Measuring Screw Sizes: How To Identify The Right Fit

There are two systems for determining screw sizes: the metric system and the imperial system. The difference lies in the unit of measurement. The imperial system measures thread size, length, and diameter in inches, while the metric system uses millimeters. To measure the length of a screw, measure from the tip of the screw to where the head rests. If the screw is countersunk, measure from the top of the head to the tip. The diameter of the screw is the width of the threaded part, measured from one side of the thread to the other. For screws using the imperial system, the diameter is represented by either a gauge number or a fraction of an inch. Metric screws are indicated by the letter 'M' followed by the number of millimeters.

Measuring screw length

Measuring the length of a screw is simple but not always obvious. The process depends on the type of screw and its head.

How to Measure Screw Length

The nominal length of a screw is the length used to define the size of the fastener. It is the name of the screw size and not necessarily the overall length of the component. Nominal lengths are usually found within product names or catalogue listings. They are simplified terms used to avoid inconvenient literal sizes or micron-accurate measurements.

The main consideration when measuring the nominal length of a screw is whether or not to include the length of the head. The nominal length of a screw usually relates to the depth of the required hole.

For instance, the length of a cap head screw is defined by the length of its shaft, because the head sits above the required hole. The length of a countersunk screw, on the other hand, is defined by its total length (including its head) because the head sits below the surface, thus making up part of the required hole.

The nominal length of a screw is usually, though not always, measured from below the head to the tip of the screw. If no head exists, such as on a threaded bar or grub screw, the screw would be measured from end to end.

Exceptions to the Rule

Some screw types have different ways of measuring length. For example, the length of a flat head, countersunk screw is measured from the top of the head to the tip. Oval head screws are measured from the widest point of the head to the tip. Self-drilling screws are measured to the end of the drill point tip.

There is also some variation in how the length of screws with raised heads, particularly countersunk screws, is measured. The raised section of a raised countersunk screw sits above the required hole, so it is not counted as part of the nominal length. This means that raised head screws cannot be measured end-to-end.

Tips for Measuring Screw Length

• The screw length is determined by the shaft size, which is the part of the screw inserted into the material. This rule generally applies to non-countersinking screws.
• Flat-head screws sink entirely into the material. Measure these from the top of the head to the tip of the threads.
• The general rule is that the screw should enter at least half of the attached material's thickness.
• The screw's thickness should match the material's width to avoid tears.

Measuring screw diameter

To measure the diameter of a screw, you need to measure the width of the threaded part of the screw. This is also known as the screw size. The method of measurement depends on whether you are using the imperial or metric system.

Imperial System

The diameter of a screw in the imperial system is represented by a gauge number or a fraction of an inch. Measure from one side of a thread to the other using a ruler or measuring tape, then use a gauge guide to match your measurement to a gauge number.

Metric System

The diameter of a screw in the metric system is the number of millimetres, preceded by the letter M. For example, M6 corresponds to 6 mm. Measure the width of the thread in millimetres to get the diameter.

Measuring thread pitch/spacing

Thread pitch, or spacing, is the distance between two consecutive peaks of a screw thread. It is measured in millimetres (mm) or inches (in). The pitch is usually less than 1 mm, so you would measure it as a decimal point of a millimetre.

There are different ways to measure the pitch of a screw thread. One way is to use a pitch gauge. A pitch gauge has several leaves with a number stamped on it. The number indicates the pitch. You can also use a ruler, but this is not as precise as using a caliper. The ruler should be high resolution and show measurements to a fraction of a millimetre.

To measure the pitch of a metric screw, measure the distance between two consecutive crests. For an imperial screw, measure the threads-per-inch (TPI). This is the number of screw threads present on an inch of the screw's shaft. TPI is typically expressed in a number followed by a T, such as 16T or 24T. A higher TPI indicates that there are more threads per inch, meaning the screw pitch is finer. For example, a screw with a TPI of 12 would have 12 threads per inch of total threading.

You can also calculate the pitch by dividing the thread length by the number of threads. For example, if a screw has a thread length of 10mm and 5 threads, then the pitch is 2mm.

Using the imperial system

To identify the size of a screw using the imperial system, you will need to take a few measurements. First, measure the length of the screw. This is done by measuring from the tip of the screw to where the screw head rests when it is fully embedded in something. The length is given in inches and does not include the head of a raised or domed screw.

Next, you will need to measure the width of the thread to get the diameter. The diameter of a screw in the imperial system is represented by either a gauge number or in fractions of an inch. A gauge number corresponds to a certain fraction of an inch in diameter. For example, a #0 gauge screw is 1/16 of an inch in diameter, while a #12 screw is 0.216 inches in diameter.

Finally, you can calculate the thread spacing value by counting the number of threads in one inch. Thread counts in the imperial system typically range from 35-40 threads per inch. This measurement is also called the thread pitch.

It is important to note that screw sizes in the imperial system are usually listed with the gauge first, followed by the length, and sometimes the thread count. For example, "10-35 x 2” indicates a screw with a #10 gauge, 35 threads per inch, and a length of 2 inches.

Using the metric system

The metric system is widely used for measuring screw sizes, with the majority of screws measured in millimetres. To determine the size of a screw in the metric system, you need to measure its length, diameter, and pitch.

Length

The length of a screw is the distance from the tip of the screw to the point where the screw head rests when it is fully embedded in a surface. The type of screw head will determine where exactly you start measuring from. For instance, flat-headed countersunk screws will rest flush with the surface, so you would measure from the top of the flat head to the tip of the screw. Round-headed countersunk screws, on the other hand, only sink partially into the surface, so you would start measuring from the bottom of the rounded top. For screws with other types of round heads that are not countersunk, measure from the flat underside of the screw head to the tip.

Diameter

The diameter of a screw is the width of its threaded part. To measure the diameter, put the end of the screw against a ruler or measuring tape and measure the width of the threaded part in millimetres. This is usually the first number in the metric screw size callout, preceded by the letter "M". For example, M6 corresponds to a diameter of 6 millimetres.

Pitch

Pitch refers to the distance between the threads of a screw. In the metric system, pitch is measured in millimetres. It is typically less than 1 millimetre and is expressed as a decimal point of a millimetre. To measure the pitch, use a ruler or measuring tape to measure the distance from one thread to the next. The pitch of a screw is usually correlated with its diameter. For example, a 2-millimetre screw typically has a pitch of 0.4 millimetres.

Frequently asked questions