Screws come in a variety of types and sizes for an endless number of construction tasks—from woodworking to metal roof installations. But, choose the wrong length or width, and it can split the wood, or affect the soundness of a structure. As withstaples, screw measurement is slightly more complicated than that of nails. Here are three essential measurements every tradesperson shouldknow.
Screw Measurement, In Three Parts
There are three main screw measurements: gauge, length, and threads per inch (TPI). When shopping for collated screws at Nail Gun Depot, for instance, you’ll find screws labeled like this:Duraspin #8 x 1-1/4" #08X125CBACTS. So, what do the #8 and 1-1/4" mean?
Screw Gauge
The first number is screw gauge, which refers to the outside thread diameter. This is also known as “major diameter.” Screws with a major diameter less than 1/4” are typically labeled in sizes #0 to #14.Screws with a 1/4" or larger major diameter are labeledin fractions of an inch.
For each gauge size, there is a decimal equivalent.Example: #1 = .073”.That number increases by .013” with each increasing size. For the #8 Duraspin screw (shown below), the decimal equivalent is 0.164”. Engineering Toolbox has a handy screw size chartthat lists screw gauges and their decimal equivalents.
Beyond major diameter, screws have other width measurements. The width beneath the threaded part of the screw is known as root diameter or “minor diameter."The measurement of the unthreaded part of the screw (if not fully threaded) is the shank diameter.
Screw Length
The next important aspect of screw measurement is shaft length. In the Duraspin screw mentioned above, the length is the second detail in its label—1-1/4".Shaft length is the part of the screw that drives into a surface.
The length measurement for a countersinking screw is the distance from the top of the head to the tip. This goes for flat-head, bugle-head, trim-head—and any other countersinking screw where the head can be driven beneath a surface.
For a non-countersinking screw, it's the distance from the bottom of the head to the tip. So for hex-, pan-, button-, round-, and truss-head screws, length is measured from directly under the head to the tip. One exception: an oval-head screw, which can be partially countersunk, is measured from the widest point of the head to the tip.
Belowis an example of two non-countersinking timber screws from Simpson Strong-Tie. The first screw has a washer head with a low profile. Thesecond screw also has a washer head, but a more prominent hex drive. Note where the length is measured on each.
Threads Per Inch (TPI)
TPI is ameasurement of the number of threads in a one-inch section of screw. The TPI measurement occasionally follows the screw gauge with a hyphen. For example, a screw labeled "#10-12" has a #10 gauge with 12 threads per inch.You may have heard the term "thread pitch," which refers to the number of threads per unit of measurement.
Check out the detailed measurements, below, for the Senco Duraspin 08X125CBACTS washer-head screw. The #8 gauge screw has a major diameter of 0.17" and 8 TPI. The screw is 1-1/4" long, a measurement taken from the bottom of the head to the point.
If you're shopping for collated screws and need help, contact Customer Service for assistance.
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FAQs
Screws that are flat on the bottom of the head (non-countersunk) – Length is measured from the flat bottom of the head to the tip or blunt end of the screw. Screws that are flat on the top of the head (countersunk) – Length is measured from the flat top of the head to the tip or blunt end of the screw.
How are screws measured? ›
Measuring Screw Length
For the vast majority of machine screws, the nominal length is measured from below the head to the tip of the screw. For countersunk fasteners, the length of the screw is generally measured from the top of the head to the tip of the screw.
How do you know screw sizes? ›
You simply count the number of thread peaks along a one-inch length. You'll also measure the major diameter of the screw, as shown above. These two measurements will combine to give you the screw size you need. For instance, if your major diameter is ¼″ and you have 20 threads per inch, then your screw size is ¼″ x 20.
What is the rule for screw size? ›
In general though, the screw should always go be sized so that the smooth part of the shank goes through the top, thinner piece of material, and the threaded part of the shank into the thicker piece. In choosing length, try to have about 2/3 of the screw threaded into the "secondary" piece.
How are metric set screws measured? ›
The size of a metric screw or bolt is specified as diameter, pitch and length, in millimeters (millimeters is abbreviated "mm"). For nuts, size will appear as diameter and pitch. There is an exception though—if pitch isn't specified, the fastener is coarse threaded.
What is a screw explained? ›
A screw is simply a cylinder surrounded by ridges called threads. The top, the part that is visible after the screw is installed, is the head. Directly below the head lies the shank, a smooth area of the screw with no threading; not all screws have one. The rest of the cylinder is called the shaft.
What determines screw head size? ›
The gauge of a screw is defined by the thickness (diameter) of the un-threaded part of the screw known as the shank. Ordinarily, in the United Kingdom the head size is roughly twice the diameter of the shank, although there are some exceptions associated with hybrid, hardened, DIN Standard (metric) and imported screws.
How are bolts sized? ›
A bolt's size is classified based on a number of dimensions, namely the bolt's diameter, length of shank, thread pitch, grade, thread length and head size. Bolts can me specified in metric using millimeters, but the standard, internationally-recognized descriptions of bolt sizes are in inches.
How is bit size determined? ›
The length of the bit corresponds to the width of your horse's mouth. This measurement is usually provided in inches and fractions of an inch, such as 5" or 5 1⁄2".
What is the rule of thumb for screws? ›
There is a rule of thumb that the anchor length (the length of threads in the board you are fastening to) should be twice that of the fastened material (the thickness of the material being fastened). So a 1-1/2" piece of material fastened to another piece of material should have 3" of threaded shaft into that material.
To accurately measure a screw, use a ruler or a tape measure. Place the screw on a flat surface and align the ruler's zero mark with the screw's head (unless it's a round-head screw, which we'll go into detail later). Measure from the beginning of the screw head to the end of the screw's thread.
What does the number before the screw size mean? ›
The first number is the diameter. The bigger the number the bigger the screw. The second number is the number of threads per inch. Check your state and local codes before starting any project. Follow all safety precautions.
What does #10 screw mean? ›
For screws measured in inches, diameters of 1/4 inch and smaller (for machine and sheet metal screws) or 5/16 inch and smaller (for wood screws) are expressed with a # and a whole number (ex., a screw with a major diameter of 3/16 inch is a #10 screw). Smaller numbers indicate smaller diameters.
What does #8 screw mean? ›
Former Engineering Technician (2002–2016) Author has. · 4y. The number 8 refers to the diameter of the screw. It's the system in the US that small screws, that are less than 1/4″ in diameter, are sized with.
What's thicker #8 or #10 screws? ›
In the Imperial system of units, a number 10 machine screw (0.190 inch major diameter) is bigger than a number 8 machine screw (0.164 inch major diameter).
How big is a #10 screw? ›
Screw Dimensions
| "A" Flat Head | "B" Body Dia |
|---|
| Size | Max | Max |
|---|
| 8 | 0.332 | 0.136 |
| 10 | 0.385 | 0.157 |
| 12 | 0.438 | 0.176 |
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