How To Measure A Nut For Socket Size

You’re not alone if you’ve ever been stumped when trying to figure out what size nut to use for a bolt, be it a standard size in imperial or a metric bolt head. The numerous sizes might be daunting and perplexing. You might get even more confused once you know your socket tools come in the US standard and metric measurement systems.

Regardless of these differences, determining the size of a nut does not have to be complicated.  You can measure a nut in several ways, and when you need to match a nut to a bolt or socket wrench, you’ll find there are generally two ways of measurement that deal with most issues.

In our guide, you can learn more about how are sockets measured and what is the easy way to find the right socket wrench size. By the end, you’ll know how to measure socket size, so you can see the difference, and picking the right wrenches or socket for the task. (Read Spark Plug Socket Sizes Guide)

nut for socket

How Are Nut Sizes Measured?

Here, you can find all you need about how to measure nuts and bolt dimensions.

Diameter Size of Bolt and Nut Size

If practicable, use a vernier; if a ruler is necessary, try to measure outside the threads.

The threaded side of the bolt, not the head, must be measured.

The letter “M” denotes the diameter of a metric bolt. If it’s imperial, an M4 bolt has a diameter of 4mm (metric units) and is measured in inches.

A 1/4′′ bolt size, for example, measures 1/4′′.

Shank Diameter of Nut or Bolt Size

The length of a bolt’s shank, measured in mm for Metric nuts, is its width.

For completely threaded bolts, the thread diameter measurement can be used. It s like the Significant or Thread diameter.

Shank Length of Nut or Bolt Size

A fastener’s length is determined by its tip. The end of the fastener is believed to be the material surface.

The measurement of a pin with a head that protrudes above the surface. This measurement is taken from the bottom of the head to the end of the fastener.

The measurement for recessed bolts is taken from the point on the base. The link ends at the outer surface.

Why Thread Size Matters

The threading of a bolt or nut is described by two words: size and threads per inch. It ensures that they will be able to work effectively.

Thread matters for measuring nuts for socket size, for example.

If bolt and nut threads are not the same, the threading will either seize or shred. As a result, the link is rendered worthless. (Read Can You Use Impact Sockets As Regular Sockets)

nuts and bolts

How to Determine Thread Size of a Nuts and Bolts

Bolts usually have four measurements, whereas nuts only have two.


  • Shank size,
  • Bolt head size,
  • Shank diameter,
  • Thread pitch (the size of the thread).

Nut size is determined by thread pitch and hex width.

The sizes of bolts and nuts are not interchangeable. Most nut and bolt sizes are available in both metric and standard sizes.

The alloy grade determines the strength of nuts and bolts.

A shank is a non-threaded part of the head, so ensure you measure a countersunk screw’s entire length.

First Step:

Place the head of a bolt or nut in a nut or bolt gauge. Most automobile parts stores carry nut and bolt gauges in both metric and standard sizes.

The bolt head or nut should fit snugly into the gauge’s holes. Within the indication, it will not wiggle.

The length of the bolt head or nut will be etched on the gauge that corresponds to the place you choose.

Second Step:

Estimate the length of the bolt from the bottom of the head using a measuring tape.

Because the variation in bolt length between metric and standard is so tiny. Most of the time, this is done in inches.

Third Step:

To establish the bolt’s width, place the head of the bolt into the bolt length gauge. This is the diameter of the shank, which you can measure in metric or standard units.

To find out what the difference is, use each. For example, a three-eighth is smaller than a 10-millimeter measurement.

Because millimeters and inches differ, you’ll need to consider using metric and standard gauges.

It’s about figuring out which one fits the bolt best. First, you will determine the bolt’s total diameter width. Then, the gauge’s hole will be inscribed with the size width.

Fourth Step:

Place the thread pitch gauge on the bolt’s threads after unfolding it because the millimeter and inch measures differ slightly.

Only one gauge key will match the threads exactly on the thread gauge. Each key will have a measure inscribed, indicating the pitch measurement.

Fifth Step:

After estimating the thread pitch size of the bolt, place a nut on the shank. Because it prevents the shank from wriggling, it is the correct nut for the bolt. The pitch appears to be identical.

A nut or bolt’s thread must be striated to screw into something with a different thread. Screw and diameter diameters vary depending on the products you’re working with.

How Do You Determine Bolt Nut Size?

A bolt is a cylindrical object with a hexagonal (occasionally square) head on one end and threads on the other (entirely or partially).

A nut is a hexagonal (occasionally square) object with threads on the inside and the bolt’s counterpart. To secure the bolt and nut, the thread pitch of the bolt and nut must be the same. (Learn How To Make Socket Sizes Readable)

bolt nut washer


The washer is another crucial object. It’s used to expand the contact area between the nut and the surface, which minimizes the surface’s stress. Washers also shield the surface from damage caused by the nut’s tightening.

A bolt, nut, and washer can secure two objects, and the best part is that the joint is temporary, meaning that it can be removed when you loosen the nut.

Drilling a hole in the items is the initial stage in joining two parts together. The hole can either be tapped or left untapped.

If the hole is tapped, the bolt can simply be threaded into the hole (as long as the hole and bolt pitch are the same) with no nut to hold the bolt. You’ll need to secure the bolt using a washer and nut if it’s a standard hole.

The diameter of the hole to be drilled must be the same as or slightly greater than the diameter of the bolt, which is an important consideration.

How to Measure Socket Drive Size

Drive Hole, L, and S are the three most important dimensions of a socket. The following critical dimensions affect how this socket is used:

  • The torque size of the driver tool you’ll need is determined by the size of the drive hole (also known as the “driver hole” or “square hole”), which is shown under Drive Hole.So you’d use a 12-inch drive socket with a 12-inch drive ratchet, for example.
  • The letter L stands for the overall length of the socket. Your working environment will dictate the length you’ll need. For example, a deep socket will be the best option for screws in deep holes.
  • S stands for the nut’s intended size, determining which socket you’ll require. A 5mm socket, for example, is required for a 5mm nut.

Drive Hole Size

The size of the drive hole will be found by your use and torque demand.

Common measurements are:

  • 1/4″ (6.3mm),
  • 3/8″ (9.5mm),
  • 1/2″ (12.7mm),
  • 3/4″ (19.0mm) and 1″ (25.4mm)

Socket Drive Hole

The hardware shop can find ratchet wrenches and other spanners with the above drive-hole sizes.

The torque resistance increases with the size of the drive hole. Therefore, for restricted places, smaller drive holes are ideal.

The most common size in the auto repair sector is 3/8 inch (9.5 mm). It varies, however, from country to country.

Socket Sizes: SAE (INCH, fraction) & Metric (mm)

There are two ways for measuring socket sizes: SAE (imperial, measured in inches) and metric (measured in millimeters) (measured in millimeters).

These two systems are used to measure both 6-point and 12-point sockets.

The 5.5-24mm range is the most popular. In the United States, the SAE system is widely utilized, but the metric system is predominantly used in Europe and Asia.

Convert: 1 inch = 25.4mm

Socket length

Sockets come in three different sizes: deep, semi-deep, and standard.

You can choose the length that best suits your working environment. The semi-deep size was created with 3/8″ DR in mind.

What Size Socket Do I Need?

Here’s how you’ll measure hex heads to find the bolt size or nut size to match your different size wrench or sockets. (Learn How To Use Brake Cleaner Without Removing Tire)

Imperial System

  1. Set the digital caliper to read in inches rather than millimeters after turning it on.
  2. Set the nut on one of the flats on a flat surface. Allow the nut to fit between the caliper jaws by opening them far enough to sit on two of the flats.
  3. Close the caliper jaws until one flat side of the nut is on opposite sides of each jaw.
  4. Look at the caliper display. The reading is a numerical representation of the nut or bolt size.

Imperial sockets are measured in thousands of an inch.

The reading will show you want a wrench; you’ll need to tighten or loosen the screws you have.

Metric System

  1. Set the digital caliper to read in millimeters rather than inches after turning it on. Set the nut flat on the surface.
  2. Allow the nut to fit between the caliper jaws by opening them far enough.
  3. Close the caliper jaws until one flat side of the nut is resting on each jaw.
  4. Take a look at the caliper display. The decimal reading represents the nut’s size.

Metric sockets are sized in millimeters.

Select the first-millimeter size slightly larger than your caliper reading for your wrench. This is the size of your nut and suitable for your socket.

tape measure

Measuring With Tape Measure

Not everyone can use a caliper, so here, you can find how to do the same using a tape measure.

Measuring Between Flats

Lay the nut on a flat surface.

  1. Place the end of your measuring tape against one of the flats on the nut’s outer perimeter.
  2. Position the tape measure across your nut’s diameter to the flat side across from the one you are pulling from.
  3. To find the measurement of a metric nut, count the number of lines on the tape measure. For example, the size is a 9-mm (millimeter) nut if you count 9 lines.
  4. Count the tiniest lines on the tape measure in sixteenths if the nut is US standard. To find the size, add the sixteenths together. For example, the nut’s size is 11/16 if you count 11 lines.
  5. You can measure from point to point to find the “Whitworth system” measurement. You measure from point-to-point on the hexagon rather than from flat to flat.

These two ways assist you in determining what size wrench or socket to use on a nut.

Measuring for Diameter

As in the previous steps, place the nut on the table. Set the tape measure’s end on one side of the nut’s inner threads.

  1. Stretch the tape measure to the other side of the nut’s inner threads.
  2. Count the lines to find the threaded hole’s diameter. Next, measure the width of the threads’ widest aperture.
  3. This measurement will assist you in matching the nut’s thread diameter to the bolt’s thread diameter.

Measuring Thread

  1. Count the number of threads that run the nut’s internal thickness length. Keep track of the threads with a magnifying glass and a pin.
  2. Make a note of the total on a piece of paper.
  3. Calculate the nut thickness in threads per inch.
  4. Coarse threads are found in threads with 16 threads per inch, while fine threads are found in threads with twenty threads per inch.
  5. The nut is coarse-threaded if it is 1/4-inch thick and that 1/4-inch has four threads.
  6. The nut is fine-threaded if it has five threads in that 1/4-inch thickness.
  7. Hold the nut between your fingers and measure the circumference parallel to the nut’s hole to measure nut thickness.

You will notice that most fractional bolts have radial lines on the head and can determine the size of the nut.

How To Measure A Nut For Socket Size (2)