Machine Screw Size Chart: #0 to 1/2 Inch UNC & UNF Reference
#6-32, #8-32, and #10-24.
What Is a Machine Screw?
A machine screw is a fastener with machine-cut threads designed to mate with a threaded nut or pre-tapped hole. Unlike wood screws, machine screws don't cut their own threads — they require a matching female thread to grip. They come in every head style: pan, flat (countersunk), oval, round, hex, socket cap, and truss.
Machine screws follow the Unified Thread Standard (UTS), which specifies two parameters: the diameter (in numbered sizes for small screws or fractional inches for larger) and the thread count (threads per inch, or TPI). You'll always see them written together: #8-32 means size #8, 32 threads per inch.
How Machine Screw Sizes Are Numbered
Sizes #0 through #12 follow a formula: major diameter (inches) = 0.060 + (0.013 × size number). A #8 screw: 0.060 + (0.013 × 8) = 0.164 inches. That's why the numbers don't map cleanly to a ruler — they're an industry numbering system, not a measurement in itself.
Above #12, the system switches to fractional inches: 1/4″, 5/16″, 3/8″, 7/16″, and 1/2″. These sizes also come in UNC and UNF thread variants.
M5 × 0.8 is metric (M5 = 5mm diameter, 0.8mm pitch). #10-24 is imperial machine screw (#10 diameter, 24 TPI). The M prefix is the key tell.
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Open Identifier →Machine Screw Size Chart: Complete Reference (#0–1/2″)
All data below follows the ASME B18.6.3 standard (machine screws) and B1.1 (unified inch threads). Clearance hole sizes are for a standard (free-fit) clearance — the hole the screw passes through freely without threading it.
| Size | Dia. (in) | Dia. (mm) | UNC TPI | UNF TPI | Clearance Drill | Nearest Metric |
|---|---|---|---|---|---|---|
| #0 | 0.0600" | 1.524 | 80 | — | #52 | M1.6 |
| #1 | 0.0730" | 1.854 | 64 | 72 | #47 | M2 |
| #2 | 0.0860" | 2.184 | 56 | 64 | #42 | M2.2 |
| #3 | 0.0990" | 2.515 | 48 | 56 | #37 | M2.5 |
| #4 | 0.1120" | 2.845 | 40 | 48 | #31 | M3 |
| #5 | 0.1250" | 3.175 | 40 | 44 | #29 | M3.2 |
| #6 | 0.1380" | 3.505 | 32 | 40 | #27 | M3.5 |
| #8 | 0.1640" | 4.166 | 32 | 36 | #18 | M4 |
| #10 | 0.1900" | 4.826 | 24 | 32 | #10 | M5 |
| #12 | 0.2160" | 5.486 | 24 | 28 | #2 | M5.5 |
| ↓ Fractional Sizes | ||||||
| 1/4" | 0.2500" | 6.350 | 20 | 28 | F (0.257") | M6 |
| 5/16" | 0.3125" | 7.938 | 18 | 24 | P (0.323") | M8 |
| 3/8" | 0.3750" | 9.525 | 16 | 24 | W (0.386") | M10 |
| 7/16" | 0.4375" | 11.113 | 14 | 20 | 29/64" | M11 |
| 1/2" | 0.5000" | 12.700 | 13 | 20 | 33/64" | M12 |
Orange TPI = most common designation for that size. Clearance drill = standard (free-fit) clearance hole. Nearest metric is approximate — threads are not compatible.
The Six Sizes You'll See Everywhere
Of the 15 standard sizes in the chart, six cover about 90% of real-world applications:
#4-40— small electronics, computer case thumbscrews, instrument panels#6-32— outlet covers, light fixtures, small appliances, PC case screws#8-32— electrical boxes, outlet boxes, general light hardware#10-24— light fixtures, cabinet hardware, larger electronics#10-32— rack mount equipment (servers, audio gear), fine-thread applications1/4-20— camera tripod mounts, structural hardware, standard machine work
If you're buying a replacement screw for an appliance or electronics item and don't have calipers, start with #6-32 and #8-32 — they're the most common by volume. Most outlets, covers, and electrical enclosures use one of those two.
UNC vs UNF: Which Thread Do You Have?
The same diameter screw comes in two thread variants. The key difference is how many threads fit in one inch.
| Property | UNC (Coarse) | UNF (Fine) |
|---|---|---|
| Thread spacing | Larger gaps | Smaller gaps |
| Assembly speed | Faster | Slower |
| Dirt/damage tolerance | More tolerant | Less tolerant |
| Vibration resistance | Lower | Higher |
| Best for | General use, wood/soft materials | Auto, instruments, thin-wall |
| Example: #10 | #10-24 UNC | #10-32 UNF |
How to Measure a Machine Screw Diameter
The best tool is a digital caliper. Measure across the outer thread crests (the highest points) — that is the major diameter and matches the size chart above.
If you don't have calipers, the easiest method is to thread the mystery screw into assorted nuts until you find a match. Match up the thread pitch by feel — a correct match threads in smoothly with no wobble. If it threads but feels slightly sloppy or too tight to start, the pitch is off (UNC vs UNF mismatch).
Machine Screw Length Measurement
Length conventions differ by head type:
- Pan, round, truss, hex, socket cap: measure from under the head to the tip of the threads
- Flat (countersunk): measure the total length including the head — the entire screw sinks into the material
- Oval (raised countersunk): measure from the maximum diameter of the head to the tip
This matters when ordering replacements. A pan-head #8-32 × 1″ and a flat-head #8-32 × 1″ have the same thread length but different overall lengths because the flat head is included in the measurement.
Machine Screws vs Machine Bolts
The terms overlap in common usage, but technically: a machine screw is fully threaded from tip to head and intended for use in tapped holes or with a nut. A machine bolt (hex bolt) has an unthreaded shank section and is designed for through-hole clearance with a nut on the other end. In practice, anything under 1/4″ is almost always called a "machine screw" regardless of threading; anything 1/4″ and above may be sold as either, depending on whether it's fully threaded.
Identify Your Fastener Now
Not sure which size you have? Measure the diameter, pick the head type, and our tool identifies your screw and gives you the full designation — no chart hunting required.
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Digital Caliper (6-inch)
A digital caliper is the fastest way to confirm a machine screw's major diameter. Reads to 0.001″ — accurate enough to distinguish every size in the chart above.
View on Amazon →Frequently Asked Questions
What diameter is a #8 machine screw?
A #8 machine screw has a major diameter of 0.164 inches (4.166 mm). The most common variant is #8-32 UNC (32 threads per inch coarse). You can verify with calipers measuring across the outer thread crests.
What is a #10 machine screw in metric?
A #10 machine screw is 0.190 inches (4.826 mm) in diameter. The closest metric size is M5 at 5.0 mm — slightly larger. The threads are not compatible, so you cannot substitute one for the other.
What is the difference between UNC and UNF machine screws?
UNC (Unified National Coarse) has fewer threads per inch — faster to assemble and more tolerant of dirt. UNF (Unified National Fine) has more threads per inch and resists vibration better. Both are available in the same numbered diameters. They are not interchangeable — always match the TPI.
How do I identify a machine screw size without calipers?
Thread the screw into nuts from a known assortment until you find a match that starts without cross-threading. You can also compare the shank to a ruler and match against the chart above. For exact measurements, a digital caliper is the most reliable approach.
What size drill bit do I need for a #6-32 clearance hole?
Use a #23 drill bit (0.154 inch) for a standard clearance hole through material. This passes the screw freely without threading. For a tapped (threaded) hole to receive a #6-32 screw, use a #36 drill bit (0.1065 inch) and then tap the hole 6-32.