Step-by-step guide to accurately measuring magnet wire diameter.

A micrometer is the most important hand tool in a motor winders toolkit. Calipers are generally not precise enough to differentiate between adjacent wire gauges, which might only differ by a few hundredths of a millimeter.

When measuring magnet wire, you must first remember that the wire is coated in a thin layer of enamel insulation (often polyurethane or polyesterimide). This enamel adds artificial thickness to the wire.

To get the true copper diameter, use a lighter to burn off the enamel at the very tip of the wire, then wipe away the ash with fine sandpaper or a cloth. Place the bare copper between the micrometer anvils and gently tighten the thimble until the ratchet clicks. Compare this exact millimeter reading against a standard SWG or AWG chart.