I went to Home Depot to pick up some utility shelves to store things in the garage, then read that there were two options for what I wanted - 18" and 24" depth.
I wasn't sure which one to get, and wanted to get a visual sense. First I estimated with my feet, but I wanted a more accurate method. I figured I was in Home Depot and there was a measuring tape somewhere. I remembered where I knew there was one. So I walked from aisle 6 to aisle 19 and stared at the measuring tape, made a decision, and came back to get the 24" option. When I got back I realized there was a better method that would have saved me the trip to aisle 19 and gotten me a better visual: