Cut the ruler in half. It is best to use a 6-inch ruler; rarely does more than 6 inches of rain fall at one time. When you choose a ruler, be sure it is plastic, as wood will not hold up in the rain and outdoor environment.
Place the ruler on the outside of a clear plastic cup with a rubber band. Line up the bottom of the ruler with the bottom of the cup. Be sure to attach the ruler so the numbers are facing out and it is aligned to measure from the bottom up. This is where you will take your reading. Most of the time, teachers ask students to measure rainfall in terms of the nearest 1/4 inch. Another way to attach the ruler is by using clear packaging tape.
Write the measurements on the cup. If you prefer not to use a ruler for this gauge, you can draw the measurements on the cup with a black permanent marker. If you do this, begin at the bottom of the cup and write in measurements up to 3 inches, marking every inch, 1/2 inch and 1/4 inch. Wrap a piece of clear tape over the markings to prevent them from being washed away by the rain.
Set the cup outside. If necessary, prop the cup with rocks in it or around it.
Check the rain gauge. After a rainfall, grab the cup and determine how much rain occurred by reading either the ruler or the markings on the cup.