How To Calculate Average Monthly Rainfall

Knowing the average monthly rainfall for a location is helpful when you're packing for a trip or considering relocating your family. Or maybe you just want to know how much it rains in your back yard. Finding the average monthly rainfall in any location is a relatively simple and straightforward calculation if you have the proper data.

Step 1

Gather the monthly rainfall data for the desired location from a website such as the National Climatic Data Center. Because of year-to-year fluctuations in precipitation, you'll want to gather at least 10 consecutive years of data for any one month to give you a better idea of the location's monthly average.

Step 2

Add together all of the monthly rainfall totals in your sample data. You'll be adding measurements in inches because rainfall is generally measured in inches in the United States.

Step 3

Divide by the number of years in your data set to arrive at the average monthly rainfall for any location. An example of the complete calculation for a 10-year average monthly rainfall should look like this: 3.2 (inches of rain) + 3.1 + 2.9 + 3.7 + 2.9 + 4.1 + 3.5 + 2.8 + 2.9 + 1.7 = 30.8 inches of rain, divided by 10 years = 3.08 inches of average monthly rainfall.

Step 1

Set up a rain gauge in any open location where trees or the overhang on a house or other building won't affect how much rain is gathered. You can buy a rain gauge at websites such as Ambient Weather.

Step 2

Take daily rainfall readings at the same time each day. You must gather the data every 24 hours to have accurate information.

Step 3

Complete the calculation in Section 1 at the end of the month to derive the average rainfall for your location.

Step 4

Continue to compile rainfall data for a longer period of time to arrive at a more precise average monthly rainfall total for your location.