Safe Period & Ovulation Calculator — Fertile Window, Next Period, Safe Days

Free online safe-period and ovulation calculator. Enter the first day of your last period, your typical cycle length (21–35 days, default 28), and your period length (3–7 days, default 5), and instantly see the next 3 predicted period start dates, your estimated ovulation day, the fertile window (5 days before ovulation through 1 day after, when sperm could meet a viable egg), and both safe windows — the pre-ovulation safe days and the post-ovulation safe days, shown separately because they are NOT equally reliable. A two-month calendar colors every day so you can see the whole cycle at a glance: red for period, orange for fertile, pink for ovulation day, green for safe. The calculation follows the calendar (rhythm) method, fixing the luteal phase at 14 days (the only biologically stable phase). All math runs in your browser — the date you enter is never uploaded, never logged, never sent anywhere. The calendar method is for cycle awareness, NOT a contraceptive — WHO data put its perfect-use effectiveness at about 76%. For contraception, use a condom, hormonal method, IUD, or another clinically validated option, and talk to your doctor about which one fits you.