Finding out how much rain has fallen during a particular storm or period of time isn't nearly as difficult as one may think. Elementary and high school science students calculate rainfall totals accurately all the time, and you can too. Doing so requires little more than a few objects you can often find lying around your house.
Set a rain gauge or a can such as a coffee can outside your home. Leave any top off of the unit so that the inside is exposed. Make sure nothing like a tree or the overhang of your house is covering the container. The opening of the container should be open to the sky.
Record the dimensions of your container. Find the total height, as well as the length and width (for square containers) or diameter (for circular containers) using a standard tape measure. Write this information down on a piece of paper.
Wait for it to rain. As it rains, your rain collection container will fill up with water.
Measure the height of the water in your container. If you're using a rain gauge, marks on the side of the gauge itself can be used to find this measurement. Most rain gauges measure the rain in inches, centimeters and millimeters. If you're using a coffee can, use a tape measure to find the distance between the bottom of the can and the top water level inside. This depth, in inches, is equal to the amount of inches of rain that fell during the storm.
Calculate how much rainfall is in your container. For circular containers, for example, divide the diameter of the container by 2 to find the radius. Multiply the radius by itself, multiply that number by the number of inches of rain in the container and multiply that by 3.14 to find the total number of cubic inches of rain in your container. For a square container, multiply the length and the width by the number of inches of rain in the container to find the square inches of rain inside the container.