How to Read Calipers

From precisely measuring a pipe's diameter to identifying microscopic machine screws to finding the exact depth of a hole—a caliper is the tool for the job. While operating a caliper is pretty self-explanatory, learning to read them properly is a different story.

However, without a thorough explanation of how to read calipers, it can be hard to decipher the precision measurement, which defeats the purpose of using the tool. Below, we'll break down the different types of common calipers such as Vernier, dial, and digital, and explain how to read them.

Types of Calipers

The most commonly used caliper types are outlined below. Each variation features a similar jaw device to obtain measurements but utilizes different readouts—each requiring a different method for deciphering the measurements.

Vernier Calipers: Vernier calipers, named after French mathematician Pierre Vernier, feature a main scale inscribed on the beam and an auxiliary scale that slides along the beam. The auxiliary scale, called a Vernier scale, has two graduated scales used to gather precise measurements.

Dial Calipers: Dial calipers have a main scale similar to the Vernier caliper, but the way in which the precision measurement is displayed is different. Instead, dial calipers have a dial that displays the secondary precision measurement, which is added to the main measurement to achieve the final measurement.

Digital Calipers: The simplest and quickest of all to read, digital calipers feature a digital readout that displays the precise measurement calculated by an internal processor.

Parts of a Caliper

The three calipers listed above feature similar construction, with many of the same parts. However, please note that not all calipers are the same and some models may feature fewer or additional components.

• Upper Jaws: The upper jaws face away from each other, allowing them to measure the internal inside of an object.
• Lower Jaws: The lower jaws face toward one another, allowing them to measure the outside of an object.
• Beam/Main Scale: The beam of the caliper is the long portion where the main scale is located.
• Lock Screw: The lock screw allows the user to lock the jaws tightly in place to hold the measurement once the caliper is removed from the object.
• Thumb Screw: The thumb screw is a rolling wheel that allows for smooth adjustment of the jaws.
• Depth Rod: The depth rod protrudes from the end of the beam and is used to measure the depth of an object.

How to Read a Vernier Caliper

Vernier calipers are the trickiest calipers to read but offer a great deal of precision. Once you learn how to read a Vernier caliper, you can easily read a digital and dial caliper. Reading a Vernier caliper is a two-step process. The first step is finding the main measurement, the second step is locating the precision measurement via the auxiliary scale and adding it to the main measurement. Many Vernier calipers measure thousandths of an inch, so this is what we will use for our example.

How to Read the Main Scale

Unlike a ruler, which is divided by sixteenths of an inch, the main scale of a caliper capable of measuring to the nearest thousands of an inch will be divided by tenths of an inch. Within each 1/10 (0.1) mark are three smaller tick marks, each indicating 25/1000 (0.025) of an inch.

How to Read the Vernier Scale

The Vernier scale, located on the sliding portion of the caliper, typically features 25 graduation marks. Each mark doesn't equal but represents 1/1000 (0.001) of an inch. Because the Vernier scale represents but doesn't equal 1/1000 of an inch, it can't be used independently to find a measurement and must be used in conjunction with the main scale. The steps below will outline how to do this.

1. Place Jaws Against the Object

   Place the jaws around the object you intend to measure (or inside if measuring the inside distance) and move the thumb wheel until the caliper rests against the walls of the object. If you desire to find the depth of a hole, place the depth rod in the hole and adjust the wheel until the rod hits the bottom of the hole.

2. Find the Main Measurement

   Look at the numbers on the main scale. Locate and identify the closest number to the left of the zero on the Vernier scale. This is your starting number. For example, let's say the closest number to the left of the Vernier scale is 75/1000 (0.075) of an inch.

   Tip

   If the zero on the Vernier scale lines up perfectly with a number on the main scale, the second step of the process is unnecessary and the measurement is reflected by the number that aligns with the zero.

3. Find the Secondary Measurement

   If the zero on the Vernier scale doesn't align with a graduation on the main scale, the measurement falls somewhere between 75/1000 (0.075) and 100/1000 (0.010). To identify the precise measurement, the Vernier scale must be used.

   Starting at zero on the Vernier scale, move right along the scale and identify the mark that aligns with a mark on the main scale. The scale is designed to only allow for one mark to align with the main scale. Disregard the mark on the main scale and only note the mark on the Vernier scale. For our example, let's say the mark that lines up is six, which represents 6/1000 (0.006).

4. Find the Final Measurement

   To find the final measurement, add the measurement taken from the main scale and add the measurement found on the Vernier scale. The sum of these two measurements is the final measurement.

   So, for our example, here is the equation and outcome:

   75/1000 (0.075) + 6/1000 (0.006) = 81/1000 (0.081)

   Tip

   If the sliding scale moved past one or more full inches on the main scale, don't forget to include this in your final measurement.

How to Read Other Calipers

Other calipers, such as dial calipers and digital calipers, are operated much like a Vernier caliper and read in a similar way. For a dial caliper, use the dial gauge to find the second measurement to add to the main scale measurement. The measuring increments will typically be listed on the face of the dial. For a digital caliper, simply read the measurement on the digital display. Reference the display or your specific tool's user manual to identify the meaning of the numbers.

Keeping Your Caliper Accurate

To ensure the accuracy of your caliper, you must check for zero before using the tool. On a digital caliper, simply close the jaws and press the "zero" button. On a dial caliper, close the jaws and check the dial for zero. If the dial is off, loosen the dial lock and adjust the dial until the needle points to zero, then tighten the lock once more.

Some Vernier calipers can be zeroed by adjusting an adjustment wheel until the zeroes of each scale align with the jaws closed. For Vernier calipers without this feature, you must find the zero error and account for it with each measurement.

To help prevent zero errors, handle your caliper with care and store it in a safe spot where it won't be bumped by other tools or dropped.

Positive Zero Error Vs. Negative Zero Error

The two types of zero errors are positive and negative. A positive zero error occurs when the zero of the Vernier scale sits to the right of the zero of the main scale and a negative zero error occurs when the opposite is true. To find a true measurement on a caliper with a zero error, a positive zero error should be subtracted from your final measurement, while a negative zero error should be added to your final measurement.

• Finding Positive Zero Error: To find a positive zero error, close the jaws of the caliper. Because the zero of the Vernier scale sits to the right of the zero on the main scale, you know the caliper has a positive zero error. Find the mark on the Vernier scale that aligns with a mark on the main scale. This is your positive zero error. For example, if the aligning mark on the Vernier scale is 3/1000 (0.003), the zero error is +0.003. So, you will need to subtract 3/1000 from your final measurement to find the true measurement.
• Finding Negative Zero Error: To find a negative zero error, close the jaws of the caliper. Because the zero of the Vernier scale sits to the left of the zero on the main scale, you know the caliper has a negative zero error. Find the mark on the Vernier scale that aligns with a mark on the main scale to identify your zero error. However, instead of counting up from zero as you did you find the positive zero error, count down from 25 to find the negative zero error. So, if the aligning mark is 20, count down from 25 to find a negative zero error of -0.005. You will need to add 5/1000 to your final measurement to find the true measurement.

When to Replace Your Caliper

If properly handled and stored, calipers can outlast a lifetime. However, if you find that your calipers are bent, damaged, or inaccurate even after zeroing—it may be time to find a replacement.