How to Read a Screw Thread Callout

A thread gauge, for measuring thread pitch.

You have a loose screw --the threaded fastener sort--and you walk into the hardware store to obtain a replacement. There, you encounter an entire aisle of screws, nuts, washers, and other small hardware. Which one do you need? If you know a little bit about how screw sizes work, the process of finding the right part will be a lot easier.

Steps

1. 

   Many different sizes.
   Read the numbers. They will look something like one of these:
   • #4-40 x .5
   • 1/4-20 x 5/8
   • M3-50 x 10
2. 

   The major diameter for the threaded portion of the screw.
   Interpret the first number. The first number gives the major, or largest, diameter.
   • In Unified threads (measured in inches) there are numbered diameters #0 through #10, with 0 the smallest and 10 the largest. (Diameters #12 and #14 may also be found, but are usually on older equipment and needed for repairs or restorations. #14 is close to, but not exactly the same as, 1/4-inch.) The major diameter in Unified threads = 0.060" + 0.013"*(numbered diameter). So #2 has a major diameter of 0.086". The odd numbers exist, but the even numbers are in far more common use.
   • For screws larger than a #10, the diameters are listed in fractional inches. For instance, a 1/4-20 screw has a 1/4-inch major diameter.
   • For metric threads, e.g. M3.5, the number following the M is the major diameter of the external thread in millimeters.
3. 

   The distance between adjacent threads, or thread pitch.
   Interpret the second number. It has to do with the distance between adjacent threads. It may be given as the number of threads per unit length; or it may be given as the distance between threads, also called the thread pitch.
   • For Unified threads, the number given is threads per inch. For instance, a 1/4-20 screw has 20 threads per inch.
   • For metric threads, the thread pitch is given in millimeters per thread. Thus, a M2 x 0.4 screw has threads every 0.4mm. Although most metric fasteners have two or more standard pitches (fine & coarse threads), the pitch is often omitted from a thread callout, it is always helpful to carry a sample with you to the hardware store.
     • There are two major metric "industrial standards": DIN Deutsches Institut für Normung (German) and the JIS Japanese Industrial Standards. Although these standards are closely related and often identical, there will be cases where say a JIS M8 bolt may not have the same pitch as a DIN M8 bolt.
4. 

   The length of most screws is measured from the bottom of the head.
   Read the length, which is generally given after the "x". The length of most screws is measured from the bottom of the head, as shown. Note, however, that a flathead screw, designed to sit flush in a countersunk material, is measured to the top of the head.
   • For unified threads the length is given in inches. A 1/4-20 x 3/4 screw is .75 inches long. The length may be given in fractional inches or the fractional equivalent.
   • For metric threads, the length is given in millimeters.
5. Understand some other nomenclature that sometimes goes with screw threads.
   • Thread classes refer to fit, how loosely or tightly the screw fits in the nut. The most common thread classes are 2A or 2B. A indicates an external thread, such as on a screw or bolt. B indicates an internal thread, such as on a nut. The 2 (or, far less commonly, 1 or 3) describes the tightness of the fit.
   • You may see the abbreviations UNC and UNF. These stand for unified coarse and unified fine, respectively, and they refer to standard series of thread pitch. Each series assigns a pitch to diameter. For instance, a #10 UNC screw has 24 threads per inch, whereas a #10 UNF screw has 32 threads per inch. If a thread is specified by its series, look for the pitch in a table.
   • Minor diameter is the smallest diameter of the thread, the innermost diameter. Major diameter is the largest diameter of the thread, the outermost diameter. The diameter given is typically the nominal major diameter of an external, or male, thread.
6. For a complete description of the screw, include the head and drive style. The Wikipedia article in the external links section includes diagrams of different heads and drives.

Tips

• If you have an unknown fastener, a thread gauge or screw checker can be a big help in determining its size. If no such instrument is available, try screwing your fastener into a known, mating thread. Stop immediately if you feel undue resistance, to avoid stripping threads.
• 

  These screws are the same.
  One way to check whether two screws are the same is to set them side by side facing opposite directions. If their threads mesh, they have the same thread pitch. This is also a quick way to check length.
• To read aloud, say these callouts as follows:
  • #4-40 x .5 -- Say "Four-forty by point five" or "Number four-forty by a half."
  • 1/4-20 x 5/8 -- Say "Quarter-twenty by five eighths."
  • M3-50 x 10 -- Say "Em three fifty by ten."
• This guide is for machine screws. Other threads, such as wood screws, follow slightly different guidelines. Other thread series, such as the PG series and British Whitworth also exist, but they are relatively rare.
• The majority of machine screws are right-handed threads, meaning that the screw will turn clockwise to insert and counterclockwise to remove. Remember, "righty, tighty; lefty loosie". One common exception is the thread holding the left pedal on a bicycle to the crank arm.
• Screws typically come in certain round-numbered lengths, so a 1/4 inch screw may be far easier to find than a 5/32 inch screw.
• Consult a size chart for more information.
• A good rule of thumb with machine screws is that a minimum of three full threads should engage the mating thread. If there are not at least three threads engaged in a thin material, use a nut or other reinforcement.

Warnings

• When specifying a fastener, make sure that the fastener is adequate for the job and compatible with the materials and environment.
• Tapered pipe threads follow entirely different rules.
• Metric standards are often very close to English standards and can often be confused. Sometimes context can be helpful (e.g. If the hardware is from a non-American car, it's probably metric).