Period Calculator
A Predictable Pattern Built From One Date
The menstrual cycle repeats on a fairly consistent schedule for most people, which means a single reliable data point — the first day of the last period — combined with a typical cycle length is enough to project forward to the next period and back to the likely ovulation window. It won't be exact for every cycle, but it gives a genuinely useful planning estimate.
The Formula
Ovulation Day (approx.) = Next Period Start − 14 days
Fertile Window = Ovulation Day − 5 days to Ovulation Day + 1 day
The 14-day offset reflects the typical luteal phase — the relatively fixed second half of the cycle between ovulation and the next period — which is why the fertile window calculation counts backward from the next period rather than forward from the last one.
Typical Cycle Length Range
A cycle length of 21–35 days is generally considered within the typical range for adults, with 28 days often used as a textbook default even though relatively few people have a cycle that lands exactly on 28 days every month.
Why Tracking This Matters
- Planning around your cycle — predicting period timing helps with everything from travel planning to symptom preparation.
- Conception planning — identifying the fertile window is one of the most common reasons people track cycle data in the first place.
- Spotting irregularity early — comparing predicted versus actual period dates over several cycles can flag a pattern worth discussing with a healthcare provider.
How to Use This Calculator
- Enter the start date of your last period.
- Enter your average cycle length in days (28 is the default).
- Select Calculate to see your predicted next period and fertile window start.
Related Calculations
For a more customizable ovulation estimate that accounts for your specific luteal phase length, see the Fertility Window Calculator.