Predict your next period, ovulation day, and fertile window from your last period date and cycle length. Private, all-local processing. | No signup · Data stays on device
Privacy Safe · Works Offline• Ovulation typically occurs 14 days before the next period (e.g., day 14 of a 28-day cycle).
• Fertile Window = 5 days before ovulation + ovulation day + 2 days after (~7 days total).
• Rhythm method for contraception is ~75% effective — not recommended as sole birth control.
If cycle length varies by more than 7 days, predictions become less accurate. Track for 3-6 months before relying on this tool. Consult a gynecologist for irregular cycles.
No. The rhythm method has a ~25% failure rate. This calculator is for cycle awareness and fertility planning only — not a substitute for reliable contraception.