Fertile Window Calculator

How the fertile window calculator works

This calculator uses the calendar method, also known as the Ogino-Knaus method. It assumes that ovulation happens roughly 14 days before the next period and that fertility depends on two survival times: sperm can live up to 5 days in the reproductive tract, while the egg survives only 12 to 24 hours. From the shortest and the longest of your recent cycles it takes the first fertile day as the shortest cycle minus 18, and the last fertile day as the longest cycle minus 11. Day 1 is the first day of bleeding.

Write down the first day of every period for 6 to 12 months, then enter the shortest and the longest cycle you observed, counting from one first day of bleeding to the day before the next one. Also enter the first day of your last period and how many days your period usually lasts. The tool projects six cycles using the average length, which is the mean of your shortest and longest cycle rounded to the nearest whole day, and marks period, fertile days and likely ovulation on a three-month calendar.

Example: the last period began on 1 October and your cycles range from 26 to 32 days. The first fertile day is 26 - 18 = day 8 and the last is 32 - 11 = day 21, so the window runs from 8 to 21 October, which is 14 days. The average cycle is 29 days, so ovulation is likely on day 15 (29 - 14), on 16 October, and the next period is expected on 30 October.

The calendar method is an estimate, not a diagnosis. On its own it is not a reliable contraceptive method: with typical use many pregnancies occur. It works poorly with irregular cycles, after childbirth, while breastfeeding, after stopping hormonal contraception, and near menopause, and stress, illness or travel can shift ovulation. For better precision combine it with ovulation (LH) tests and talk to a doctor, especially when trying to conceive for more than a year or choosing contraception.