How the Predictions Work
This calculator uses the first day of your last period plus your average cycle length to project future periods, ovulation, and fertile windows. Ovulation is estimated at cycle length − 14 days (not 14 days after the period starts), because the luteal phase after ovulation is consistently ~14 days. The fertile window spans the 5 days before ovulation through ovulation day — this reflects the lifespan of sperm (up to 5 days) and the egg (12–24 hours).
Cycle Phase Breakdown
A typical 28-day cycle has four phases: Menstrual (days 1–5) — uterine lining sheds; Follicular (days 6–13) — follicles mature and estrogen rises; Ovulatory (around day 14) — LH surge triggers egg release; Luteal (days 15–28) — progesterone rises to prepare for implantation. The chart visualizes these four phases for your specific cycle length.
Calculation Example
Last period: March 1. Cycle length: 28 days. Period duration: 5 days.
Next period: March 29. Ovulation: March 15. Fertile window: March 10–15.
Expert Tips
- Track 3+ consecutive cycles before relying on predictions — a single data point can’t show your personal pattern.
- Ovulation tests (LH strips) give real-time confirmation; use them alongside calendar predictions if conception timing matters.
- Cycle length varies by up to ±7 days even in healthy women — small month-to-month variation is normal.
- Cycles consistently shorter than 21 days, longer than 35 days, or varying by 7+ days cycle-to-cycle are worth discussing with a gynecologist.