Project the next six period ranges and a calendar estimate of ovulation from your last period, typical bleeding length, and average cycle length.
How to use this calculator
Enter the first day of bleeding as the cycle start and use the length of a complete cycle. Check the projected dates against your own recent records. A calendar projection can shift when cycle lengths vary; it does not confirm ovulation.
Examples
September 8, 2026; five-day period; 28-day cycle
The next period is estimated for October 6–10. Estimated ovulation is September 22, with a calendar range of September 20–24.
Formula
periodStart(i) = lastPeriod + i × cycleLength
periodEnd(i) = periodStart(i) + periodLength − 1
nextPeriod = lastPeriod + cycleLength
estimatedOvulation = nextPeriod − 14
ovulationRange = estimatedOvulation − 2 … estimatedOvulation + 2
Every future row repeats the same average. Real cycle and bleeding lengths can change, so do not use these dates to diagnose a condition or prevent pregnancy.
When the estimate is less useful
Irregular cycles, pregnancy, perimenopause, postpartum changes, illness, stress, and some medicines can make an average-cycle forecast inaccurate. Record actual start dates and seek care for concerning or persistent changes.
Sources
- US Office on Women's Health: Your menstrual cycle and your health — cycle and ovulation context.
- American Society for Reproductive Medicine: Optimizing natural fertility — fertile-window timing and variability.