Fertility Calculator

A fertility calculator can help you figure out when you will likely ovulate, and therefore be the most fertile, during your menstrual cycle. This is useful when you want to keep track of the fertile time of your cycle whether you are trying to conceive or avoid getting pregnant.

Using the Calculator

An online digital fertility calculator simplifies the task of estimating your ovulation day, which is the day you are the most fertile. To use the calculator above:

  1. In the first three boxes, select the month, the date of the first day, and the year of your last period from the drop-down lists.
  2. In the next box, enter in days the average length of your menstrual cycle.
  3. Click "Calculate." This will produce the most likely date of your next ovulation.
  4. Click the "Reset" button to perform a new calculation.

How It Works

The calculator counts back 14 days from the date of your next expected period to determine your ovulation day for the current cycle. It works based on the following assumptions and facts about the menstrual cycle:

  • If your cycles are regular, you can assume the date you will get your next period by counting forward by the number of days of the average length of your cycles.
  • The most predictable part of your cycle is the second half (the luteal phase) after ovulation.
  • No matter how long the first half of your cycle is, you will get your next period 12 to 14 days after the day you ovulate.
  • By counting backwards 12 to 14 days from the expected date of your next period, you will arrive at the days you will most likely ovulate your egg for that cycle.

For simplicity, the calculator assumes a 14 day luteal phase and that your cycles are regular, lasting between 19 and 45 days in length.

Cycle Lengths and Ovulation

OBGYN explaining menstrual cycle

Keep the following in mind about cycle lengths and ovulation:

  • If your menstrual cycle lengths are regular, you will ovulate around the same day in your cycle each month, although this can vary by a few days.
  • If your cycles vary in length, the calculator is not a reliable predictor of when you might ovulate each cycle.

Regardless of the pattern of your cycle lengths, no method can precisely predict the day of ovulation. You can use other aids or fertility signs, such as an ovulation predictor kit, to help you narrow down your ovulation day, whether you are trying to get pregnant or avoid it.

Your Most Fertile Days

If your menstrual cycles last the average 28 days, you will ovulate around days 12 to 14. You are most likely to conceive if you have intercourse the day the calculator predicts you might ovulate and on the day or two before.

Note, however, that your chance of getting pregnant starts five days before ovulation - a six day fertile window - according to a revealing study published in 1995 in The New England Journal of Medicine. This is because sperm can survive up to five days in a woman's reproductive tract after ejaculation.

Simplify the Calculation

The fertility calculator makes it simpler and easier for you to narrow down the day you might ovulate in a cycle. This helps you time intercourse to improve your chances of getting pregnant or know when you have to be more cautious if you want to avoid getting pregnant.

Fertility Calculator