which change earlier in the cycle than LH. While these may predict ovulation slightly sooner, they require more consistent tracking and interpretation.

In short:
The Ovulation Calculator can give you an estimate of your fertile window, but combining it with cycle tracking, temperature monitoring, or test kits provides a more reliable picture.