Measuring a screw thread requires three key pieces of information: the diameter of the screw, the pitch (spacing between threads), and whether the thread is metric or imperial. With a set of calipers and optionally a thread pitch gauge, you can identify almost any screw thread in a few minutes.
Measure the Diameter First
The most important measurement is the major diameter, which is the widest point across the threads of a bolt or screw. Place the flat jaws of a caliper across the outside of the threads so each jaw rests on the outermost crest on opposite sides. For a metric screw, this measurement in millimeters gives you the “M” number. A reading of roughly 8mm means you likely have an M8 bolt. For imperial screws, you’ll measure in inches, and common sizes like 1/4″, 5/16″, and 3/8″ are easy to spot.
For internal threads, like the inside of a nut or a tapped hole, you need the minor diameter instead. Insert the inner jaws of your caliper into the threaded hole, expand them until they contact the valleys on both sides, and read the measurement. If that’s tricky, a simpler approach is to thread in a bolt that fits smoothly, then measure the bolt’s major diameter instead.
Determine the Thread Pitch
Pitch is the distance from one thread crest to the next. Metric and imperial systems express this differently, and knowing which system you’re dealing with changes how you measure.
Metric Threads
For metric screws, pitch is measured in millimeters. Place your caliper on two adjacent thread crests and read the distance between them. Common metric pitches include 1.0mm, 1.25mm, 1.5mm, and 2.0mm. A standard M8 bolt, for example, has a coarse pitch of 1.25mm. If your caliper reading falls neatly on one of these common values, you can cross-reference it with a metric thread chart to confirm.
Imperial Threads
Imperial screws use threads per inch (TPI) rather than a millimeter pitch value. To find TPI, count the number of thread crests along a one-inch span. If your screw is shorter than an inch, count the threads along whatever length you have, measure that length in inches, and divide: TPI equals the number of threads divided by the length in inches. A 1/4″ bolt with 20 threads per inch is a 1/4-20, one of the most common imperial fasteners.
Using a Thread Pitch Gauge
A thread pitch gauge is a set of small metal blades, each cut with a different thread profile. You press each blade against the threads until you find the one that fits perfectly into the grooves with no light visible between the blade and the screw. The number stamped on that blade is your pitch (in millimeters) or TPI. This is significantly more reliable than measuring pitch with calipers, since calipers can slip across the angled thread faces and give you a slightly off reading. For anything beyond casual identification, a pitch gauge is the better tool.
Fine vs. Coarse Threads
Most bolts come in either coarse or fine thread versions. Coarse threads have a larger pitch, meaning fewer threads per inch or a greater distance between crests. They’re the default for general-purpose fastening. Fine threads pack more threads into the same length, which makes them stronger in tension and allows more precise adjustments, but they’re also more prone to cross-threading, galling, and damage. If two bolts have the same diameter but different pitches, you need to know which one you have, because they won’t interchange. A 1/4-20 (coarse) bolt will not thread into a 1/4-28 (fine) nut.
Tapered vs. Parallel Threads
Not all threads are straight along their length. Pipe threads, particularly NPT (National Pipe Thread), are tapered, meaning the diameter gradually increases from one end to the other. To check whether your thread is tapered or parallel, measure the diameter at the first crest, the fourth crest, and the last crest with your calipers. If all three readings are the same, the thread is parallel (straight). If you get three progressively different measurements, the thread is tapered. This distinction matters because tapered threads seal by wedging tighter as they’re tightened, while parallel threads rely on a gasket or O-ring to seal.
Reading a Thread Callout
Once you have your measurements, you can match them to a standard thread designation. These callouts follow a predictable format depending on the system.
Metric threads are written as the letter M followed by the diameter and pitch: M8 x 1.25 means 8mm major diameter with a 1.25mm pitch. If the pitch is omitted, it’s the standard coarse pitch for that size.
Imperial Unified threads list the diameter first, then TPI and the series: 1/4-20 UNC means a quarter-inch bolt with 20 threads per inch in the Unified National Coarse series. UNF indicates fine threads. For numbered sizes smaller than 1/4″, the diameter is given as a gauge number rather than a fraction, so a #10-32 is a number 10 screw with 32 TPI.
Pipe threads add their own layer. NPT designations use a nominal pipe size that doesn’t correspond to any actual measurement on the fitting. A 1/2″ NPT fitting, for instance, has a major diameter closer to 0.84 inches. If you suspect you’re dealing with pipe threads, measure the diameter and TPI, then look them up in an NPT reference table rather than trying to guess the nominal size.
Measuring Without Specialized Tools
If you don’t have calipers or a pitch gauge, you can still get a workable measurement. For diameter, a ruler held across the threads will get you in the right ballpark. For pitch, lay a ruler along the threads and count how many crests fall within 10mm or one inch, then divide accordingly. The results won’t be as precise, but they’re often enough to narrow your bolt down to one or two possible sizes.
Another practical method is the trial-and-fit approach. Thread your unknown bolt into a nut of a known size. If it screws in smoothly with no wobble or resistance, the threads match. Hardware stores often have thread-checking stations with labeled nuts and bolts for exactly this purpose. For internal threads in a hole where you can’t easily get calipers inside, this is often the fastest way to get an answer.
Common Mistakes to Avoid
The most frequent error is measuring pitch with calipers instead of a thread gauge. Because thread crests are angled rather than flat, caliper jaws tend to ride up the flanks and produce a reading that’s slightly too large. If your pitch measurement doesn’t match any standard value, try the nearest standard size with a pitch gauge to confirm.
Another common mistake is confusing metric and imperial threads that are close in size. An M10 bolt (10mm diameter, roughly 0.394 inches) is very close to a 3/8″ bolt (0.375 inches), and trying to force one into the other’s nut will damage both. When your measurement falls near the boundary between a metric and imperial size, check the pitch as well. Metric and imperial pitches almost never align, so the pitch measurement will tell you which system you’re dealing with even when the diameters are ambiguous.
Worn or damaged threads also throw off measurements. If the crests are visibly flattened, corroded, or chipped, your diameter reading will be smaller than the original specification. Measure across the least damaged section you can find, and keep in mind that the true size may be one step larger than what your calipers show.