The tap drill for an M3 x 0.5 thread is a 2.5 mm drill (0.0984 in), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 2.51 mm, and 2.5 mm is the nearest standard metric drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 2.68 mm | 2.7 mm | 2.70 mm / 0.1063 in | 46% |
| 60% | 2.61 mm | 2.6 mm | 2.60 mm / 0.1024 in | 62% |
| 65% | 2.58 mm | 2.6 mm | 2.60 mm / 0.1024 in | 62% |
| 70% | 2.55 mm | 2.55 mm | 2.55 mm / 0.1004 in | 69% |
| 75% | 2.51 mm | 2.5 mm | 2.50 mm / 0.0984 in | 77% |
The drills either side of the target are 8 points of thread apart, so either is a reasonable choice. 2.5 mm (0.0984 in) gives 77% and a stronger but harder-to-tap thread; 2.55 mm (0.1004 in) gives 69% and less tapping torque. On M3 x 0.5 the choice is mostly about tap breakage.
M3 is the workhorse of electronics enclosures, brackets, 3D printers and lab fixtures, and it is often the smallest size a shop taps on a machine. Its basic pitch diameter is 2.68 mm and its tensile stress area is 5.0 mm². A bolt of this size passes through a 3.40 mm normal-fit clearance hole (ISO 273).
Example: M3 x 0.5 at 75% thread
Result: 2.5 mm (0.0984 in), 77% thread
| Major (nominal) diameter | 3.00 mm / 0.1181 in |
|---|---|
| Pitch | 0.5 mm |
| Basic pitch diameter (d − 0.6495·P) | 2.68 mm |
| Basic minor diameter, internal (d − 1.0825·P) | 2.46 mm |
| Tensile stress area | 5.0 mm² |
| Fit | Hole diameter |
|---|---|
| Close | 3.20 mm / 0.1260 in |
| Normal | 3.40 mm / 0.1339 in |
| Loose | 3.60 mm / 0.1417 in |
All Metric coarse sizes:
Use 2.5 mm (0.0984 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 2.51 mm. For an easier-to-tap 50% thread, 2.7 mm (0.1063 in) gives 46%.
2.5 mm is 0.0984 in. The nearest inch drill to the 75% target is #39 (0.0995 in), which gives 73% thread.
A forming tap needs a bigger hole: 2.78 mm for 65% thread, so drill 2.8 mm (0.1102 in). Forming taps work in ductile metals such as aluminum and mild steel.
For M3 x 0.5, ISO 273 gives close fit 3.20 mm, normal fit 3.40 mm and loose fit 3.60 mm. Normal fit suits most bolted joints; close fit locates parts more accurately.
5.0 mm², from As = π/4 × (d − 0.9382 × P)². Multiply it by the bolt material's proof or tensile strength to get the load the thread can carry.
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