The Goldilocks Screw Gauge™ is precision milled stainless steel probe graduated in six "steps" of increasing diameter. Insert the gauge through the barrel's bottom thread leaf until it feels snug. Note which "step" is in the barrel. Compare that 'Step " to the sizing chart provided to determine the proper replacement screw diameter. The colors on the chart correspond with most major screw suppliers' color code on the cap of the screw vial. If your color coding is different, their is a chart on the reverse that you can customize with your own color code.