Engineering Reference
Thread Callout Decoder
Paste the callout as it appears on the drawing. Get the diameter, pitch, tap drill, and what the class actually means.
1/4-20 UNC-2B · Unified inch thread
Major diameter
0.2500 in
6.35 mm
Pitch
20 threads per inch
1.270 mm between crests
Tap drill
0.2000 in
approx. 75% thread
Class 2B
internal, standard fit
Major diameters per ASME B1.1 / ISO 261. Tap drills are workshop reference values; confirm against your tap maker's chart for hard materials.
Tap drill sizes, at a glance
The numbers the decoder returns, as a plain table you can read without typing anything. Tap drills are the standard 75 percent thread-engagement sizes. Inch diameters are in inches, metric in millimetres.
Unified coarse (UNC)
| Callout | Major Ø | TPI | Tap drill |
|---|---|---|---|
| #4-40 UNC | .1120 | 40 | #43 (.089) |
| #6-32 UNC | .1380 | 32 | #36 (.1065) |
| #8-32 UNC | .1640 | 32 | #29 (.136) |
| #10-24 UNC | .1900 | 24 | #25 (.1495) |
| 1/4-20 UNC | .2500 | 20 | #7 (.201) |
| 5/16-18 UNC | .3125 | 18 | F (.257) |
| 3/8-16 UNC | .3750 | 16 | 5/16 (.3125) |
| 7/16-14 UNC | .4375 | 14 | U (.368) |
| 1/2-13 UNC | .5000 | 13 | 27/64 (.4219) |
| 5/8-11 UNC | .6250 | 11 | 17/32 (.5312) |
| 3/4-10 UNC | .7500 | 10 | 21/32 (.6562) |
| 1-8 UNC | 1.0000 | 8 | 7/8 (.875) |
Unified fine (UNF)
| Callout | Major Ø | TPI | Tap drill |
|---|---|---|---|
| #10-32 UNF | .1900 | 32 | #21 (.159) |
| 1/4-28 UNF | .2500 | 28 | #3 (.213) |
| 5/16-24 UNF | .3125 | 24 | I (.272) |
| 3/8-24 UNF | .3750 | 24 | Q (.332) |
| 1/2-20 UNF | .5000 | 20 | 29/64 (.4531) |
| 3/4-16 UNF | .7500 | 16 | 11/16 (.6875) |
ISO metric coarse
| Callout | Major Ø | Pitch | Tap drill |
|---|---|---|---|
| M3 x 0.5 | 3.00 | 0.50 | 2.50 |
| M4 x 0.7 | 4.00 | 0.70 | 3.30 |
| M5 x 0.8 | 5.00 | 0.80 | 4.20 |
| M6 x 1.0 | 6.00 | 1.00 | 5.00 |
| M8 x 1.25 | 8.00 | 1.25 | 6.80 |
| M10 x 1.5 | 10.00 | 1.50 | 8.50 |
| M12 x 1.75 | 12.00 | 1.75 | 10.20 |
| M16 x 2.0 | 16.00 | 2.00 | 14.00 |
| M20 x 2.5 | 20.00 | 2.50 | 17.50 |
This tool decodes one callout at a time. Mavlon prices the whole bid package.
Mavlon’s AI Quoting Engine turns a customer’s bid package into a priced draft quote in your own pricing within minutes.
Book a Demo15 minutes, run by the founder
Frequently asked questions
What does 1/4-20 UNC-2B mean?
Quarter-inch major diameter, 20 threads per inch, Unified National Coarse, class 2B: an internal thread (tapped hole) at the standard fit. The matching bolt is 1/4-20 UNC-2A.
2A vs 2B vs 3A vs 3B?
The letter is the side: A external, B internal. The number is the fit: 2 standard commercial, 3 tighter with reduced allowance, common on aerospace fasteners. The class changes the pitch diameter limits you inspect against.
What does M8x1.25-6H mean?
Metric thread, 8 mm major diameter, 1.25 mm pitch (coarse for M8), class 6H: the standard internal fit. Written as just M8, the coarse pitch is implied.