Thread Size Chart
Complete reference for metric and imperial thread sizes. Includes tap drill sizes and clearance hole dimensions.
Thread Size Reference Chart
Tap drill sizes for ~75% thread engagement
| Thread Size | Major Ø | Pitch | Tap Drill(75%) | Clearance Close | Clearance Normal |
|---|---|---|---|---|---|
| M1.6 | 1.6mm | 0.35mm | 1.25mm | 1.7mm | 1.8mm |
| M2 | 2.0mm | 0.4mm | 1.6mm | 2.2mm | 2.4mm |
| M2.5 | 2.5mm | 0.45mm | 2.05mm | 2.7mm | 2.9mm |
| M3 | 3.0mm | 0.5mm | 2.5mm | 3.2mm | 3.4mm |
| M3.5 | 3.5mm | 0.6mm | 2.9mm | 3.7mm | 3.9mm |
| M4 | 4.0mm | 0.7mm | 3.3mm | 4.3mm | 4.5mm |
| M5 | 5.0mm | 0.8mm | 4.2mm | 5.3mm | 5.5mm |
| M6 | 6.0mm | 1.0mm | 5.0mm | 6.4mm | 6.6mm |
| M8 | 8.0mm | 1.25mm | 6.8mm | 8.4mm | 9.0mm |
| M10 | 10.0mm | 1.5mm | 8.5mm | 10.5mm | 11.0mm |
| M12 | 12.0mm | 1.75mm | 10.2mm | 13.0mm | 13.5mm |
| M14 | 14.0mm | 2.0mm | 12.0mm | 15.0mm | 15.5mm |
| M16 | 16.0mm | 2.0mm | 14.0mm | 17.0mm | 17.5mm |
| M18 | 18.0mm | 2.5mm | 15.5mm | 19.0mm | 20.0mm |
| M20 | 20.0mm | 2.5mm | 17.5mm | 21.0mm | 22.0mm |
| M22 | 22.0mm | 2.5mm | 19.5mm | 23.0mm | 24.0mm |
| M24 | 24.0mm | 3.0mm | 21.0mm | 25.0mm | 26.0mm |
Understanding the Chart
- • Tap Drill: Hole size before threading (~75% thread engagement)
- • Clearance Close: Tight fit, minimal play
- • Clearance Normal: Standard fit for most applications
- • Pitch: Distance between threads in millimeters
Most Common Metric
M3, M4, M5, M6, M8
Most Common UNC
#4-40, #6-32, #8-32, 1/4-20
Metric ≈ Imperial
M4 ≈ #8, M6 ≈ 1/4"
Understanding Thread Sizes
Metric Thread Designation
Metric threads are designated by the letter M followed by the nominal diameter in millimeters. The thread pitch (distance between threads) may be specified after an "x". For example:
- M6 — 6mm diameter, standard (coarse) pitch (1.0mm assumed)
- M6 x 1.0 — 6mm diameter, 1.0mm pitch (coarse)
- M6 x 0.75 — 6mm diameter, 0.75mm pitch (fine)
Imperial Thread Designation
Imperial threads use two main systems: UNC (Unified National Coarse) and UNF (Unified National Fine). The designation includes:
- Nominal diameter (fraction or number)
- Threads per inch (TPI)
- Thread series (UNC or UNF)
Examples: 1/4-20 UNC (1/4" diameter, 20 TPI, coarse),1/4-28 UNF (1/4" diameter, 28 TPI, fine)
Tap Drill Selection
The tap drill creates the hole before threading. The drill size determines the thread engagement percentage:
- 75% thread — Standard recommendation, good balance of strength and ease of tapping
- 65% thread — Easier tapping, suitable for harder materials
- 83% thread — Maximum practical thread, harder to tap but stronger
The tap drill sizes in our chart are for approximately 75% thread engagement.
Clearance Hole Sizing
Clearance holes allow a bolt or screw to pass through freely. There are three standard fits:
- Close fit: Minimal clearance, good alignment
- Normal fit: Standard clearance for most applications
- Free fit: Generous clearance for easy assembly
Common Thread Applications
| Size | Common Uses |
|---|---|
| M2, M2.5 | Electronics, small mechanisms, PCB standoffs |
| M3, #4-40 | Electronics enclosures, small appliances |
| M4, #8-32 | Computer cases, rack equipment |
| M5, #10-32 | Industrial equipment, fixtures |
| M6, 1/4-20 | General machinery, camera tripods (1/4-20) |
| M8, 5/16-18 | Structural, automotive |
Frequently Asked Questions
What is the tap drill size for M3?
The tap drill size for M3 x 0.5 (standard pitch) is 2.5mm. This creates approximately 75% thread engagement, which is ideal for most applications. For M3 x 0.35 (fine pitch), use a 2.65mm drill.
What is the difference between UNC and UNF threads?
UNC (Unified National Coarse) threads have fewer threads per inch and are more common for general fastening. UNF (Unified National Fine) threads have more threads per inch, providing greater strength and better resistance to vibration loosening. UNF is often used in automotive and aerospace applications.
How do I determine clearance hole size?
Clearance holes should be slightly larger than the bolt/screw diameter to allow the fastener to pass through freely. For a close fit, add 0.1-0.2mm (metric) or use the next drill size up (imperial). For a free fit, add 0.3-0.5mm or go 2-3 drill sizes larger.
What does M4 x 0.7 mean?
M4 x 0.7 is a metric thread designation where "M4" indicates a 4mm nominal diameter and "0.7" is the thread pitch in millimeters (distance between threads). This is the standard/coarse pitch for M4. Fine pitch M4 would be M4 x 0.5.
What is thread pitch?
Thread pitch is the distance from one thread crest to the next, measured parallel to the thread axis. In metric threads, pitch is expressed in millimeters (e.g., 0.5mm, 1.0mm). In imperial threads, it's expressed as threads per inch (TPI), such as 20 TPI or 32 TPI.
How do I convert between metric and imperial thread sizes?
Metric and imperial threads are not directly interchangeable. However, some sizes are close: M3 ≈ #4, M4 ≈ #8, M5 ≈ #10, M6 ≈ 1/4", M8 ≈ 5/16", M10 ≈ 3/8". Always verify compatibility as thread angles differ (60° for both, but pitch/TPI won't match).
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