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Calculate thread dimensions instantly for metric, UNC, UNF, NPT, and BSPT threads. Get major diameter, pitch diameter, minor diameter, thread depth, and tap drill sizes.
(Internal thread fit)
Metric (ISO 68):
H = Pitch × 0.866025
Thread Depth = H × 0.625
UN (Unified):
H = (1/TPI) × 0.866025
Thread Depth = H × 0.625
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A thread is a helical ridge on a cylinder or cone. Threads are essential components in fastening, power transmission, and mechanical assembly. The quality and accuracy of threads determine the reliability of assemblies.
Thread dimensions include major diameter (largest diameter), pitch diameter (effective diameter), minor diameter (smallest diameter), and thread depth. These measurements ensure proper fit and function in assemblies.
Key Parameters:
| Thread Type | Region | Common Use | Pitch |
|---|---|---|---|
| UNC | Imperial | General fastening | Coarse |
| UNF | Imperial | Precision work | Fine |
| UNEF | Imperial | Ultra-precision | Extra Fine |
| Metric | Metric | International standard | Variable |
| NPT | Imperial | Pipe fittings | Coarse |
| BSPT | British | British pipe fittings | Coarse |
| Size | TPI | Pitch |
|---|---|---|
| #4 | 40 | 0.025" |
| #6 | 32 | 0.0313" |
| 1/4" | 20 | 0.050" |
| 5/16" | 18 | 0.0556" |
| 3/8" | 16 | 0.0625" |
| 1/2" | 13 | 0.0769" |
| Size | Pitch | Designation |
|---|---|---|
| M6 | 1.0mm | M6x1.0 |
| M8 | 1.25mm | M8x1.25 |
| M10 | 1.5mm | M10x1.5 |
| M12 | 1.75mm | M12x1.75 |
| M16 | 2.0mm | M16x2.0 |
| M20 | 2.5mm | M20x2.5 |
The tap drill size is approximately 75% of the major diameter, creating the correct depth for standard threads. Using the correct tap drill size ensures proper thread strength and prevents bolt breakage.
| Thread Size | Tap Drill Size | Thread Size | Tap Drill Size |
|---|---|---|---|
| 1/4"-20 UNC | 7 (0.201") | M6x1.0 | 5.0mm |
| 5/16"-18 UNC | "F" (0.257") | M8x1.25 | 6.8mm |
| 3/8"-16 UNC | 5/16" (0.3125") | M10x1.5 | 8.5mm |
| 7/16"-14 UNC | 3/8" (0.375") | M12x1.75 | 10.2mm |
| 1/2"-13 UNC | 27/64" (0.422") | M14x2.0 | 12.0mm |
| 1/4"-28 UNF | 3 (0.213") | M16x2.0 | 14.0mm |
Pitch is the distance between thread crests (measured in mm for metric threads). TPI (Threads Per Inch) is the number of threads in one inch of length. They're inversely related: TPI = 1 / Pitch (in inches).
Thread class defines how tight or loose the fit is between a bolt and nut. Classes range from 1 (loose fit) to 3 (tight fit). Letter A indicates external threads (bolts), B indicates internal threads (nuts). 2B is the most common for general fastening.
Use the formula: Tap Drill Size = Major Diameter - (2 × Depth of Thread). A good rule of thumb is approximately 75% of the major diameter. Using the correct size prevents stripped threads and weak connections.
NPT (National Pipe Taper) threads are used for pipe fittings and connections. The slight taper (1:16) creates a seal when assembled. NPT is used in hydraulic, pneumatic, and plumbing applications.
UNF (fine pitch) has more threads per inch, making it ideal for precision work, vibration resistance, and thin-walled applications. UNC (coarse pitch) is faster to assemble and better for general fastening.
No. Metric and imperial threads are not compatible due to different pitches and angles (60° vs 55°). Always match thread standards. Use adapters if mixing is necessary.
Thread stripping occurs from over-tightening, using the wrong tap drill size (too large), material incompatibility, or corrosion. Prevent it by using correct torque specs, proper drilling, and appropriate materials.
A good rule is at least 1.5 times the bolt diameter. For critical applications, use 2-3 times. Minimum engagement ensures adequate strength without over-stressing the hole.
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