You need the equipment which can measure the fat % like what the other have posted. There is no way to do manual calculation like BMI stuff. I used to have the thought it can be calculated using some math equation but the answer is no. The gym will have the weight measuring device capable to measure the body fat %. I usually measure it when I go to gym.

12~15% very rip built up already. If device showing 12~15% but don't have visible abs, can through the device into dustbin already.