Health and Fitness Calculators
Period & Fertile Window Estimator
Estimate your next period, ovulation date, and fertile window from the start date of your last period and your average cycle length.
This is a rough estimate based on a typical 14-day luteal phase, not a personalized prediction — actual cycles vary. Logging your cycle in the FitnessSyncer Notebook builds a personalized prediction from your own logged history instead of this generic estimate.
Your next period is estimated to start around -.
Ovulation is estimated around -.
Your estimated fertile window is -.
How it works
The estimate works backward from your next period rather than forward from your last one, because the second half of the cycle varies far less between people than the first half does.
Ovulation = start of last period + (cycle length - 14 days)
With a 28-day cycle starting on the first of the month, ovulation is estimated around the fifteenth, and the fertile window runs from the tenth to the sixteenth.
What this number is not
It is not contraception and it is not a prediction for you. It is the average cycle applied to your dates, and anyone whose cycles vary will find it wrong often.
How accurate it is
The fourteen-day luteal phase this assumes is a population average, and a real one runs anywhere from ten to sixteen days. Logging your own cycles in the Notebook builds a prediction from your history instead, which is a better answer for you than any formula.
Common questions
How accurate is the fertile window?
It is a reasonable first guess and no more. Sperm can survive several days, which is why the window opens before the estimated date, but the estimate itself can be out by days in either direction.
Can FitnessSyncer predict this from my own history?
Yes. Log your cycles in the Notebook, or connect a service that records them, and your predictions come from your own recorded cycle lengths rather than from a 28-day average.
What if my cycles are irregular?
Then this calculator will not help much, and neither will any formula. Tracking basal body temperature and cervical mucus alongside your dates gives a far better picture, and both can be logged in the Notebook.
