Free Delivery on UK Orders Over £40 14 Day No-Quibble Returns Discreet Packaging

Are ovulation calculators accurate?


Ovulation calculators work by entering your last menstrual period and your average cycle length to predict when you will ovulate. For a woman with a regular 28 day cycle, the ovulation calculator would identify the typical fertile window of day 10 – 15. This includes 5 days before ovulation and the day of ovulation. 

Although this can give you an idea of when you may ovulate, every woman is different and a normal cycle length can be between 21 and 35 days and even vary from month to month.

Many of the ovulation calculators and apps also assume a luteal phase of 14 days. The luteal phase is the length of time between ovulation and the first day of your menstrual cycle. The normal length can be as short as 10 days or as long as 15.

A research study in the US1 found that websites and apps that predict women’s most fertile days can often be inaccurate. The research, which tested 53 fertility calculators, found that only 4 predicted the woman’s precise fertile window. 

From the 20 websites and 33 apps assessed only one website and three apps identified that fertile window. They included the site and the apps Clue; My Days - Period & Ovulation; and Period Tracker. Clue uses an algorithm to calculate and predict your own unique cycle, which means it gets more accurate the more you use it. 

To identify when you are ovulating and to understand your own personal cycle, we recommend using fertility products, such as an ovulation test or fertility monitor, and/or using natural tracking techniques, including monitoring changes in basal body temperature, cervical fluid and cervical position. 



1. Robert Setton, M.D., obstetrics and gynaecology New York-Presbyterian Hospital/Weill Cornell Medical College, New York City; Jennifer Wu, M.D., obstetrician-gynecologist, Lenox Hill Hospital, New York City; May 15, 2016, presentation American College of Obstetricians and Gynecologists annual meeting, Washington, D.C.