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Ovulation Calculator

Predict your ovulation date and fertile window based on the first day of your last menstrual period and your average cycle length. Understanding your fertile days can help you plan or prevent pregnancy.

The year your last menstrual period started.

The month your last menstrual period started (1-12).

The day of the month your last menstrual period started.

Your average menstrual cycle length in days. The standard is 28 days.

This calculator estimates when you are most likely to ovulate by subtracting 14 days from the end of your cycle. It then identifies your fertile window, which spans from five days before ovulation to one day after, giving you a six-day window when conception is most likely.

How It Works

Ovulation Estimation Formula

Ovulation Date = LMP + Cycle Length - 14 days

The estimated ovulation date is calculated by adding your cycle length to the first day of your last period, then subtracting 14 days. The fertile window spans from 5 days before to 1 day after ovulation.

Ovulation typically occurs 14 days before the start of the next period

Ovulation date = LMP + cycle length - 14 days

Fertile window starts 5 days before ovulation (sperm can survive up to 5 days)

Fertile window ends 1 day after ovulation (egg survives 12-24 hours)

Next period = LMP + cycle length

Important Notes:

  • •This method assumes ovulation occurs 14 days before the next period, which is an average
  • •The luteal phase (after ovulation) is relatively constant at about 14 days for most women
  • •Actual ovulation timing can vary by a few days even with regular cycles
  • •Ovulation predictor kits (OPKs) and basal body temperature tracking can provide additional confirmation

Worked Example

A woman whose last period started on April 1, 2026, with a 28-day cycle.

Inputs:

  • last Period Year:2,026
  • last Period Month:4
  • last Period Day:1
  • cycle Length:28

Result:

Her estimated ovulation date is April 15, 2026. The fertile window runs from April 10 to April 16. Her next period is expected around April 29, 2026.

Who Is This Calculator For?

  • women trying to conceive
  • couples planning pregnancy
  • women tracking their cycle

Frequently Asked Questions

In a standard 28-day cycle, ovulation typically occurs around day 14. However, ovulation actually happens about 14 days before the start of your next period, so women with longer or shorter cycles will ovulate on different days. For example, in a 32-day cycle, ovulation would occur around day 18.
The fertile window is approximately six days long: the five days before ovulation and the day of ovulation itself. Sperm can survive in the reproductive tract for up to five days, while the egg is viable for about 12 to 24 hours after release. The two to three days just before ovulation are the most fertile.
This calculator should not be relied upon as a contraceptive method. Ovulation timing can vary from cycle to cycle due to stress, illness, travel, and hormonal fluctuations. If you want to use fertility awareness for birth control, work with a healthcare provider and use multiple tracking methods together.
If your cycles vary significantly in length, predicting ovulation with a calculator alone is less reliable. Consider tracking ovulation with additional methods such as basal body temperature charting, cervical mucus monitoring, or over-the-counter ovulation predictor kits. Consult your doctor if your cycles are consistently irregular.
Common signs of ovulation include a slight increase in basal body temperature (about 0.4 to 1.0 degrees Fahrenheit), changes in cervical mucus that becomes clear and stretchy like egg whites, mild pelvic pain or cramping on one side (called mittelschmerz), and increased libido. Not all women notice these signs.
Yes. The follicular phase (before ovulation) varies in length, but the luteal phase (after ovulation) is relatively constant at about 14 days. So in a 30-day cycle, ovulation likely occurs around day 16, while in a 26-day cycle it would be around day 12. This calculator adjusts for your specific cycle length.

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Last updated: April 20, 2026