Ovulation Calculator

Ovulation date and fertile window calculator (based on last period and cycle length)

This ovulation calculator is designed for one practical job: estimate your likely ovulation day and your fertile window using the first day of your last period and your typical cycle length. Most people search for this when they want a quick, reasonable estimate to plan intercourse timing, understand cycle patterns, or decide when to start using ovulation tests. The output is an estimate, not a diagnosis, but it is usually good enough for planning purposes when your cycles are fairly regular.

The calculator works by estimating when your next period would start, then counting backward by your luteal phase length (the time from ovulation to the next period). Many cycle trackers use a default luteal phase of about 14 days, so this calculator does the same unless you choose a different value. Once ovulation is estimated, it calculates a fertile window that starts five days before ovulation and ends one day after, because sperm can survive for several days and the egg’s viable window is short.

For day-to-day use, you only need one required input: the first day of your last period. If you do not know your cycle length, use a reasonable default of 28 days. If you have more accurate data (for example, you consistently have a shorter or longer cycle, or you know your luteal phase from testing), add it in the advanced field to tighten the estimate. The results include the estimated ovulation date, the fertile window date range, and the approximate next period start date so you can sanity-check whether the estimate fits your typical pattern.

Assumptions and how to use this calculator

• This calculator assumes you are estimating ovulation for the current cycle using the first day of your last period (LMP) as day 1.
• Cycle length is treated as the number of days from the start of one period to the start of the next. If you are unsure, 28 days is used as a practical default.
• Luteal phase length defaults to 14 days unless you specify another value. This directly shifts the ovulation estimate earlier or later.
• The fertile window is estimated as ovulation minus 5 days through ovulation plus 1 day. This is a planning range, not a guarantee of fertility on every day.
• Irregular cycles, recent hormonal contraception, breastfeeding, postpartum changes, illness, travel, or high stress can shift ovulation. If that applies, treat results as a rough guide and rely on tests or clinical advice for accuracy.

Common questions

Is this calculator accurate if my cycles are irregular?

Not reliably. Irregular cycles reduce the usefulness of any method that depends on an “average” cycle length. If your cycle varies by many days month to month, the estimated ovulation date can be off by a week or more. In that situation, use the output only as a loose planning range and consider ovulation predictor kits (LH tests), cervical mucus observations, or basal body temperature tracking to identify ovulation more directly.

What if I do not know my average cycle length?

Use 28 days as a starting estimate. Then compare the “next period start” date shown in the results to what typically happens for you. If your period often arrives earlier, reduce the cycle length. If it often arrives later, increase it. Even a small adjustment (for example, 26 vs 28 days) can shift the estimated ovulation date by two days, which matters when you are timing a fertile window.

What does luteal phase length mean, and should I change it?

The luteal phase is the time between ovulation and the start of the next period. Many people are close to 14 days, which is why it is the default. If you have tracked ovulation with tests or temperature and noticed a consistent pattern (for example, your period starts about 12 days after ovulation), entering that value will make this calculator more consistent with your body. If you do not know it, leave it blank.

Why is the fertile window several days long if ovulation is one day?

Because sperm can survive inside the reproductive tract for multiple days, while the egg is viable for a much shorter time after ovulation. The highest probability is usually in the days leading up to ovulation and the day of ovulation itself. The window is intentionally broader to reflect normal biological variability and to keep the tool useful for planning.

Does this calculator confirm ovulation or pregnancy?

No. It only estimates dates based on averages and assumptions. It does not confirm that you ovulated, and it cannot confirm pregnancy. If you are trying to conceive, treat it as a planning aid and consider tests (LH tests for ovulation, pregnancy tests for pregnancy). If you are trying to avoid pregnancy, do not use this estimate as birth control. Use reliable contraception and professional guidance.