The least count of any measuring equipment is the smallest quantity that can be measured accurately using that instrument. Thus Least Count indicates the degree of accuracy of measurement

Measurement

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

 that can be achieved by the measuring instrument.

All measuring instruments used in physics have a least count. A meter ruler's least count is usually 0.1 centimeter; an electronic scale commonly has a least count of 0.001g, although this may vary; a vernier

Vernier scale

A vernier scale is an additional scale which allows a distance or angle measurement to be read more precisely than directly reading a uniformly-divided straight or circular measurement scale...

 caliper

Caliper

A caliper is a device used to measure the distance between two opposing sides of an object. A caliper can be as simple as a compass with inward or outward-facing points...

 has a least count of 0.02 millimeters frequently, although this too may vary; and micrometer screwgauge's least count is usually 0.001 millimeter and of course a conventional ruler has .01m frequently.

The least count is the discrimination of a voltmeter

Voltmeter

A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to...

. All measuring instruments used in the subject of physics can be used to measure various types of objects, but all do so without considering the detail of accuracy.

No measuring instrument used in physics is accurate and always has an error when readings are taken. Even the latest technology used in measuring objects also have an error where reading are concerned. Various names can be given to this error. The least count, uncertainty or maximum possible error are the terms normally used in a physics course, although this may vary with different syllabuses.

The error made in an instrument can be compared with another by calculating the percentage uncertainty of each of the readings obtained. The one with the least uncertainty is always taken to measure objects, as all measurements are required with accuracy in mind. The percentage uncertainty is calculated with the following formula: *100

The smaller the measurement, the larger the percentage uncertainty. The least count of an instrument is indirectly proportional to the accuracy of the instrument.

The least count of the vernier can be measured by using the formula stated below.

Least count (L.C) = 1 M.S.D - 1 V.S.D

Example:

vernier scale is constructed by taking 49 main scale divisions dividing them into 50 divisions

ie. 49mm divided into 50 parts

therfore

1 vsd=49/50 mm=0.98mm

1 MSD=1mm

substituting in formula

L.C = 1 M.S.D - 1 V.S.D

L.C=1-0.98=0.02mm

Least count error

The smallest value that can be measured by the measuring instrument is called its least count. All the readings or measured values are good only up to this value. The least count error is the error associated with the resolution of the instrument.

For example, a vernier callipers has the least count as 0.01 mm; a spherometer may have a least count of 0.001 mm. Least count error belongs to the category of random errors but within a limited size; it occurs with both systematic and random errors. If we use a metre scale for measurement of length, it may have graduations at 1 mm division scale spacing or interval.

Using instruments of higher precision, improving experimental techniques, etc., we can reduce the least count error. Repeating the observations several times and taking the arithmetic mean of all the observations, the
mean value would be very close to the true value of the measured quantity.