Period Calculator
Predict your next few periods and fertile days from your last period and cycle length.
PregnancyScience-backed formula
Enter your details
Fill in the fields, then calculate
Please choose a date.
Enter your cycle length.
Enter your period length.
Awaiting Data
Enter your details and tap Calculate to see your results appear here.
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ResultYour result appears here.
What This Calculator Does
This tool projects your upcoming periods from your last start date and your average cycle. It helps you plan ahead and spot your fertile days.
Key dates line up into an easy week by week timeline.
Cycles vary, so treat the dates as a guide. Tracking over months sharpens the pattern.
The tool also marks the fertile window around ovulation.
How to Use This Calculator
- Choose the first day of your last period.
- Set your cycle and period length.
- Tap Calculate to see the next three cycles.
The Formula Used
Next period = last period + cycle length
Repeat to project future cycles
Repeat to project future cycles
- cycle
- Days from one period to the next.
- period length
- How many days bleeding lasts.
Understanding Your Results
Short cycle
Under 24 days
May need a provider's review.
Typical cycle
24 to 38 days
A normal adult range.
Long cycle
Over 38 days
Worth tracking and discussing.
Tips to Improve
- Log start dates each month.
- Note flow and symptoms for patterns.
- See a provider about very irregular cycles.
- Use tracking to plan around your period.
Frequently Asked Questions
Most adult cycles run 24 to 38 days, and some natural variation is healthy.
Stress, weight change, and hormones can all shift timing. Persistent issues deserve a checkup.
It projects your next three cycles from your inputs.
Scientific References
- Bull, J. R., et al. (2019). Real-world menstrual cycle characteristics. npj Digital Medicine, 2, 83.
- Fehring, R. J., et al. (2006). Variability in the phases of the menstrual cycle. JOGNN, 35(3), 376–384.
- American College of Obstetricians and Gynecologists. (2015). Menstruation in girls and adolescents.