Ovulation Calculator

It doesn't just add 14 days to your last period. It first projects your next period by adding your average cycle length to your last period's start date, then subtracts your luteal phase length (14 days by default) from that projected date. This backward calculation is more accurate for cycles that aren't exactly 28 days, because the luteal phase tends to be more consistent than the days leading up to ovulation.

This calculator's 6-day fertile window (5 days before ovulation through ovulation day) is based on a 1995 New England Journal of Medicine study by Wilcox, Weinberg and Baird, which measured ovulation directly using daily hormone tracking in 221 women. They found essentially no pregnancies from intercourse after ovulation day — the egg is viable for only about 12-24 hours — so the window doesn't extend beyond it.

Rather than showing a specific percentage for a given day, this calculator sorts each day relative to your estimated ovulation date into a band: 'Highest' for the 1-2 days before ovulation, 'High' for ovulation day, 'Moderate' for 3-5 days before, and 'Very low' outside that window. This follows the general pattern the Wilcox study found — a precise percentage would overstate how exact a calendar estimate can be.

The calculator adjusts for your entered average cycle length, but it assumes that average is representative of your current cycle. If your cycle length varies a lot from month to month, the underlying assumption weakens and the estimate becomes less reliable. In that situation, ovulation predictor kits or basal body temperature tracking add real signal that a calendar-only calculation can't provide.

No. It's a calendar-based educational estimate, not a diagnostic tool or a hormone measurement. If you've been trying to conceive for 12 months without success (or 6 months if you're over 35), or if your cycles are consistently irregular, that's a reason to speak with a healthcare provider rather than rely on calendar predictions alone.