What is Tapping Drill Sizes?
Tapping drill sizes refer to the size of the hole that needs to be drilled in order to tap threads into a material. The size of the hole is determined by the size of the tap that will be used.
Tapping drill size formula
The recommended tap drill size for a standard ( UNC/UNF) thread is calculated by subtracting 1/64 inch from the major diameter of the tap for sizes up to 1 inch and subtracting 1/32 inch for sizes above 1 inch.
For example, if you want to tap a 1/4-20 thread, the recommended tapping drill size would be 0.209 inches (1/4 inch – 1/64 inch).
For metric thread, the recommended tap drill size is calculated by subtracting 0.8 mm from the major diameter of the tap for sizes up to M12 and subtracting 1.0 mm for sizes above M12.
Tapping drill sizes for metric threads
Read more: What are Fasteners & their types?
recommended tapping drill sizes for metric threads:
| Metric Thread Size | Recommended Tap Drill Size |
|---|---|
| M2 x 0.4 | 1.6 mm |
| M2.5 x 0.45 | 2.05 mm |
| M3 x 0.5 | 2.5 mm |
| M4 x 0.7 | 3.3 mm |
| M5 x 0.8 | 4.2 mm |
| M6 x 1.0 | 5.0 mm |
| M8 x 1.25 | 6.8 mm |
| M10 x 1.5 | 8.5 mm |
| M12 x 1.75 | 10.2 mm |
| M14 x 2.0 | 11.9 mm |
| M16 x 2.0 | 13.5 mm |
| M18 x 2.5 | 15.2 mm |
| M20 x 2.5 | 16.8 mm |
| M22 x 2.5 | 18.4 mm |
| M24 x 3.0 | 20.0 mm |
| M27 x 3.0 | 22.5 mm |
| M30 x 3.5 | 25.0 mm |
| M33 x 3.5 | 27.5 mm |
| M36 x 4.0 | 30.0 mm |
Tapping drill sizes Chart
Here is a chart of recommended tapping drill sizes for both Metric and UNC/UNF thread:
| Thread Type | Thread Size | Recommended Tap Drill Size |
|---|---|---|
| Metric | M3 x 0.5 | 2.5mm |
| Metric | M4 x 0.7 | 3.3mm |
| Metric | M5 x 0.8 | 4.2mm |
| Metric | M6 x 1.0 | 5.0mm |
| Metric | M8 x 1.25 | 6.8mm |
| Metric | M10 x 1.5 | 8.5mm |
| Metric | M12 x 1.75 | 10.2mm |
| Metric | M14 x 2.0 | 11.9mm |
| Metric | M16 x 2.0 | 13.5mm |
| Metric | M18 x 2.5 | 15.2mm |
| Metric | M20 x 2.5 | 16.8mm |
| Metric | M22 x 2.5 | 18.4mm |
| Metric | M24 x 3.0 | 20.0mm |
| Metric | M27 x 3.0 | 22.5mm |
| Metric | M30 x 3.5 | 25.0mm |
| Metric | M33 x 3.5 | 27.5mm |
| Metric | M36 x 4.0 | 30.0mm |
| UNC/UNF | 1/16-27 | 0.0595 inches (1.5126 mm) |
| UNC/UNF | 1/8-27 | 0.095 inches (2.413 mm) |
| UNC/UNF | 1/4-18 | 0.209 inches (5.309 mm) |
| UNC/UNF | 5/16-18 | 0.3145 inches (7.994 mm) |
| UNC/UNF | 3/8-16 | 0.3175 inches (8.07 mm) |
| UNC/UNF | 7/16-14 | 0.4275 inches (10.795 mm) |
| UNC/UNF | 1/2-13 | 0.4535 inches (11.50 mm) |
Tapping drill size for m6
The recommended tapping drill size for an M6 metric thread is 5.0 mm. This is calculated by subtracting 0.8 mm from the major diameter of the tap (6.0 mm – 0.8 mm = 5.2 mm).
Tapping drill size for m8
The recommended tapping drill size for an M8 metric thread is 6.8 mm.
This is calculated by subtracting 0.8 mm from the major diameter of the tap (8.0 mm – 0.8 mm = 7.2 mm).
Tapping drill size for 1/8 npt
The recommended tapping drill size for a 1/8 NPT (National Pipe Taper) thread is 0.125 inches (3.175mm).
This is calculated by subtracting 0.013 inches (0.33mm) from the major diameter of the tap (1/8 inch – 0.013 inches = 0.125 inches).
Tapping drill size for 1/4 bsp
The recommended tapping drill size for a 1/4 BSP (British Standard Pipe) thread is 0.2065 inches (5.2375mm).
This is calculated by subtracting 0.02 inches (0.508 mm) from the major diameter of the tap (1/4 inch – 0.02 inches = 0.2065 inches)
Tapping drill size for 1/2 bsp
The recommended tapping drill size for a 1/2 BSP (British Standard Pipe) thread is 0.4535 inches (11.50mm).
This is calculated by subtracting 0.02 inches (0.508mm) from the major diameter of the tap (1/2 inch – 0.02 inches = 0.4535 inches).
Tapping drill size for 3/8 bsp
The recommended tapping drill size for a 3/8 BSP (British Standard Pipe) thread is 0.3175 inches (8.07mm).
This is calculated by subtracting 0.02 inches (0.508mm) from the major diameter of the tap (3/8 inch – 0.02 inches = 0.3175 inches).
Tapping drill size for m12
The recommended tapping drill size for an M12 metric thread is 10.2 mm. This is calculated by subtracting 1.0 mm from the major diameter of the tap (12.0 mm – 1.0 mm = 11.0 mm).
Tapping drill size for m24
The recommended tapping drill size for an M24 metric thread is 20.0 mm. This is calculated by subtracting 1.0 mm from the major diameter of the tap (24.0 mm – 1.0 mm = 23.0 mm).
Tapping drill size for m4
The recommended tapping drill size for an M4 metric thread is 3.3 mm. This is calculated by subtracting 0.8 mm from the major diameter of the tap (4.0 mm – 0.8 mm = 3.2 mm).
