An ovulation calculator is a simple tool that helps in the calculation of fertile days for a particular menstrual cycle. The accuracy of an ovulation calculator may be off sometimes, but most of the times you can determine the ovulation day or date with the help of this simple calculator.

Your fertile window—the best time to get pregnant—starts on the 20th day (on average) of your menstruation cycle and lasts for up to 6 days. If this is not your typical ovulation cycle, chances are you may still be able to conceive as the fertile window is of about 6 days; if you include the lifespan of the sperm, it gives you a pretty good chance of conceiving.

Here’s a simple formula to calculate the ovulation day:

First day of the fertile window = (Number of days in your menstrual cycle) − 20 (average ovulation cycle) + (First day of the last period)

For example, if you started your period on 5th February and your menstruation cycle is 26 days long, then the first day of your fertile window will be:

24 − 20 + 5 = 11, i.e., 11th February

Since the fertile window is usually 6-day long, 11th to 17th February will be your intercourse window for pregnancy.

*Approximate ovulation date is the last day of the fertile window, which is 17th February in the above-mentioned example.

Let’s assume you started your period on 24th April and your menstruation cycle is 30 days long, then the first day of your fertile window will be:

30 − 20 + 24 = 34, i.e., 4 days after 30th April or 4th May

The fertile window will be from 4th to 10th May, and 10th May will be the approximate ovulation date.

You can use this simple ovulation date calculator to find the day of ovulation and the fertile window, but there is no such thing as ‘100 percent accurate ovulation calculator’ because many factors affect the ovulation. Please note if you are having any trouble conceiving, it is better to consult a gynaecologist.