Do it against a wall in a gym. Get one side of your body against the wall with your arm stretched as high as you can reach with both feet flat on the floor, and make a measurement of how high that mark is. Then space yourself from the wall maybe two or three inches. Stand with both feet shoulder width apart or something similar to that, and leap as high as you can straight up use chalk or wet your finger tips to leave a mark on the wall. Measure this height then do the math this is how you do it without those special vertical leap measuring things. If you take a run at it or even one or two steps it's not a true vertical leap measurement, it must be from a stand still off both feet.