About OvulationCalc

OvulationCalc is a small, free calculator for the questions people actually ask about a cycle: when do I ovulate, which days are fertile, when is my period due, and when is it worth taking a pregnancy test?

The method

The calculator uses the standard calendar model, applied carefully. Ovulation is estimated by counting backwards from the expected next period across the luteal phase — the comparatively stable second half of the cycle, set to 14 days by default and adjustable if you have measured your own. The fertile window is the six days ending on ovulation day, reflecting sperm survival of up to five days and an egg life of about a day. Due date estimates use conception + 266 days, the same identity clinical dating is built on.

The odds figures

The per-day conception probabilities shown in the results come from Wilcox AJ, Weinberg CR, Baird DD, "Timing of sexual intercourse in relation to ovulation", New England Journal of Medicine 1995; 333:1517–1521 — 221 women, with ovulation day confirmed by urinary hormone metabolites. We show the dip three days before ovulation exactly as published rather than smoothing it away, and we say plainly that these are group averages, not a personal forecast.

How it is built

The site is a static page set with no accounts, no database and no analytics on your inputs: every calculation runs in JavaScript in your own browser, and the dates you type never leave your device. OvulationCalc is part of a family of small free tools built by an independent developer; the sister tool for the other end of the same timeline is DueDateCalc.

Limits

Calendar estimates project your average cycle forward. They cannot detect this month's actual ovulation, diagnose anything, or substitute for a clinician — and, to repeat the point made across the site, they must never be used as a contraception method.