Ovulation Calculator

Understanding your fertile window

Pregnancy is only possible during a relatively narrow window each cycle — the five days before ovulation plus the day of ovulation itself, since sperm can survive in the reproductive tract for up to five days while the egg survives roughly 12–24 hours after release. Identifying this window is the basis of most fertility awareness methods, whether you're trying to conceive or trying to understand your cycle better for other reasons.

How ovulation timing is estimated

This calculator uses the luteal phase method: ovulation is estimated to occur 14 days before your next expected period, since the luteal phase (the time between ovulation and the next period) is relatively consistent at around 14 days for most people, even when the first half of the cycle (the follicular phase) varies more. By entering the first day of your last period and your typical average cycle length, we can estimate both your likely ovulation date and the fertile days leading up to it.

Why cycle length matters so much

A common misconception is that ovulation always happens on day 14 of every cycle, but that's only true for a textbook 28-day cycle. Someone with a 32-day cycle typically ovulates later, around day 18, while someone with a 24-day cycle typically ovulates earlier, around day 10 — which is why entering your own average cycle length, rather than assuming 28 days, produces a meaningfully more accurate estimate.

What to do with your estimated fertile window

Calendar-based estimates like this one are a helpful starting point, but they work best for people with fairly regular cycles. For a more precise, real-time signal, many people combine calendar estimates with ovulation predictor kits (which detect the LH hormone surge) or basal body temperature tracking. If you've been tracking ovulation and trying to conceive for 12 months (or 6 months if over 35) without success, it's a good time to speak with a healthcare provider.