Ovulation Calculator

Fertile window and likely ovulation day.

Estimate your ovulation date and six-day fertile window from your last period and cycle length, with the luteal phase explained.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

First day of your last period

Average cycle length28 days

Calendar prediction is only moderately reliable, ovulation shifts month to month even in regular cycles. It is not adequate as contraception.

Likely ovulation

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How the Ovulation Calculator works

Ovulation is usually about 14 days before the next period starts, not 14 days after the last one began, a distinction that matters for anyone whose cycle is not 28 days. The fertile window spans roughly five days before ovulation and the day itself, because sperm survive far longer than the egg.

Also known as: fertile window calculator · when am I ovulating · fertility calendar calculator

How the prediction is made

The calculation works backwards from the expected next period rather than forwards from the last one. The luteal phase, between ovulation and the next period, is relatively consistent at 12 to 16 days in most women, averaging 14.

The follicular phase, from the period to ovulation, is the variable part. It can be a week or three weeks, and it is what makes cycles of different lengths different.

So ovulation is estimated as roughly 14 days before the next period is due, not 14 days after the last one began. For a 28 day cycle those coincide; for a 35 day cycle they are a week apart, which is why the standard advice misleads anyone whose cycle is not 28 days.

The fertile window

Sperm survive in the female reproductive tract for up to five days in favourable conditions. An egg survives roughly 12 to 24 hours after release.

That asymmetry means the fertile window runs from about five days before ovulation to one day after, giving roughly six days. Intercourse after ovulation has essentially no chance of conception.

The probability is not uniform across those six days. It is highest in the two days before ovulation and on the day itself, and considerably lower at the start of the window.

Why calendar prediction is unreliable

Cycle length varies between women and within the same woman between cycles. Studies tracking ovulation directly have found that only a minority of women ovulate on day 14, and that the timing varies substantially even in women who describe their cycles as regular.

Stress, illness, travel, shift work, weight change and intensive exercise all shift ovulation, sometimes by a week or more.

Which means a calendar-based prediction identifies a probable window rather than a date. For couples trying to conceive that is usually good enough as a starting point. For anyone trying to avoid pregnancy it is not remotely good enough.

Signs that indicate ovulation more directly

Cervical mucus changes predictably: it becomes clearer, more slippery and stretchier in the days approaching ovulation, resembling raw egg white. That change is one of the more reliable non-technological indicators.

Basal body temperature rises by roughly 0.3 degrees Celsius after ovulation and stays elevated. It confirms that ovulation happened rather than predicting it, which limits its use for timing but makes it useful for learning your own pattern.

Ovulation predictor kits detect the luteinising hormone surge that precedes ovulation by 24 to 36 hours, which does predict rather than confirm. They are the most reliable home method and they can produce false positives in polycystic ovary syndrome.

When to seek advice

General guidance is to consult a doctor after twelve months of trying without conceiving, or after six months if the woman is over 35, because time matters more with age.

Earlier consultation is appropriate where there are known risk factors: irregular or absent periods, known endometriosis or polycystic ovary syndrome, previous pelvic surgery or infection, or a known male factor.

This calculator estimates a window from dates and nothing more. It cannot detect whether ovulation is occurring, and anovulatory cycles produce periods that look normal. Persistent difficulty conceiving is a reason to see a clinician rather than to keep refining the timing.

Where to go next

The Ovulation question rarely arrives on its own. These are the ones that usually come with it:

Period Calculator — Next several periods from your cycle length.
Conception Date Calculator — Estimated conception date from a due date or last period.
Pregnancy Due Date Calculator — Due date, current week, and trimester milestones.
BMI Calculator — Body mass index with category and healthy weight range.

Sources

Wilcox et al., BMJ 2000, the fertile window (PubMed)

Not medical advice. This calculator gives a general estimate, not medical advice. It cannot account for your individual health, medical history, or medication. Talk to a qualified healthcare professional before acting on any result.

Frequently asked questions

When do I ovulate?

Roughly 14 days before your next period. On a 28-day cycle that is day 14, but on a 35-day cycle it is day 21. The luteal phase after ovulation is relatively fixed at 12-16 days, while the follicular phase before it varies.

How long is the fertile window?

About six days: the five days before ovulation plus ovulation day. Sperm survive up to five days in fertile cervical mucus, while the egg is viable for only 12-24 hours, which is why the window sits mostly before ovulation rather than after.

How accurate are calendar-based predictions?

Only moderately. Ovulation timing varies month to month even in regular cycles, and one study found the predicted fertile window is accurate for a minority of cycles. Ovulation predictor kits, basal body temperature or cervical mucus tracking are considerably more reliable.

Can I get pregnant outside the fertile window?

It is much less likely but not impossible, because ovulation can occur earlier or later than predicted. Calendar-based methods are not reliable contraception on their own.