How does a Caliper (King’s Foot) work?

Caliper , or King’s Foot , is a measuring tool that allows us to measure (redundancy aside) pieces, both from their internal and external faces, with a scale of up to one millimeter divided into ten parts (0.1 mm).

In this article we will learn what it is, how to use it (both use and reading), what its parts are, and most importantly: how to take care of it so that it does not bend/break?

The quality of a product is due, on many occasions, to the accuracy and precision of the size of different parts (such as  screws , shafts, gears , plugs, among others), and to audit whether the shape/thickness of different parts is correct, It can be corroborated with this tool: caliber ,  vernier caliper or vernier .

What is a caliber?

caliber, vernier caliper or vernier is a measuring tool that offers a pattern with which this pattern is compared with what you want to measure. It is mostly used to measure inside and outside diameters with greater precision than a common ruler.

Accuracy of a gauge

The precision of a caliber is one tenth of a millimeter, that is, it is capable of dividing a millimeter into 10 parts . Let us compare this precision with that of a common ruler. If we look at a common ruler we will be able to appreciate a value such as 1.5 cm, which is equal to 15 millimeters. While with the caliber we will be able to appreciate a value of 1.54 cm which is equal to 15.4 mm.

On the other hand, there are even more precise gauges that can have precisions of one twentieth of a millimeter (they divide the millimeter into 20 parts) and one fiftieth of a millimeter (they divide the millimeter into 50 parts).

What is a gauge used for?

gauge , as we mentioned at the beginning of this article, is a measurement tool that was developed in order to measure certain lengths with high precision. The gauge also allows for “internal” and “external” measurements thanks to its double mouth.

How does a gauge work?

The operation of the caliber is very simple. We are going to explain how to use it to obtain different measurements of the same piece. Let’s imagine a washer, we can obtain two measurements of this piece with the gauge.

First of all, we open the external jaws and place the washer between them. This measurement will give us the outer diameter of the washer. Then we do the same with the ears to measure interiors. In this case we close the ears, place the washer, and then open the ears until they both butt against the ends inside the washer. In this way we can obtain the measurement of the internal diameter of the washer.

How to read a gauge

Once we have the measurement  made as explained in the previous paragraph, it is time to read it. For this we have 3 different scales. A main scale divided into cm and mm, a scale in inches, which will not interest us unless we want to measure in inches, and a vernier of millimeters (number 6 in the figure above).

Let’s see it with an example that will be easier. Let us imagine that we have the following measurement.

We can see from the main scale that we are talking about 3.8 cm or 38 mm. What we must determine now is the tenth part of the millimeter and we will determine this with the vernier . We can see that in the vernier we have a scale from 0 to 10. We must look for the line that coincides with one of the lines of the main scale. In case it is not possible to appreciate they coincide in the 3rd line of the vernier.

With this we can conclude that the tenth part of the millimeter coincides with 3/10, so the exact measurement of this image is 3.83 cm or 38.3 mm .

parts of a gauge

    1. External jaws. For measurements of external parts of a piece
    2. Internal ears. For measurements of internal parts of a piece
    3. Needle for depth measurement. When you want to determine the depth of the hole in a part.
    4. Main scale. Scale with divisions in millimeters and centimeters with which we determine the first part of the measurement.
    5. Secondary scale. Contains divisions in inches to be able to return measurement values ​​in inches.
    6. Vernier in millimeters. To determine the reading of the fraction of millimeters. Depending on how many fractions the vernier is divided into will determine the precision of the caliber
    7. Vernier in inches. To perform the same function as the vernier in millimeters only when a measurement is made in inches.


  1.  sliding or brake. When open, it allows the moving part of the gauge to slide. When it is closed, it does not allow the movement of the mobile part.

Types of calibers

According to the measurement to be made 

  • Standard gauges : Gauges that serve as a reference to manufacture other gauges.
  • Verification Calipers : Generally used for measuring finished parts.
  • Working gauges: Used during work.

According to its operation

  • Universal vernier calipers: These are the traditional calibers. They have a great duration and a low cost since they do not use a lot of tooling.

  • Vernier caliper with clock. They are similar to the universal vernier caliper, except that they have a clock that allows immediate reading of the measurement value. It has the advantage of greater clarity in the readings, but it has the disadvantage that it does not have the possibility of measuring in inches.
  • Digital vernier caliper. They are the most modern gauges on the market. They have the great advantage that the measurement is shown directly on the screen and does not lead to observational errors. It has the disadvantage that they are the most expensive on the market.

Differences between gauge and micrometer

The caliper and the micrometer  are both very precise measuring tools. Both of them measure lengths and return values ​​in centimeters. What differentiates one from the other is the precision with which they measure. While the gauge has a maximum precision of 1/50 mm, the micrometer  works with a precision of 1/100 to 1/1000 mm.

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