How is the safe period calculated?
Calendar method: ovulation ≈ 14 days before the next period, fertile window around ovulation, and why it is not reliable contraception.
A common rule assumes a ~14-day luteal phase, so ovulation is about cycle length minus 14 (day 14 in a 28-day cycle).
Sperm may live 3–5 days and the egg about 1 day, so the fertile window is often ovulation minus 5 through ovulation plus 1.
Other days are sometimes called “safer,” but irregular cycles, illness and stress shift ovulation — failure rates are high.
Use proven contraception if needed, and treat this estimate as educational only.