Micrometre. View of a micrometre screw gauge, a tool used to measure the size of small objects with very high precision. The user has three scales to read - the first is on the inner spindle of the micrometre and is used for the largest units (typically reading to the nearest 0.5mm), the next is in the barrel of the micrometre (reading to the nearest 0.01mm), finally a vernier scale on the spindle gives a reading to the nearest 0.001mm, or micron.