How the Fertile Window Works
Ovulation happens about 14 days before the next period — not 14 days after the last one (that only matches 28-day cycles). The egg lives ~24 hours, but sperm survive up to five days, so the fertile window spans roughly five days before ovulation through the day after. Intercourse in the two days before ovulation has the highest conception probability.
Ovulation ≈ LMP + (cycle length − 14) days
Worked Example
Last period started June 1, cycle length 30 days → ovulation ≈ June 17, fertile window ≈ June 12–18, next period ≈ July 1.
Calendar Method vs. Body Signals
Calendar math assumes your cycles are regular — real ovulation shifts with stress, travel, illness and naturally irregular cycles. For higher confidence, combine the calendar with:
- LH tests (ovulation strips): detect the hormone surge 24–36 h before ovulation.
- Basal body temperature: rises ~0.3 °C after ovulation — confirms it happened.
- Cervical mucus: becomes clear and stretchy ("egg-white") around your most fertile days.
This calculator supports conception planning. It is NOT reliable as a contraception method — cycle variability makes calendar-based birth control one of the least effective options.