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The tap drill for a metric coarse thread is the nominal diameter minus the pitch: M6 x 1.0 takes a 5.0 mm drill, M8 x 1.25 takes 6.8 mm and M10 x 1.5 takes 8.5 mm. The tables below give the drill for each metric coarse size from M2 to M24, common metric fine sizes, and BSW sizes from 1/8" to 3/4".
Rule used: tap drill = major diameter minus pitch, rounded to a standard drill size (M12 x 1.75 gives 10.25, so the chart value is 10.2 mm).
Thread
Pitch (mm)
Tap drill (mm)
M2
0.4
1.6
M2.5
0.45
2.05
M3
0.5
2.5
M4
0.7
3.3
M5
0.8
4.2
M6
1.0
5.0
M8
1.25
6.8
M10
1.5
8.5
M12
1.75
10.2
M14
2.0
12.0
M16
2.0
14.0
M18
2.5
15.5
M20
2.5
17.5
M22
2.5
19.5
M24
3.0
21.0
Metric fine tap drill sizes
Thread
Tap drill (mm)
M6 x 0.75
5.25
M8 x 1
7.0
M10 x 1
9.0
M10 x 1.25
8.8
M12 x 1
11.0
M12 x 1.5
10.5
M14 x 1.5
12.5
M16 x 1.5
14.5
M20 x 1.5
18.5
M24 x 1.5
22.5
BSW tap drill sizes
BSW is British Standard Whitworth. The drills below are the sizes listed in BS 1157:1975, which give 77 to 87% of a full thread.
Size
Threads per inch
Tap drill (mm)
1/8"
40
2.55
3/16"
24
3.70
1/4"
20
5.10
5/16"
18
6.50
3/8"
16
7.90
7/16"
14
9.30
1/2"
12
10.50
9/16"
12
12.10
5/8"
11
13.50
11/16"
11
15.00
3/4"
10
16.25
Percentage of thread
Percentage of thread is how much of the full thread depth the drilled hole leaves for the tap to cut. A smaller drill leaves more thread to cut. A larger drill leaves less, so the tap works less and the thread has less engagement. The chart values above are the standard starting point; a tap maker's own chart takes priority where it differs.
Which BVF products fit
BVF lists metric taps from M2.5 to M18 and BSW tap sets. Each size below has its drill in the tables above.
Single HSS SPPT taps: M2.5 x 0.45, M3 x 0.5, M4 x 0.7, M5 x 0.8, M6 x 1.0, M8 x 1.25, M10 x 1.5, M12 x 1.75, M16 x 2 and M18 x 2.5 (see Taps & Tap Sets).
The IT Brand HSS jobber drills on BVF run from 1.00 mm to 13.00 mm, sold in packs by size range. They include the tap drills for M3 to M12 and for the BSW sizes from 3/16" to 1/2". The 2.05 mm drill for M2.5, and the drills for M14 and larger and for BSW 5/8" (13.5 mm), are not in these packs. Examples: HSS Jobber Drill 6.00mm - 6.90mm for M8 (6.8 mm) and HSS Jobber Drill 8.00mm - 8.90mm for M10 (8.5 mm).
