Editing Micrometer (device) (section)

===Customary/Imperial system===
[[File:276inch-micrometer.jpg|thumb|Imperial unit micrometer thimble showing a reading of 0.2760&nbsp;in. The main scale reads 0.275&nbsp;in (exact) plus 0.0010&nbsp;in (estimated) on the secondary scale (the last zero is an estimated tenth). The reading would be 0.2760&nbsp;±&nbsp;0.0005&nbsp;in, which includes plus/minus half the width of the smallest ruling as the error. Here it has been assumed that there is no zero point error (often untrue in practice).]]
The spindle of a micrometer graduated for the [[Imperial and US customary measurement systems]] has 40 threads per inch, so that one turn moves the spindle axially 0.025&nbsp;inch (1 ÷ 40 = 0.025), equal to the distance between adjacent graduations on the sleeve. The 25 graduations on the thimble allow the 0.025&nbsp;inch to be further divided, so that turning the thimble through one division moves the spindle axially 0.001&nbsp;inch (0.025 ÷ 25 = 0.001). Thus, the reading is given by the number of whole divisions that are visible on the scale of the sleeve, multiplied by 25 (the number of [[thousandth of an inch|thousandths of an inch]] that each division represents), plus the number of that division on the thimble which coincides with the axial zero line on the sleeve. The result will be the diameter expressed in thousandths of an inch. As the numbers 1, 2, 3, etc., appear below every fourth sub-division on the sleeve, indicating hundreds of thousandths, the reading can easily be taken.

Suppose the thimble were screwed out so that graduation 2, and three additional sub-divisions, were visible on the sleeve (as shown in the image), and that graduation 1 on the thimble coincided with the axial line on the sleeve. The reading would then be 0.2000 + 0.075 + 0.001, or 0.276&nbsp;inch.