Ovulation Calculator

How this is worked out

Ovulation is counted back from your next period, not forward from your last:

ovulation = next period − luteal phase

This is the opposite of how most calculators do it, and it is the correct way round. A cycle has two phases. The follicular phase, before ovulation, varies a great deal between people and between cycles. The luteal phase, after ovulation, is stable at around 14 days for almost everyone. So the reliable anchor is the end of the cycle, not the start.

Counting forward from day 1 puts ovulation on day 14 for everybody, which is right only for a 28-day cycle. On a 35-day cycle ovulation is around day 21; on a 24-day cycle, around day 10.

The fertile window is then asymmetric: it opens five days before ovulation and closes about a day after. Sperm survive in the reproductive tract for up to five days; an egg is viable for around twenty-four hours. Being "on time" is already late.

A worked example

The window opens before ovulation, not after

This is the single most useful thing on the page. Sperm can survive in the reproductive tract for up to five days waiting for an egg. An egg, once released, is viable for about twenty-four hours. So the fertile window runs from five days before ovulation to roughly one day after — and the most fertile days are the two immediately before, not the day itself.

The practical consequence is that couples who wait for a positive ovulation test before trying have already missed the best days. A positive LH test indicates ovulation is likely within 24 to 36 hours, which puts you at the end of the window rather than the start.

Trying every day or two across the whole window is more effective than trying to hit a single day precisely, and it removes the pressure of aiming at a date that a calculator can only estimate.

Why counting forward from day 1 is wrong

The "day 14" rule comes from taking a 28-day cycle and halving it. It has become so entrenched that many calculators apply it to every cycle length, which produces a fertile window that is simply wrong for anyone whose cycle is not 28 days.

What actually happens is that the luteal phase — from ovulation to the next period — is remarkably consistent at around 14 days. What varies is how long the follicular phase takes before ovulation happens. Someone with a 35-day cycle is not ovulating late in a normal follicular phase; they have a longer follicular phase and a normal luteal one.

So the honest calculation counts back. This calculator does, and it lets you set the luteal phase yourself if you have tracked it — a short luteal phase, under about ten days, is worth mentioning to a clinician.

A calendar is an estimate, and the body is not a calendar

Stress, illness, travel, poor sleep and significant changes in training or weight can all delay ovulation in a given cycle. When ovulation is late, the period is late — the luteal phase stays roughly the same length, so the whole cycle stretches.

That means a calendar prediction is a starting point rather than a schedule. If you want a better signal, basal body temperature confirms ovulation after it has happened (a sustained rise of a few tenths of a degree), and cervical mucus changes — becoming clear and stretchy — a few days before, which is genuinely predictive.

Ovulation predictor kits detect the LH surge and give a day or two of warning. Used alongside a calendar estimate they narrow the window considerably; used alone they can miss a cycle where ovulation shifts.

When to stop using a calculator and ask someone

The general guidance is a year of trying under 35, or six months over 35, before seeking investigation. Those thresholds are not arbitrary — most couples conceive within that window, and the ones who do not are more likely to have something identifiable and treatable.

Some things are worth raising sooner regardless of how long you have been trying: cycles consistently shorter than 21 days or longer than 35, a luteal phase under about ten days, cycles that have become irregular when they were not, very heavy or very painful periods, or a known condition such as PCOS or endometriosis.

This calculator is a planning aid. It cannot tell you whether you are ovulating at all, which is the question that actually matters if a calendar has not worked.

Assumptions and sources

Ovulation timing

Counted back from the next expected period by the luteal phase length, which is stable at approximately 14 days.

Fertile window

Five days before ovulation to approximately one day after — sperm survive up to five days, an ovum about 24 hours. checked 2026-08

Cycle variation

Normal adult cycles range from about 21 to 35 days, with most of the variation in the follicular phase.

When to seek advice

Conventional guidance is 12 months of trying under 35, or 6 months over 35.