Ovulation Calculator

Estimate your ovulation day and fertile window from your last period and cycle length — plus your next period and a due date if you conceive. Runs in your browser.

This is an estimate for general information and planning only, not medical advice. Cycles vary and ovulation timing is not exact. If you are trying to conceive or have concerns about your cycle, consult a healthcare provider.

How your fertile window is estimated

Ovulation — when an ovary releases an egg — happens about 14 days before your next period starts, because the luteal phase (the time between ovulation and menstruation) stays fairly constant at around 14 days. So on a regular 28-day cycle you ovulate near day 14, and on a longer cycle it comes later. The calculator estimates ovulation as your last period plus your cycle length minus 14. The fertile window is the six-day span ending on ovulation day: the five days before, plus the day itself. Sperm can survive up to about five days, so intercourse in this window is what most often leads to conception, even though the egg itself only lives for roughly a day.

Ovulation, next period and a possible due date

The results above show the estimated ovulation day, the fertile window as a date range, the date your next period is expected (your last period plus a full cycle), and — if you conceive during this cycle — an estimated due date, counted as 266 days from ovulation. These are averages: real ovulation can shift by several days from month to month. Ovulation predictor kits, basal body temperature charting and cervical mucus tracking give a more personal picture.

Frequently Asked Questions

How is the fertile window calculated?
Ovulation is estimated by counting back about 14 days from your next expected period — the luteal phase is fairly constant — so with an average cycle it falls near the middle of your cycle. The fertile window is the ovulation day plus the five days before it, giving a six-day span. This works because sperm can survive in the reproductive tract for up to about five days, while an egg is viable for roughly 12 to 24 hours after release, so intercourse in the days leading up to ovulation is what most often results in conception.
How accurate is an ovulation calculator?
An ovulation calculator gives an estimate based on cycle averages, not a measurement of your body. It assumes a roughly regular cycle and a typical 14-day luteal phase, so the true ovulation day can vary by several days, especially if your cycles are irregular, you are stressed or ill, or you are approaching menopause. Tracking methods such as ovulation predictor kits (which detect the LH surge), basal body temperature charting, and cervical mucus observation pinpoint ovulation more accurately for your own body.
When is the best time to conceive?
The days most likely to lead to pregnancy are the two to three days immediately before ovulation and the day of ovulation itself. Many couples aim for intercourse every one to two days across the fertile window so that sperm are present when the egg is released. Because the egg only lives about a day, timing before ovulation tends to work better than trying immediately after.
Does cycle length change the ovulation day?
Cycle length changes the estimated ovulation day directly. Ovulation is tied to when your next period will start, not to a fixed day 14. Because the luteal phase stays close to 14 days, a longer cycle pushes ovulation later and a shorter cycle brings it earlier. On a 28-day cycle ovulation is around day 14, while on a 32-day cycle it is closer to day 18. That is why entering your own average cycle length gives a better estimate than assuming 28 days.
Is this a substitute for medical advice?
This ovulation calculator is not a substitute for medical advice. It gives an estimate for general information and family-planning purposes only. It is not medical advice and does not replace care from a qualified professional. Cycles vary from person to person and month to month, and ovulation timing is not exact. If you are trying to conceive, having trouble conceiving, or have concerns about your cycle, speak with your own healthcare provider.

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