Fertility Calculator

It works backward, not forward. It first adds your average cycle length to the start date of your last period to estimate your next period, then subtracts your luteal phase length (14 days by default) from that date. This is more reliable than simply adding 14 days to your last period, because the luteal phase tends to stay more consistent cycle to cycle than the days leading up to ovulation.

Because sperm can survive for several days in fertile cervical mucus, intercourse in the days before ovulation can still result in pregnancy, even though the egg itself is only viable for about 12-24 hours after release. This calculator's 6-day window (5 days before ovulation through ovulation day) is based on a 1995 study in the New England Journal of Medicine (Wilcox, Weinberg & Baird) that measured ovulation directly via daily hormone tracking.

Instead of showing a specific percentage for a given day, this calculator classifies each day relative to your estimated ovulation date into a band. 'Highest' covers the 1-2 days before ovulation, 'High' covers ovulation day itself, 'Moderate' covers 3-5 days before ovulation, and 'Very low' covers everything else. These bands follow the general pattern found in the Wilcox study rather than a precise personal probability, since day-by-day odds vary a lot by age and individual fertility.

The calculator adjusts automatically as long as you enter your actual average cycle length — a 32-day cycle shifts the estimated ovulation date later than a 28-day cycle would. What it can't adjust for is genuine irregularity: if your cycle length varies a lot month to month, the estimate becomes less reliable, and tracking methods like ovulation predictor kits or basal body temperature will narrow it down better than a calendar estimate alone.

No. This tool estimates a fertile window from calendar math, which is not the same as clinically verified fertility awareness-based contraception — it doesn't detect your actual hormone levels or confirm ovulation occurred when predicted. If you're trying to avoid pregnancy, talk to a healthcare provider about a method with a verified effectiveness rate.