Estimate the 'safer' and fertile days of your menstrual cycle based on the rhythm method for fertility awareness.
"Safe days" refer to the time in a woman's menstrual cycle when the chances of getting pregnant are at their lowest. This calculation is based on the **rhythm method**, a form of fertility awareness that tracks the cycle to predict fertile and infertile phases.
The rhythm method is **not a reliable form of contraception**. Ovulation can be affected by stress, illness, and other factors, making predictions inaccurate. For effective birth control, please consult a healthcare provider.
Ovulation typically occurs about 14 days before your next period — not 14 days after your last one, a common mix-up. Because sperm can survive up to 5 days in the reproductive tract and the egg remains viable for about 24 hours after release, the fertile window stretches from roughly 5 days before ovulation through 1 day after. The days outside that window are the so-called "safe days."
Example: 28-day cycle → ovulation around day 14 → fertile window roughly days 9–15 → "safer" days fall before day 9 and after day 15.
This is calendar math, not a measurement: it assumes your cycles are regular and that ovulation lands exactly when predicted. Both assumptions fail often enough to matter — see below.
As contraception: not effective enough to rely on. Even with perfect tracking, a meaningful share of users conceive each year — and typical use, where cycles are imperfectly tracked and ovulation shifts with stress, illness, travel, intense exercise, or perimenopause, fails far more often. Health authorities rank fertility-awareness methods among the least effective reversible contraceptive methods. Never rely on cycle math alone to prevent pregnancy.
Sources: see the NHS guide to natural family planning. This calculator is educational only — speak to a healthcare provider about contraception.
Maintained by Live Fitness Check. Last updated: September 1, 2026. How we test our calculators .
Estimates lower- and higher-fertility days from menstrual-cycle dates and average cycle length.
The calculator estimates the likely ovulation period from cycle timing and marks surrounding days according to its configured fertility-window rules. The calculation uses last menstrual period date and average cycle length.
Calendar estimates cannot reliably guarantee contraception because ovulation can vary even in usually regular cycles. Use the number as an estimate and consider professional advice when the result may affect health, treatment, pregnancy, nutrition, exercise safety or another important decision.
