Simply insert gauge into each tank until it touches the bottom, place finger firmly over the opening at the top and remove to read usable gallons. Calibrated for the newer Cessna 182s with steel tanks holding 43.5 gallons of usable fuel per tank. All others Cessna 182 aircraft owners should use the UNIVERSAL FUELHAWK and calibrate it to their specific tank size. Calibration cards, and simple instructions are supplied with each gauge. Measurements are in US Gallons. Gauge is 11 inches long.