What Is an 8-32 Screw? Meaning and Size Explained

An 8-32 screw is a #8 machine screw with 32 threads per inch, giving it a major diameter of about 0.164 in (4.17 mm) in the coarse UNC thread series.

A stripped hole, a nut that won’t thread, a drawer of leftovers — the numbers on a screw tell you exactly what will and won’t fit. An 8-32 screw is a US inch-series machine screw. The “8” is a size gauge, not a millimeter measurement, and the “32” counts threads per inch. Get those numbers right and you avoid the most common hardware mistake: buying a screw that looks identical to the one you lost but refuses to thread.

What Do the Numbers “8-32” Actually Mean?

The first number is a screw size gauge mapping to a fixed diameter; the second is threads per inch (TPI). Neither is metric. A #8 screw has a major diameter of 0.1640 in — roughly 4.17 mm — and 32 TPI converts to a thread pitch of 0.03125 in, or about 0.79375 mm between crests.

The gauge number is not linear. Screw gauges follow their own scale, so a #8 is not “twice” a #4. For the exact diameter, look it up or measure it; don’t assume the gauge number multiplies into inches.

What Size Is an 8-32 Screw in Inches and Millimeters?

An 8-32 screw measures 0.1640 in across its major diameter, equal to about 4.17 mm, with 32 threads per inch. That diameter matters when checking fit, because thread pitch only tells you how closely threads are spaced.

Is 8-32 the Same as 8-36?

No. They share a diameter but not a thread count, so they are not interchangeable. An 8-32 screw threads into a #8-32 tapped hole or nut and nothing else — an 8-36 fine-thread fastener will bind or cross-thread in the same hole even though the shaft looks the same width.

Coarse (UNC) and fine (UNF) threads are a deliberate design choice. Coarse threads like the 8-32 engage faster and resist stripping in softer materials; fine threads hold better under vibration and give more precise adjustment on machinery. When matching a replacement, read both numbers off the original screw or its mating nut — never guess from diameter alone.

What Do You Use an 8-32 Screw For?

An 8-32 screw is a machine screw, built for a tapped hole, nut, or threaded insert rather than biting into raw wood. Product listings show it sold in fully threaded, right-hand, coarse-thread variants as socket head cap screws, hex head screws, and other head styles.

Head and length vary by product family, so two boxes of “8-32” screws can look and behave nothing alike. One common socket head cap screw family lists a head diameter of 0.27 in and a head height of 0.164 in — but that’s that product, not a rule. A #29 drill (0.136 in) is the standard tap drill for cutting #8-32 threads, and a #16 drill (0.177 in) gives a normal-fit clearance hole.

Material and thread-fit grade change strength and corrosion resistance, and those come from the specific screw, not the sizing. Stainless steel resists rust better than plain steel, but the 8-32 label tells you nothing about load rating. Using an under-spec’d screw in a stressed joint can strip threads or fail, so match material and grade to the job.

If you’re stocking up, a tested roundup of the best 8-32 screws to buy can save you a return trip.

How Do You Read Any US Screw Size?

Split the designation into diameter and thread count, then confirm both against the hole or nut it must fit. The US inch system uses gauge numbers for diameter, and the second figure is always TPI. Metric fasteners work the opposite way: diameter in millimeters first, thread pitch in millimeters second, like M4 x 0.7. That difference trips up plenty of people sorting a mixed bin.

“8” is not 8 mm, and a 4.17 mm screw is not a 4 mm metric bolt. Grab the right replacement by measuring major diameter and counting threads per inch, then hand-thread the screw a few turns before tightening — a correct fit spins in with light finger pressure. If it seats without resistance, you’ve got the right match and can torque it down.