Menstrual & Fertility Tracking
Cycle Calculator
Predict your next period, ovulation day, and fertile window from your last period date. Runs entirely in your browser nothing leaves your device.
Next period
May 29, 2026
Ovulation day
May 15, 2026
Fertile window
May 10, 2026 → May 16, 2026
Current period ends
May 5, 2026
The Cycle math
Next period = last period + cycle length
Ovulation = next period − 14 days (luteal phase is the constant)
Fertile window = ovulation − 5 days to ovulation + 1 day
If you don't know your typical cycle length, log 3 cycles first. The average across cycles is more reliable than any single one.