To calculate the number of jellybeans in a jar, count the number of jellybeans on the top layer of the jar. Then, count the number of jellybeans going down the side of the jar, and multiply that number by the number of jellybeans in the top layer to get an estimate of the number of jellybeans in the jar.
Another way to estimate the number of jellybeans in a jar is to make a guess based on the volume of the jar. There are approximately 930 jellybeans in a gallon. Look at the jar to estimate its volume, and make a guess accordingly. For instance, if the jar is about the size of a quart, there would be approximately 233 jellybeans in the jar. This is because a quart is 1/4 gallon, and 930 jellybeans divided by four is roughly 233.
Jellybeans can be irregularly shaped, and therefore, they may not fill the jar tightly. Rather, there may be air holes or gaps in the jar. If estimating the number of jellybeans in a jar based on the estimated volume of the jar, it is best to multiply the estimate by 80 percent. That leaves a 20 percent margin of error for empty space in the jar.