Paleontologists first measure the distance between footprints in a trackway (stride length), as well as the length of the footprints themselves. The estimated leg length is then determined by multiplying the footprint length by a known constant. For example, the constant for theropods is approximately 4.5. The relative stride length is determined by dividing the measured stride length by this estimated leg length. The resulting number would be referenced to a standard graph to obtain a dimensionless speed for the particular dinosaur. To get a real world speed, this dimensionless speed is used, along with the estimated leg length and the acceleration of gravity. This results in a more familiar number: speed in miles per hour or kilometers per hour.