Using an Ovulation Calculator to Predict Your Fertile Days

Updated May 15, 2019
woman using calendar on digital tablet

If you're trying to get pregnant, then the ability to predict when you ovulate can be very helpful. This ovulation calculator can make your task easier.

Ovulation Calculator

This calculator can predict when you are most likely to become pregnant. Ovulation occurs when a woman's ovary releases an egg. This usually happens about once a month. Women are most likely to become pregnant if they engage in sexual intercourse in the days around ovulation.

Making the Calculation

Use the calculator below to determine when you will most likely ovulate.

Using the Calculator

Women who are trying to become pregnant can use this ovulation calculator to determine when sexual intercourse is most likely to result in pregnancy. However, if you are trying to avoid pregnancy, you shouldn't use the ovulation calculator as your only source. To avoid pregnancy, use reliable birth control.

Information You Need

Before using this calculator, you will need some basic information:

  • The start date of your last menstrual period
  • The average length of your cycle in days

If you don't know the exact start date of your last period, it is important you begin tracking it in order to get the most accurate results on this calculator. Likewise, you may wish to monitor periods for a few months and average the number of days they last.


On average, ovulation occurs about 13-15 days before a woman's period starts. The calculator uses this information plus your own cycle length to estimate when you will ovulate.

  • The calculator supplies the date you are most likely to ovulate based on a 14-day luteal phase.
  • The ovulation date may not be exact.
  • Sperm remain active in a woman's body for up to five days after intercourse.
  • Eggs live about 12 to 24 hours after leaving your ovary.
  • You are most fertile about three days before and one day after you ovulate.
  • Start a few days before the date suggested and continue a few days past it to increase your chances of getting pregnant.


The accuracy of this ovulation calculator depends on how regular your menstrual cycles are and whether you ovulate at the expected time.

  • This calculator provides an educated guess about when ovulation will occur.
  • The calculator will work best if you have a regular menstrual cycle lasting around the average of 28 days.
  • Women with much shorter or longer cycles, or those with irregular cycles, will experience less reliability using this calculator.
  • Ovulation within a cycle can vary from one woman to another, and even from cycle to cycle.
  • Use this calculator in conjunction with other methods to recognize the signs of ovulation, such as charting changes in your body temperature, observing changes in vaginal mucus, or performing monthly urine tests. By combining multiple methods, you may be more likely to predict ovulation accurately.

March of Dimes Ovulation Calendar

The March of Dimes website offers a very informative, easy-to-use ovulation calendar which will help pinpoint when you are most fertile and when ovulation may occur. When using this ovulation calendar, you will enter:

  • The first day of your last period
  • How many days there are in your cycle
  • How many days you have your period
  • Then click on 'calculate'

Various icons will then display on the calendar to indicate the days you have your period, the days your are most fertile and your prime ovulation day.

The Start of a Healthy Pregnancy

While this calculator can help you determine when you are likely to get pregnant, it is no substitute for your doctor's advice. Talk to your doctor before you plan a pregnancy in order to ensure your baby the best possible start.

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Using an Ovulation Calculator to Predict Your Fertile Days