Ovulation Calculator

Estimate your ovulation day and fertile window from the first day of your last period and your average cycle length. See ovulation, fertile days, and the next period date for the coming three cycles.

First day of your last period

Average cycle length (days)

Luteal phase length (days)

Most cycles are 21–35 days. The luteal phase (ovulation to next period) is about 14 days for most people — leave it at the default if you're unsure.

Understanding the Ovulation Calculator

The Ovulation Calculator estimates your most fertile days from two simple facts: the first day of your last menstrual period and your average cycle length. Because ovulation is tied to the days before your next period rather than the days after your last one, the tool works backwards from your predicted period to pinpoint ovulation, then maps the surrounding fertile window. It projects the next three cycles so you can plan ahead, showing ovulation day, the fertile range, and the expected start of each upcoming period. Everything runs in your browser, nothing is stored or sent anywhere, and the result is an educational estimate for family planning — not a contraceptive method or medical advice.

How it works

The calculator splits your cycle into two phases. The follicular phase runs from the first day of your period up to ovulation and varies in length. The luteal phase runs from ovulation to the next period and is more consistent, averaging about 14 days. To find ovulation it subtracts the luteal phase from your cycle length and counts that many days from the start of your period. The fertile window covers the five days before ovulation plus ovulation day, because sperm can survive up to about five days in the body. The next period is simply your cycle length added to the period start, and the whole pattern repeats for three cycles.

Ovulation = LMP + (cycle length − luteal phase) · Fertile window = (ovulation − 5 days) to (ovulation + 1 day) · Next period = LMP + cycle length

Worked example

Say your last period started on June 1 with a 28-day cycle and a 14-day luteal phase. Ovulation is estimated at June 1 + (28 − 14) = day 14, around June 15. The fertile window opens five days earlier on June 10 and closes June 16. Your next period is predicted for June 1 + 28 days = June 29, which starts the following cycle. The tool then repeats this for July and August so you can see ovulation and fertile days for three cycles at a glance.

Tips & common mistakes

Track several cycles and use your real average length rather than assuming 28 days — even a few days' difference shifts ovulation noticeably.
If you know your luteal phase from temperature or LH-test charting, enter it; otherwise leave the 14-day default, as it suits most people.
Plan intercourse across the whole fertile window, not just ovulation day — the days leading up to ovulation are often the most fertile.
Irregular, very short, or very long cycles make calendar estimates less reliable; consider ovulation predictor kits or basal body temperature tracking.
Do not rely on these dates to avoid pregnancy — fertile-window timing varies and is not a contraceptive method.
See a healthcare provider if your cycles are consistently outside the 21–35 day range or you have been trying to conceive without success.

Sources & methodology

• American College of Obstetricians and Gynecologists (ACOG) — guidance on the menstrual cycle and fertility awareness
• Office on Women's Health (U.S. Department of Health and Human Services) — ovulation and fertile window basics
• NICE / NHS guidance — fertile window spans roughly the six days ending on ovulation day; luteal phase averages about 14 days

Reviewed by the TopOpenTools editorial team · Last updated June 2026. These tools provide general estimates for educational purposes only and are not financial, tax, insurance, investment, or medical advice. Verify important decisions with a qualified professional.

How to Use This Calculator

1Select the first day of your last period using the date picker.
2Set your average cycle length (and luteal phase if you know it), then click Estimate Ovulation.
3Review your fertile window for the next three cycles — then track daily needs with the BMI calculator.

Frequently Asked Questions

When do I ovulate?

Ovulation usually happens about 14 days before your next period starts — not 14 days after your period begins. For a 28-day cycle that falls around day 14, but if your cycle is longer or shorter the date shifts accordingly.

How long is the fertile window?

The fertile window spans roughly six days: the five days before ovulation plus ovulation day itself. Sperm can survive up to about five days, so intercourse in the days leading up to ovulation can still result in pregnancy.

Is this reliable for birth control?

No. This calculator gives an estimate based on average cycle math, but real ovulation varies from cycle to cycle. It is a family-planning aid, not a contraceptive method. Use a proven birth-control method if you are trying to avoid pregnancy.