There is no formula for calculating the length of dog ramps. Every pet has their own limitations to what angle they can climb with any ramp. As a pet owner, you know your dog's abilities better than anyone. The easiest way to determine the length of dog ramp needed is to hold a tape measure at the top of the vehicle's floor/step and pull the tape measure tight to the ground at an angle. This will allow you to see the angle of the dog ramp to determine what length you will require. Another way to is to set a piece of wood up on the vehicle or step that is cut to the the length of the ramp in question. The telescoping dog ramp allows you to adjust the different lengths to not worry about the exact length of pet ramp you need. Please keep in mind the longer the dog truck ramp is, the easier it will be to use for your four-legged friend.