Ovulation Calculator — find your fertile days

How it works

A menstrual cycle has two parts. The follicular phase (from your period to ovulation) varies in length, but the luteal phase (from ovulation to your next period) is fairly constant at about 14 days. That is why ovulation is estimated by counting back 14 days from your next expected period — or equivalently, last period + (cycle length − 14).

Per the Office on Women's Health, the average cycle is 28 days (normal range 21–35). On a 28-day cycle that puts ovulation around day 14; on a shorter cycle it is earlier, and on a longer cycle later.

Your fertile window is wider than the single ovulation day. Sperm "can live for up to three days" in the body, and the egg stays fertile for about a day after release, so the calculator marks the window from roughly 3 days before ovulation to 1 day after. Having intercourse during this window gives the best chance of conception.

This is a calendar estimate. Real ovulation shifts with cycle variation, stress and illness; ovulation predictor kits or basal body temperature tracking pinpoint it more precisely.

Worked example

For a last period starting January 1, 2026 on a 28-day cycle, ovulation is estimated on Jan 15, 2026 (day 14). The fertile window runs from Jan 12, 2026 to Jan 16, 2026, and the next period is expected around Jan 29, 2026.

Frequently asked questions

How do I calculate my ovulation day?

Add your cycle length to the first day of your last period, then subtract 14 days — the length of the luteal phase. For a 28-day cycle starting January 1, ovulation is around January 15.

When am I most fertile?

In the days just before and including ovulation. Because sperm can survive up to three days and the egg about one day, the fertile window spans roughly three days before ovulation through one day after. That is the best time to try to conceive.

Why is ovulation counted back from the next period?

The luteal phase — from ovulation to the next period — is fairly constant at about 14 days, while the earlier follicular phase varies. Counting back 14 days from the expected period is therefore more reliable than counting forward from the last one.

Does cycle length change when I ovulate?

Yes. Longer cycles push ovulation later and shorter cycles bring it earlier, because the extra or missing days fall in the follicular phase before ovulation. The 14-day luteal phase stays about the same.

How accurate is a calendar estimate?

It is a good starting point but not exact. Ovulation can shift with cycle irregularity, stress or illness. Ovulation predictor kits and basal body temperature charting detect it more precisely.

Can this help me avoid pregnancy?

Calendar estimates are not a reliable form of contraception on their own because ovulation timing varies. If you are avoiding pregnancy, use a proven contraceptive method and talk to a healthcare provider.