From:
https://www.hunker.com/12001429/how-to-choose-rivets
"According to The Sheet Metal Handbook, the length of a rivet is measured from the underside of the head to the tip of the stem. The head itself is not included in this measurement. The length of the rivet should be equal to the thickness of both objects you are fastening, plus 1.5 times the diameter of the rivet's stem. For example, a 1/2-inch diameter rivet being used to fasten two one-inch thick plates will have to be 2 3/4 inches long."
This matched what I remembered but I wanted to be able to give you a source.