Free tool

Tap Drill Size Calculator

Every printed tap drill chart assumes 75% thread engagement and never says so. That assumption is what snaps taps in stainless. Pick your thread, set the engagement your material actually wants, and get the hole size — 87 threads across metric, UNC, UNF, BSW, BSP and NPT.

Calculator

Tap drill size, for the engagement you actually want

Type a thread — M8, 1/4-20, G1/2, 10-32 — then set the thread engagement. Charts only ever give you 75%; stainless usually wants 60%.

Thread engagement75%

50% easier to tap85% stronger thread

M8 × 1.25Metric coarse

Tap drill: 6.8 mm
Exact size: 6.78 mm
Inch equivalent: H
Pitch: 1.25 mm

Actual engagement: 74% with the 6.8 mm bit you can actually buy — stock sizes come in 0.1 mm steps, and this rounds up, because an undersized tap drill is what breaks taps.

Clearance hole: 8.4 / 9 / 10 mm (close / medium / free)

M8 × 1.0Metric fine

Tap drill: 7.0 mm
Exact size: 7.03 mm
Inch equivalent: J
Pitch: 1 mm

Actual engagement: 77% with the 7.0 mm bit you can actually buy — stock sizes come in 0.1 mm steps, and this rounds up, because an undersized tap drill is what breaks taps.

Clearance hole: 8.4 / 9 / 10 mm (close / medium / free)

What thread engagement actually buys you

Thread engagement is how much of the theoretical thread profile the tap has to cut. It is the single most useful number in tapping, and almost nobody adjusts it, because charts hide it. The relationship is badly non-linear, and that is the whole point:

Engagement	Thread strength	Tapping torque	When to use it
55%	~90%	Lowest	Hardened and tool steel, deep blind holes
60%	~93%	About half of 75%	Stainless 304 / 316 — the tap-saving setting
70%	~96%	Low	Mild steel, everyday shop work
75%	100% (reference)	Standard	What every printed chart assumes
80%	~102%	High	Aluminium, brass, plastics — soft material needs the bite
85%	~103%	Very high	Rarely worth it. 3% more strength, far more broken taps

Thread strength versus tapping torque by engagement

Read the third row against the fifth. Dropping from 80% to 60% costs about 9% of thread strength and saves more than half the torque. In practice the joint fails at the bolt, not the thread, long before that 9% ever matters — so on anything that work-hardens, the low setting is simply the better engineering choice.

Engagement by material

Stainless steel (304 / 316) — 60%. Work-hardens and grabs. Lower engagement is the single best defence against a snapped tap.
Mild steel — 70%. The everyday default. 70% carries about 95% of the strength of a full thread.
Cast iron — 75%. Chips break short and clear easily, so it takes standard engagement without complaint.
Aluminium / brass — 80%. Soft and free-cutting; the extra engagement compensates for weaker material.
Hardened / tool steel — 55%. Torque rises steeply with engagement. Go low, use spiral point taps and cutting fluid.
Plastics — 80%. Threads strip easily in plastic — take the extra material, and never over-torque the screw.

Full tap drill table

Every thread in the calculator, at the standard 75% engagement, with the drill you would actually order. 87 threads across seven standards.

Thread	Pitch / TPI	Tap drill 75%	Exact	Inch	Clearance (medium)
M1.6 × 0.35	0.35 mm	1.3 mm	1.26	#55	1.8 mm
M2 × 0.4	0.4 mm	1.6 mm	1.61	#52	2.4 mm
M2.5 × 0.45	0.45 mm	2.1 mm	2.06	#46	2.9 mm
M3 × 0.5	0.5 mm	2.5 mm	2.51	#39	3.4 mm
M3.5 × 0.6	0.6 mm	2.9 mm	2.92	#32	3.9 mm
M4 × 0.7	0.7 mm	3.3 mm	3.32	#30	4.5 mm
M5 × 0.8	0.8 mm	4.2 mm	4.22	#19	5.5 mm
M6 × 1.0	1 mm	5.0 mm	5.03	#8	6.6 mm
M7 × 1.0	1 mm	6.0 mm	6.03	B	7.6 mm
M8 × 1.25	1.25 mm	6.8 mm	6.78	H	9 mm
M10 × 1.5	1.5 mm	8.5 mm	8.54	R	11 mm
M12 × 1.75	1.75 mm	10.3 mm	10.30	13/32"	13.5 mm
M14 × 2.0	2 mm	12.1 mm	12.05	15/32"	15.5 mm
M16 × 2.0	2 mm	14.1 mm	14.05	35/64"	17.5 mm
M18 × 2.5	2.5 mm	15.6 mm	15.56	39/64"	20 mm
M20 × 2.5	2.5 mm	17.6 mm	17.56	11/16"	22 mm
M22 × 2.5	2.5 mm	19.6 mm	19.56	49/64"	24 mm
M24 × 3.0	3 mm	21.1 mm	21.08	53/64"	26 mm
M27 × 3.0	3 mm	24.1 mm	24.08	61/64"	30 mm
M30 × 3.5	3.5 mm	26.6 mm	26.59	1/1"	33 mm
M4 × 0.5	0.5 mm	3.5 mm	3.51	#28	4.5 mm
M5 × 0.5	0.5 mm	4.5 mm	4.51	#16	5.5 mm
M6 × 0.75	0.75 mm	5.3 mm	5.27	#4	6.6 mm
M8 × 1.0	1 mm	7.0 mm	7.03	J	9 mm
M10 × 1.25	1.25 mm	8.8 mm	8.78	11/32"	11 mm
M10 × 1.0	1 mm	9.0 mm	9.03	T	11 mm
M12 × 1.5	1.5 mm	10.5 mm	10.54	Z	13.5 mm
M12 × 1.25	1.25 mm	10.8 mm	10.78	27/64"	13.5 mm
M14 × 1.5	1.5 mm	12.5 mm	12.54	1/2"	15.5 mm
M16 × 1.5	1.5 mm	14.5 mm	14.54	37/64"	17.5 mm
M18 × 1.5	1.5 mm	16.5 mm	16.54	21/32"	20 mm
M20 × 1.5	1.5 mm	18.5 mm	18.54	47/64"	22 mm
M24 × 2.0	2 mm	22.1 mm	22.05	7/8"	26 mm
#4-40 UNC	40 tpi	2.2 mm	2.23	#43	#30
#6-32 UNC	32 tpi	2.7 mm	2.73	#36	#27
#8-32 UNC	32 tpi	3.4 mm	3.39	#29	#18
#10-24 UNC	24 tpi	3.8 mm	3.79	#25	#9
#12-24 UNC	24 tpi	4.5 mm	4.46	#16	#2
1/4-20 UNC	20 tpi	5.1 mm	5.11	#7	F
5/16-18 UNC	18 tpi	6.6 mm	6.56	F	P
3/8-16 UNC	16 tpi	8.0 mm	7.98	5/16"	W
7/16-14 UNC	14 tpi	9.3 mm	9.34	U	29/64"
1/2-13 UNC	13 tpi	10.8 mm	10.80	27/64"	33/64"
9/16-12 UNC	12 tpi	12.2 mm	12.23	31/64"	37/64"
5/8-11 UNC	11 tpi	13.6 mm	13.63	17/32"	41/64"
3/4-10 UNC	10 tpi	16.6 mm	16.58	21/32"	49/64"
7/8-9 UNC	9 tpi	19.5 mm	19.48	49/64"	57/64"
1-8 UNC	8 tpi	22.3 mm	22.31	7/8"	1 1/64"
#4-48 UNF	48 tpi	2.3 mm	2.33	#42	#30
#6-40 UNF	40 tpi	2.9 mm	2.89	#33	#27
#8-36 UNF	36 tpi	3.5 mm	3.48	#29	#18
#10-32 UNF	32 tpi	4.1 mm	4.05	#21	#9
#12-28 UNF	28 tpi	4.6 mm	4.60	#14	#2
1/4-28 UNF	28 tpi	5.5 mm	5.47	#3	F
5/16-24 UNF	24 tpi	6.9 mm	6.91	I	P
3/8-24 UNF	24 tpi	8.5 mm	8.49	Q	W
7/16-20 UNF	20 tpi	9.9 mm	9.88	25/64"	29/64"
1/2-20 UNF	20 tpi	11.5 mm	11.46	29/64"	33/64"
9/16-18 UNF	18 tpi	12.9 mm	12.91	33/64"	37/64"
5/8-18 UNF	18 tpi	14.5 mm	14.50	37/64"	41/64"
3/4-16 UNF	16 tpi	17.5 mm	17.50	11/16"	49/64"
1-12 UNF	12 tpi	23.3 mm	23.34	59/64"	1 1/64"
1/8-40 BSW	40 tpi	2.6 mm	2.57	#38	—
3/16-24 BSW	24 tpi	3.7 mm	3.75	#26	—
1/4-20 BSW	20 tpi	5.1 mm	5.13	#7	—
5/16-18 BSW	18 tpi	6.6 mm	6.58	G	—
3/8-16 BSW	16 tpi	8.0 mm	8.00	O	—
7/16-14 BSW	14 tpi	9.4 mm	9.37	U	—
1/2-12 BSW	12 tpi	10.7 mm	10.67	27/64"	—
5/8-11 BSW	11 tpi	13.7 mm	13.66	17/32"	—
3/4-10 BSW	10 tpi	16.6 mm	16.61	21/32"	—
1-8 BSW	8 tpi	22.4 mm	22.35	7/8"	—
G1/8 (BSP)	28 tpi	8.9 mm	8.86	S	—
G1/4 (BSP)	19 tpi	11.9 mm	11.87	15/32"	—
G3/8 (BSP)	19 tpi	15.4 mm	15.38	39/64"	—
G1/2 (BSP)	14 tpi	19.2 mm	19.21	3/4"	—
G3/4 (BSP)	14 tpi	24.7 mm	24.70	31/32"	—
G1 (BSP)	11 tpi	31.0 mm	31.03	1/1"	—
G1-1/4 (BSP)	11 tpi	39.7 mm	39.69	1/1"	—
G1-1/2 (BSP)	11 tpi	45.6 mm	45.59	1/1"	—
G2 (BSP)	11 tpi	57.4 mm	57.40	1/1"	—
1/8-27 NPT	27 tpi	8.6 mm	—	R	—
1/4-18 NPT	18 tpi	11.1 mm	—	7/16"	—
3/8-18 NPT	18 tpi	14.5 mm	—	37/64"	—
1/2-14 NPT	14 tpi	17.9 mm	—	45/64"	—
3/4-14 NPT	14 tpi	23.2 mm	—	29/32"	—
1-11.5 NPT	11.5 tpi	29.2 mm	—	1/1"	—

Common questions

What is the tap drill size for M8?+

6.8 mm for M8 × 1.25 coarse at the standard 75% engagement, and 7.0 mm for M8 × 1.0 fine. In stainless, drop to 60% engagement and use 7.0 mm — the thread is barely weaker and the tap is far less likely to snap.

What is the tap drill size for M10?+

8.5 mm for M10 × 1.5 coarse. For M10 × 1.25 fine use 8.8 mm, and for M10 × 1.0 use 9.0 mm. Coarse is the default unless the drawing says otherwise.

What tap drill do I use for 1/4-20 UNC?+

A #7 gauge drill (0.201", 5.11 mm) at 75% engagement. The metric substitute is 5.1 mm, which is within 0.01 mm — effectively exact.

Why does 75% thread engagement matter?+

Because it is the number every printed chart silently assumes. Going from 75% to 60% engagement costs only about 5% of thread strength but cuts tapping torque roughly in half. In stainless and hardened steel that is the difference between a finished part and a broken tap in a finished part.

Is a G1/2 BSP thread half an inch across?+

No — and this catches people out constantly. BSP sizes name the nominal bore of the pipe, not the thread. A G1/2 thread measures 20.96 mm across the crests, roughly 13/16", and its tap drill is about 19 mm.

Why is the NPT engagement slider disabled?+

NPT is a tapered thread, so the hole diameter is fixed and the fit comes from how deep the tap goes rather than how much material the drill left. The calculator shows the standard drill size instead.

Related guides

From our range

Taps, dies and the drills that feed them

HSS ground-thread taps in spiral point and spiral flute, adjustable dies, and the 4.2 / 5.1 / 6.8 / 8.5 / 10.2 mm bits no standard drill set contains. Factory-direct, OEM/ODM, mixed cartons.

Taps & Dies

Industrial HSS Spiral Point Machine Tap (Through Hole Dedicated) – OEM

Taps & Dies

Industrial M8x1.25 HSS Spiral Flute Machine Tap (Blind Hole) – OEM

Taps & Dies

Industrial HSS Straight Flute Machine Tap (Through Hole) – OEM Sizes

Taps & Dies

M16 X 1.5 Industrial HSS Adjustable Round Threading Die (Metric) – OEM Sizes

HSS Twist Drill Bits

Industrial Titanium Coated HSS Twist Drill Bits (DIN338, Metric Sizes) – OEM

M35 Cobalt Drill Bits

Industrial M35 Cobalt HSS Twist Drill Bits for Stainless Steel (Metric Sizes)

Get a quote in 24h All taps & dies