Fertile Window Estimate Calculator

About this calculator

The fertile window is the span of days in a menstrual cycle when pregnancy is possible. It's estimated with the standard luteal-phase method: ovulation typically occurs about 14 days before the next period starts, regardless of how long the overall cycle is, because the luteal phase (post-ovulation) is fairly fixed while the follicular phase (pre-ovulation) varies.

Since sperm can survive in the reproductive tract for up to 5 days and a released egg is viable for roughly 12–24 hours, the fertile window is usually counted as the 5 days before ovulation plus the day of ovulation itself. This estimate is widely used in fertility awareness methods by people trying to conceive or trying to avoid pregnancy naturally, and is a starting reference point OB-GYNs and midwives use before more precise tracking (basal body temperature, ovulation predictor kits, or ultrasound).