Menstrual Cycle Tracker & Predictor
Predict next 3 periods, fertile window, and ovulation from last period and cycle length.
What this calculates
A regular cycle is 21-35 days, averaging 28. Ovulation typically falls 14 days before the next period. This calculator projects the next 3 periods, your fertile window, and ovulation day from your most recent period start and average cycle length.
Formula & how it works
Next period = last_start + cycle_length. Ovulation = next_period − 14 days. Fertile window = ovulation − 5 to ovulation + 1.
Worked example
Last period May 1, cycle 28 days. Next: May 29, Jun 26, Jul 24. Ovulation: May 15. Fertile: May 10-16.
Frequently asked questions
How accurate?
±2 days for regular cycles. Irregular cycles need 3-6 months of tracking to find an average. For conception planning, combine with OPK or basal body temp.
What if I'm pregnant?
Period delay >7 days past expected with regular cycles warrants a pregnancy test.
Cycle length changes?
Stress, illness, travel, weight changes, hormonal contraceptives all shift timing. Track over 3+ cycles.
Disclaimer: Predictions are estimates. Not for use as contraception.
Related calculators
- OvulationEstimate ovulation date and fertile window from your last period and cycle length. Useful for trying to conceive or natural cycle tracking.
- Pregnancy Due DateEstimate your pregnancy due date and how many weeks pregnant you are. Supports last menstrual period (LMP), conception date, and ultrasound dates.