Ovulation Calculator
Estimate your ovulation date and fertile window from your last period and cycle length.
Assumptions used in this calculation
- Luteal phase: Assumes a standard 14-day luteal phase (time from ovulation to next period), which is fairly consistent even when cycle length varies.
About this calculator
Predicting the days you're most likely to conceive means working backward from your next expected period by a fixed 14-day luteal phase, then extending a window around that date to account for sperm survival time, a calculation that's easy to get wrong when counting on a paper calendar. This calculator takes the first day of your last period and your average cycle length, and calculates your estimated ovulation date as (last period + cycle length − 14 days), then builds a fertile window spanning 5 days before ovulation through 1 day after. The window is asymmetric on purpose: sperm can survive in the reproductive tract for up to 5 days, while a released egg is only viable for about 24 hours, so most of the fertile window falls before ovulation, not after. Because predictions rely on a single average cycle length, accuracy drops for anyone with irregular cycles, tracking several cycles or pairing this with ovulation predictor kits improves reliability.
Worked example
Last period Jan 1, 2026, 28-day cycle
Result: Ovulation ≈ Jan 15; fertile window Jan 10-16
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