What is Least Count and how to calculate it

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Least count of an instrument is used to select right instrument for your applications. For example if you are selecting an instrument to measure a dimension 6.2 ± 0.25 mm. You can not use ruler. In this article we will discuss what is lease count and how to calculate it.

What is Least Count (L.C.) ?

Least Count of an instrument is the smallest measurement that can be taken using an instrument. In other words, it is the smallest change an instrument can measure. For example, using ruler shown below we can measure minimum 1 mm length. Therefore ruler L.C. is 1 mm.

Least Count vs Precision

L.C. is directly related to precision. If an instrument has small L.C., that means it is more precise.

For example, if you compare digital and mechanical vernier caliper. Digital Vernier L.C. is 0.01 mm. Whereas mechanical vernier L.C. is 0.02 mm. Therefore digital vernier is more precise compared to mechanical vernier. 

Least Count vs Accuracy 

L.C. is the minimum dimension that can be measured using an instrument. Whereas Accuracy is how close a measured value is to actual value when that value is measured repeatedly. Read this article for more details on accuracy.

How to Calculate Least Count?

L.C. for an instrument is calculated by dividing main scale reading with number of divisions.

Main Scale L.C. = Smallest Reading on Main Scale / Number of Divisions
Instrument L.C. = Main Scale L.C. / Divisions in secondary Scale

Examples
Scale Least Count Calculations

Main Scale Reading = 1 cm

Number of Divisions in main Scale = 10

L.C. (Scale) = 1 / 10 = 0.1 cm = 1 mm

Mechanical Vernier Caliper Least Count Calculations

Main Scale Reading = 1 cm

Number of Divisions in main Scale = 10

Main Scale L.C. = 1 / 10 = 0.1 cm

Number of Divisions in secondary Scale = 50

Vernier Scale L.C. = 0.1 / 50 = 0.002 cm

Vernier Scale Least Count = 0.1 / 50 = 0.02 mm

Mechanical Micrometer Least Count Calculations

Main Scale Reading = 5 mm

Number of Divisions in main Scale = 10

Main Scale L.C. = 5 / 10 = 0.5 mm

Number of Divisions in secondary Scale = 50

Micrometer Least Count = 0.5 / 50 = 0.01 mm

Conclusion

To sum up, least count of an instrument is the minimum value that we can measure with an instrument. It is used in selection of right instrument for your application. 

Got Questions?  We will be happy to help.

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