threadsizes

M3 Tap Drill Size (M3 x 0.5)

ISO metric coarse · 0.5 mm pitch · ISO 261 / 965-1, class 6H

What size drill bit for a M3 tap?

2.5 mmmetric drill
0.0984inch
2.50mm
77%thread
No 2.5 mm drill? Use #39 (73%).

To scale section of the tapped hole

Section of a M3 x 0.5 tapped hole to scale: 2.5 mm (0.0984) drill, thread length 1.5 times the diameter. ⌀3 mm P 0.5 mm DRILL 2.5 mm Thread 1.5 × D +5 P drill
Drawn from the M3 x 0.5 basic profile. Hatched area is the part; the thread runs 1.5 × D and the drill goes 5 pitches deeper for a plug tap.

Drill options % of thread

Drill options for M3 x 0.5
DrillInchmmThread6H
2.45 mm 0.0965 2.45 85% ✗
#40 0.0980 2.49 79% ✓
2.5 mmUse 0.0984 2.50 77% ✓
#39 0.0995 2.53 73% ✓
2.55 mm 0.1004 2.55 69% ✓
#38 0.1015 2.58 65% ✓
2.6 mm 0.1024 2.60 62% ✗
#37 0.1040 2.64 55% ✗

By material Aluminum, brass, mild steel: 2.5 mm (77%) · Stainless, tool steel: #38 (65%)

75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 6H minor diameter limits (2.46 mm to 2.6 mm).

M3 x 0.5 ISO 261, 965-1, class 6H

Major diameter3 mm / 0.1181 in
Pitch0.5 mm
Pitch diameter (basic)2.675 mm
Minor diameter 6H2.459–2.599 mm
Thread-forming tap drill2.8 mm 0.1102 in

Clearance hole ISO 273

Close3.2 mm3.2 mm
Normal3.4 mm3.4 mm
Loose3.6 mm3.6 mm

For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.

Hole depth rule of thumb

  • 3 mmthread length for full strength in steel (1 × D)
  • 4.5 mmin cast iron or brass (1.5 × D), up to 6 mm in aluminum
  • +2.5 mmextra drill depth below the thread for a plug tap (5 pitches), +1 mm with a bottoming tap

Shop notes

Drill 2.5 mm and you get about 77% thread, squarely inside the 6H limits. In an inch shop the #39 (0.0995 in, 73%) is the one to use; the #40 (0.0980 in, 79%) also passes but makes a small tap work harder than it needs to.

The fine pitch is what makes M3 unforgiving. Going from 2.5 mm to 2.55 mm moves the thread from 77% to 69%, so a few hundredths of a millimeter matter here in a way they never do on an M10. On the loose side, a #38 (65%) is the last drill that still clears the minor diameter check, and it is a sensible choice in stainless or other hard stock where tap breakage is the bigger worry. The 2.6 mm that sits right next to it in a metric set is already outside the limits.

On the tight side, 2.45 mm gives about 85% thread and fails the check. It buys nothing in holding power and costs a lot in tapping torque.

Do not confuse M3 with #4-40. The 4-40 is slightly smaller (0.112 in against 0.1181 in) and coarser (0.635 mm pitch against 0.5 mm). An M3 screw will not go into a 4-40 hole without damage, and a 4-40 in an M3 hole is a sloppy fit that strips early.