What tap drill size is used for M10 x 1.5?

A common reference tap drill for M10 x 1.5 is 8.5 mm. This follows the practical major-diameter-minus-pitch approximation.

How do you calculate metric tap drill size?

A common starting approximation is tap drill diameter = major diameter minus thread pitch.

How do you calculate imperial tap drill size?

A simplified approximation is tap drill diameter = major diameter minus one divided by TPI, with the result converted to the selected imperial unit.

Does percentage thread change the tap drill size?

Yes. A lower percentage of theoretical thread generally uses a larger drill. The exact relationship depends on the thread geometry, tolerance and tapping method.

Can this replace a tap manufacturer's drill chart?

No. It is an educational calculator. Production tapping should follow the tap manufacturer's recommended drill size for the exact thread, material, tolerance and tap type.

Tap Drill Size Calculator – Metric & Imperial Tapping

Tap drill calculator

Thread system Common thread preset

Major diameter

Diameter unit

Thread pitch

millimeters per thread

Target theoretical thread percentage

Common metric tap drill sizes

Thread	Pitch	Common drill
M3 × 0.5	0.50 mm	2.5 mm
M4 × 0.7	0.70 mm	3.3 mm
M5 × 0.8	0.80 mm	4.2 mm
M6 × 1.0	1.00 mm	5.0 mm
M8 × 1.25	1.25 mm	6.8 mm
M10 × 1.5	1.50 mm	8.5 mm
M12 × 1.75	1.75 mm	10.2 mm
M14 × 2.0	2.00 mm	12.0 mm
M16 × 2.0	2.00 mm	14.0 mm
M18 × 2.5	2.50 mm	15.5 mm
M20 × 2.5	2.50 mm	17.5 mm
M22 × 2.5	2.50 mm	19.5 mm
M24 × 3.0	3.00 mm	21.0 mm

For common metric cutting taps, the familiar starting rule is approximately major diameter − pitch.

Common imperial tap drill references

Thread	TPI	Typical drill
#4-40	40	#30
#6-32	32	#36
#6-40	40	#33
#8-32	32	#29
#10-24	24	#25
#10-32	32	#21
1/4-20 UNC	20	#7
1/4-28 UNF	28	#3
5/16-18 UNC	18	F
5/16-24 UNF	24	I
3/8-16 UNC	16	5/16 in
3/8-24 UNF	24	Q
7/16-14 UNC	14	25/64 in
7/16-20 UNF	20	Y
1/2-13 UNC	13	27/64 in
1/2-20 UNF	20	29/64 in

How tap drill size works

Metric 100% reference

D − P

Imperial 100% reference

D − 1/TPI

Larger drill

Lower thread percentage

Smaller drill

Higher thread percentage

Example: M10 × 1.5

10.0 − 1.5 = 8.5 mm theoretical reference drill.

Example: 1/4-20

0.250 − 0.050 = 0.200 in theoretical reference drill.

Cutting taps vs forming taps

Cutting tap: removes material to produce the internal thread.
Forming tap: displaces material instead of cutting conventional chips.
Forming taps generally require different hole-size recommendations.
Blind holes need additional depth for the drill point, tap lead and chip space.
Material ductility and work-hardening behavior affect practical tapping conditions.
Lubrication and tapping speed can affect torque and thread quality.

For production machining, use the tap manufacturer's recommended drill diameter for the specific tap, thread tolerance and material. This calculator is an estimate, not a replacement for a manufacturer's tap chart.

Frequently Asked Questions

What drill size do I use for M10 × 1.5? The common reference drill is 8.5 mm.

What drill size do I use for M8 × 1.25? The common reference drill is 6.8 mm.

What drill size do I use for M6 × 1? The common reference drill is 5.0 mm.

What drill size do I use for 1/4-20? A common reference drill is #7, approximately 0.201 inch.

Why use less than 100% thread? A lower theoretical thread percentage generally reduces tapping torque and can make tapping easier while still providing useful thread engagement.

Does material change tap drill size? The basic thread geometry remains the same, but practical production recommendations can vary with material, tap style, tolerance, lubrication and process conditions.

Can I use this for forming taps? Use caution. Form taps have different hole-size requirements because material is displaced rather than removed.

Is a tap drill the same as a clearance drill? No. A tap drill prepares material for an internal thread. A clearance hole is intentionally larger so the screw or bolt passes through without engaging the thread.

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