Tapping Drill Sizes

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 SizeRecommended Tap Drill Size
M2 x 0.41.6 mm
M2.5 x 0.452.05 mm
M3 x 0.52.5 mm
M4 x 0.73.3 mm
M5 x 0.84.2 mm
M6 x 1.05.0 mm
M8 x 1.256.8 mm
M10 x 1.58.5 mm
M12 x 1.7510.2 mm
M14 x 2.011.9 mm
M16 x 2.013.5 mm
M18 x 2.515.2 mm
M20 x 2.516.8 mm
M22 x 2.518.4 mm
M24 x 3.020.0 mm
M27 x 3.022.5 mm
M30 x 3.525.0 mm
M33 x 3.527.5 mm
M36 x 4.030.0 mm

Tapping drill sizes Chart

Here is a chart of recommended tapping drill sizes for both Metric and UNC/UNF thread:

See also  Spring Steel Properties and Uses
Thread TypeThread SizeRecommended Tap Drill Size
MetricM3 x 0.52.5mm
MetricM4 x 0.73.3mm
MetricM5 x 0.84.2mm
MetricM6 x 1.05.0mm
MetricM8 x 1.256.8mm
MetricM10 x 1.58.5mm
MetricM12 x 1.7510.2mm
MetricM14 x 2.011.9mm
MetricM16 x 2.013.5mm
MetricM18 x 2.515.2mm
MetricM20 x 2.516.8mm
MetricM22 x 2.518.4mm
MetricM24 x 3.020.0mm
MetricM27 x 3.022.5mm
MetricM30 x 3.525.0mm
MetricM33 x 3.527.5mm
MetricM36 x 4.030.0mm
UNC/UNF1/16-270.0595 inches (1.5126 mm)
UNC/UNF1/8-270.095 inches (2.413 mm)
UNC/UNF1/4-180.209 inches (5.309 mm)
UNC/UNF5/16-180.3145 inches (7.994 mm)
UNC/UNF3/8-160.3175 inches (8.07 mm)
UNC/UNF7/16-140.4275 inches (10.795 mm)
UNC/UNF1/2-130.4535 inches (11.50 mm)
Image: Jarvistools

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).

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