Use the old one
To measure your body for optimal belt size, start by threading the tape measure through the belt loops of your trousers. It’s important to stand in a relaxed, natural position–do not push out your belly or suck it in–when taking the measurement. The measuring tape should be snug. Measure in centimeters and round to the centimeter. Choose the belt size closest to this measurement.
Attention: Over time, leather belts tend to stretch. It’s good to keep in mind that a belt that fits snug initially will likely wear more comfortably down the road.