Least count is the smallest possible measurement that a tool or measuring instrument can resolve. For example, if a measuring tape has graduations of 1/8” then the least count would be 1/8”. As the least count of a measuring tool gets smaller, it is capable of taking more accurate and precise measurements. A measuring tool such as a micrometer or digital caliper will have a smaller least count than a standard ruler. This means it is able to measure in smaller increments and therefore is more accurate.

## What is the formula for calculating least count?

Least count is calculated by taking a known interval and dividing by the number of divisions.

Least count = known interval / number of division

In the case of the ruler above, in each 1 inch interval there are eight divisions. This means that the least count of the inch ruler is 1/8”.

For the mm side of the ruler, in each 1 mm interval there are ten divisions. This means that the least count of the mm ruler is 0.1mm.

## Micrometer least count formula

The formula for calculating the least count of a micrometer is:

Least count of micrometer = least count of main scale / number of divisions on secondary scale

This is slightly different than our ruler example above. This is because a micrometer usually will have a secondary scale. The primary scale is along the sleeve of the micrometer while the secondary scale is around the thimble.

For most inch micrometers, the least count of the main scale is 0.025” and the number of divisions on the secondary scale is 25 therefore the least count would be 0.001”. Some micrometers have a tertiary scale around the sleeve. This third scale has 10 divisions. This allows the secondary scale to be read with greater accuracy. The least count of the secondary scale (0.001”) / number of divisions on tertiary scale (10) = 0.0001”.

For most metric micrometers, the least count of the main scale is 0.5mm and the number of divisions on the secondary scale is 50 therefore the least count would be 0.01mm.

## Least count of vernier caliper

The formula for calculating the least count of a micrometer is:

Least count of vernier caliper = least count of main scale / number of divisions on secondary scale

For most inch vernier calipers, the least count of the main scale is .025” and the number of divisions on the secondary scale is 25 therefore the least count would be .001”.

For most metric vernier calipers, the least count of the main scale is 1mm and the number of divisions on the secondary scale is 50 therefore the least count would be .02mm.

## Least count of common length measuring instruments

Measuring instrument | Least count in inches | Least count in mm |

Micrometer | .0001" | .001mm |

Digital Micrometer | .0001" | .001mm |

Dial Caliper | .001" | .01mm |

Digital Caliper | .0005" | .01mm |

Vernier Caliper | .001" | .02mm |

Ruler | .100" | .1mm |

Measuring tape | 1/16" | 1mm |

Height gauge | .001" | .01mm |

Dial indicator | .001"/.0001" | .01mm/.001mm |

Spherometer | .0001" | .001mm |

Micrometer screw | .0001" | .001mm |

## Related articles

For more information see these related articles: