Lunia · Free tools
Menstrual Cycle Length Calculator
Measure a completed cycle from two consecutive period start dates.
Your details
Runs entirely in your browser. Nothing you type is sent anywhere or saved.
Your result
Ready to calculate
Enter your details, then calculate to see your result here.
How this calculator works
Cycle length is the calendar-day difference between consecutive period start dates. The next-start illustration adds that observed length to the latest start. It does not average your full history or assume every future cycle will be identical.
A worked example
Previous period: first day: Aug 1, 2026 · Latest period: first day: Aug 30, 2026
- Observed cycle length
- 29 days
- Next start if this length repeats
- Sep 28, 2026
- First day of the latest cycle
- Aug 30, 2026
These are illustrative inputs, not personal recommendations.
Common questions
Does the last day of bleeding determine cycle length?
No. Count from the first day of one period to the first day of the next. Bleeding duration is a separate measurement.
How should I use the result?
Calendar dates cannot confirm ovulation, fertility or hormone levels. Irregular cycles and hormonal contraception make calendar predictions less reliable. Do not use this tool for contraception.
Sources & method references
Formula-based results are estimates, not individual medical advice.