Ovulation Calculator

How to use

1. Enter the average cycle length and the last period date.
2. Read the estimated ovulation day and fertile window.
3. Check the next period date and the cycle-length comparison.
4. Use ovulation tests alongside it if cycles are irregular.

How it works

How the ovulation day is projected

The convention is "14 days before the next period": the luteal phase (ovulation to menstruation) averages about 14 days in the population and is relatively stable, so the next period date is set by the cycle length and the ovulation day is back-solved. A 28-day cycle ovulates on day 14; a 35-day cycle on day 21; longer cycles ovulate later.

Why the fertile window is 5+1 days

The fertile window is 5 days before to 1 day after ovulation, based on gamete survival: sperm survive in the female reproductive tract about 3-5 days, and the egg can be fertilized within about 24 hours of release. So the highest conception probability isn't ovulation day itself but 1-2 days before. For a 28-day cycle with an LMP of August 1, ovulation is August 15 and the fertile window August 10-16.

When not to use it

The failure scenarios must be stated: with irregular cycles (PCOS, thyroid issues, breastfeeding, perimenopause) the "fixed 14-day luteal phase" assumption notably breaks down; with cycles shorter than 21 days or longer than 40, the tool refuses outright, as that's a range needing medical evaluation. More importantly: **it can't be used as contraception** — the calendar method's typical-use annual failure rate can exceed 20%.

Calculation basis: ovulation = next period − 14 days (the average luteal phase assumption), with a fertile window from 5 days before to 1 day after ovulation. The method is markedly less accurate for irregular cycles and must not be used as contraception; cycle lengths are limited to 21–40 days, and anything outside that range suggests seeing a doctor.

Code example

const DAY = 86400000;

function ovulation(lmp, cycleDays) {
  const t = new Date(lmp).getTime();
  const next = t + cycleDays * DAY;
  const ovu = next - 14 * DAY;              // the luteal phase is a fixed 14 days
  return {
    ovulation: new Date(ovu),
    window: [new Date(ovu - 5 * DAY), new Date(ovu + 1 * DAY)],
    nextPeriod: new Date(next)
  };
}

from datetime import date, timedelta

def ovulation(lmp, cycle_days):
    t = date.fromisoformat(lmp)
    ovu = t + timedelta(days=cycle_days - 14)
    return {"ovulation": ovu,
            "window": (ovu - timedelta(days=5), ovu + timedelta(days=1)),
            "next": t + timedelta(days=cycle_days)}

# Back-solved from a fixed 14-day luteal phase; the error grows with irregular cycles

FAQ