There are many types of ovulation calculator available today that estimate when your fertile days will occur – a form of ‘calendar method’. Usually they are based on three pieces of information:-

  1. 1. The day your last period started
  2. 2. Your usual cycle length
  3. 3. The fact that ovulation normally occurs 12-16 days before your next period starts.

The most basic version is a calendar or diary plus a calculator and you work it out for yourself, but there are widgets on the internet and apps that that will do it for you too.

The issue with these ovulation calculators is that they are based on information that is variable – almost half of women have cycles that vary by 7 days or more i and the timing of ovulation varies by a few days, even in regular cycles.

The calendar method has been found to predict the correct fertile days in only 1/3 of cyclesii.

If you want to accurately identify your fertile days each cycle with confidence, use an ovulation test or fertility monitor that detects the changes in your key fertility hormones in urine – this has been shown to be more accurate than either calendar or temperature methodsii.