Threads and Tapping Basics (Metric Threads & Tap Drill Chart)

Tapping cuts an internal thread into a pilot hole. Choosing the right pilot (tap drill) diameter and tap type is key to thread accuracy and tool life. This article compiles the basics of threads and tapping.

Thread basics

• Metric thread (M): the most common. "M6" means a 6mm nominal diameter
• Pitch: distance between thread crests. Coarse (standard) and fine exist
• Designation: M6×1.0 (diameter × pitch). Fine is written like M8×1.0
• Pipe threads (G, R): for piping (tapered/parallel)

Common thread standards

Beyond metric threads, several standards appear in practice. Knowing each symbol and thread angle prevents mix-ups when looking up or ordering parts.

• Metric thread (M): the most common; prefixed with "M" (e.g., M3 = 3mm nominal diameter). Pitch is written P○○, 60° thread angle. Coarse and fine pitches exist
• Inch threads: sized as fractions of one inch (≈25.4mm), usually in 1/8 steps. Pitch is given as threads per inch (TPI), not as a distance like metric. Includes Unified, Whitworth, and pipe threads
• Unified thread (UN): a US standard, 60° angle. Coarse = UNC, fine = UNF (extra-fine UNEF also exists). Common in cars, motorcycles, and machinery
• Whitworth thread (W): written W1/2, etc., 55° angle. The different angle makes it not interchangeable with Unified at the same size (except a few sizes with matching TPI) — use a thread gauge or spec sheet to tell them apart
• Pipe threads: for joining pipes, 55° angle, in tapered and parallel types

Pipe threads: tapered vs parallel

• Tapered pipe thread (JIS B0203 / ISO 7/1): conical, taper 1/16, used for watertight/airtight joints. External R (old PT), internal Rc (old PT); the parallel internal thread used with a tapered external thread is Rp (old PS)
• Parallel pipe thread (JIS B0202 / ISO 228/1): cylindrical, used for mechanical joints. Symbol G (old PF). External threads carry a tolerance class A or B (B has twice the tolerance of A), e.g., G3/8A

Pipe thread type	Symbol (old)	Angle
Tapered — external tapered	R (PT)	55°
Tapered — internal tapered	Rc (PT)	55°
Tapered — internal parallel	Rp (PS)	55°
Parallel — external	G (PF), class A/B	55°
Parallel — internal	G (PF)	55°

Tap drill chart (metric coarse)

Tap drill diameter ≒ nominal diameter − pitch.

Thread	Pitch	Tap drill (approx.)
M2	0.4	1.6mm
M3	0.5	2.5mm
M4	0.7	3.3mm
M5	0.8	4.2mm
M6	1.0	5.0mm
M8	1.25	6.8mm
M10	1.5	8.5mm
M12	1.75	10.2mm

Full dimension tables (shop-floor ready)

These tables are usable right at the machine: pick a thread and you get the tap drill, the internal-thread (nut) dimensions, and the external-thread (bolt) dimensions with standard fit tolerances. For metric threads the min–max follow the most common 6g (external) / 6H (internal) classes (ISO 965 / JIS B0209). Units: mm.

Reading the columns (metric): d = external major diameter (turn the thread down to this); d₂ = D₂ = pitch diameter (governs the fit — the key gauged dimension); D₁ = internal minor diameter (the allowed range of the hole after tapping — the tap drill must fall inside it); D (internal major) min = nominal diameter. External threads sit below nominal, internal above — this creates the assembly clearance.

Fig. 1 — Telling the diameters apart. On an external thread (bolt) the crest is the largest OUTER diameter d and the root is d₁. On an internal thread (nut/hole) the crest is the smallest INNER diameter D₁ (the bore) and the root is D. d₂/D₂ is the pitch diameter (governs the fit); P is the pitch.

Metric coarse — M1.6–M39, tolerance 6g/6H

Thread	Pitch P	Tap drill	d ext max	d ext min	d₂ ext max	d₂ ext min	D₂ int max	D₂ int min	D₁ int max	D₁ int min
M1.6	0.35	1.25	1.581	1.496	1.354	1.291	1.458	1.373	1.321	1.221
M2	0.40	1.60	1.981	1.886	1.721	1.654	1.830	1.740	1.679	1.567
M2.5	0.45	2.05	2.480	2.380	2.188	2.117	2.303	2.208	2.138	2.013
M3	0.50	2.50	2.980	2.874	2.655	2.580	2.775	2.675	2.599	2.459
M3.5	0.60	2.90	3.479	3.354	3.089	3.004	3.222	3.110	3.010	2.850
M4	0.70	3.30	3.978	3.838	3.523	3.433	3.663	3.545	3.422	3.242
M5	0.80	4.20	4.976	4.826	4.456	4.361	4.605	4.480	4.334	4.134
M6	1.00	5.00	5.974	5.794	5.324	5.212	5.500	5.350	5.153	4.917
M8	1.25	6.80	7.972	7.760	7.160	7.042	7.348	7.188	6.912	6.647
M10	1.50	8.50	9.968	9.732	8.994	8.862	9.206	9.026	8.676	8.376
M12	1.75	10.20	11.966	11.701	10.829	10.679	11.063	10.863	10.441	10.106
M14	2.00	12.00	13.962	13.682	12.663	12.503	12.913	12.701	12.210	11.835
M16	2.00	14.00	15.962	15.682	14.663	14.503	14.913	14.701	14.210	13.835
M18	2.50	15.50	17.958	17.623	16.334	16.164	16.600	16.376	15.744	15.294
M20	2.50	17.50	19.958	19.623	18.334	18.164	18.600	18.376	17.744	17.294
M22	2.50	19.50	21.958	21.623	20.334	20.164	20.600	20.376	19.744	19.294
M24	3.00	21.00	23.952	23.577	22.003	21.803	22.316	22.051	21.252	20.752
M27	3.00	24.00	26.952	26.577	25.003	24.803	25.316	25.051	24.252	23.752
M30	3.50	26.50	29.947	29.522	27.674	27.462	28.007	27.727	26.771	26.211
M33	3.50	29.50	32.947	32.522	30.674	30.462	31.007	30.727	29.771	29.211
M36	4.00	32.00	35.940	35.465	33.342	33.118	33.702	33.402	32.270	31.670
M39	4.00	35.00	38.940	38.465	36.342	36.118	36.702	36.402	35.270	34.670

Metric fine — tolerance 6g/6H

Thread	Pitch P	Tap drill	d ext max	d ext min	d₂ ext max	d₂ ext min	D₂ int max	D₂ int min	D₁ int max	D₁ int min
M8x1	1.00	7.00	7.974	7.794	7.324	7.212	7.500	7.350	7.153	6.917
M10x1	1.00	9.00	9.974	9.794	9.324	9.212	9.500	9.350	9.153	8.917
M10x1.25	1.25	8.80	9.972	9.760	9.160	9.042	9.348	9.188	8.912	8.647
M12x1.25	1.25	10.80	11.972	11.760	11.160	11.028	11.368	11.188	10.912	10.647
M12x1.5	1.50	10.50	11.968	11.732	10.994	10.854	11.216	11.026	10.676	10.376
M14x1.5	1.50	12.50	13.968	13.732	12.994	12.854	13.216	13.026	12.676	12.376
M16x1.5	1.50	14.50	15.968	15.732	14.994	14.854	15.216	15.026	14.676	14.376
M18x1.5	1.50	16.50	17.968	17.732	16.994	16.854	17.216	17.026	16.676	16.376
M20x1.5	1.50	18.50	19.968	19.732	18.994	18.854	19.216	19.026	18.676	18.376
M20x2	2.00	18.00	19.962	19.682	18.663	18.503	18.913	18.701	18.210	17.835
M22x1.5	1.50	20.50	21.968	21.732	20.994	20.854	21.216	21.026	20.676	20.376
M24x2	2.00	22.00	23.962	23.682	22.663	22.493	22.925	22.701	22.210	21.835
M30x2	2.00	28.00	29.962	29.682	28.663	28.493	28.925	28.701	28.210	27.835
M36x3	3.00	33.00	35.952	35.577	34.003	33.803	34.316	34.051	33.252	32.752

Fine pitches offer several pitches per diameter — always state the pitch (e.g. M12×1.5).

Fig. 2 — Flank angle by type. Metric and Unified use 60°; Whitworth and pipe threads use 55°. Same size but a different angle means not interchangeable. In every case the crest is the outer Ø and the root the inner Ø.

Unified coarse — UNC (60° angle)

Thread	TPI	Pitch P (mm)	Major d (mm)	Pitch dia d₂ (mm)	Int. minor D₁ (mm)	Tap drill ≈ (mm)
No.4-40	40	0.635	2.845	2.433	2.158	2.26
No.6-32	32	0.794	3.505	2.989	2.646	2.71
No.8-32	32	0.794	4.166	3.650	3.307	3.45
No.10-24	24	1.058	4.826	4.139	3.680	3.80
1/4-20	20	1.270	6.350	5.525	4.975	5.11
5/16-18	18	1.411	7.938	7.021	6.410	6.53
3/8-16	16	1.587	9.525	8.494	7.806	7.94
7/16-14	14	1.814	11.113	9.935	9.149	9.35
1/2-13	13	1.954	12.700	11.431	10.585	10.72
5/8-11	11	2.309	15.875	14.375	13.375	13.49
3/4-10	10	2.540	19.050	17.400	16.300	16.66
7/8-9	9	2.822	22.225	20.392	19.170	19.45
1"-8	8	3.175	25.400	23.338	21.963	22.25

Unified fine — UNF (60° angle)

Thread	TPI	Pitch P (mm)	Major d (mm)	Pitch dia d₂ (mm)	Int. minor D₁ (mm)	Tap drill ≈ (mm)
No.10-32	32	0.794	4.826	4.310	3.967	4.04
1/4-28	28	0.907	6.350	5.761	5.368	5.41
5/16-24	24	1.058	7.938	7.251	6.792	6.91
3/8-24	24	1.058	9.525	8.838	8.379	8.43
7/16-20	20	1.270	11.113	10.288	9.738	9.91
1/2-20	20	1.270	12.700	11.875	11.325	11.51
9/16-18	18	1.411	14.288	13.371	12.760	12.93
5/8-18	18	1.411	15.875	14.958	14.347	14.51
3/4-16	16	1.587	19.050	18.019	17.331	17.46
1"-12	12	2.117	25.400	24.025	23.109	23.25

For inch threads, d₂ and D₁ are basic dimensions from the 60° (Unified) / 55° (Whitworth) profile. Tap drill is a ~75%-thread reference (pick the nearest drill). Fit tolerances follow their own systems: Unified uses classes 2A/2B (or 3A/3B) per ASME B1.1.

Whitworth coarse — BSW (55° angle)

Thread	TPI	Pitch P (mm)	Major d (mm)	Pitch dia d₂ (mm)	Int. minor D₁ (mm)	Tap drill ≈ (mm)
1/8-40	40	0.635	3.175	2.768	2.362	2.55
3/16-24	24	1.058	4.763	4.085	3.408	3.70
1/4-20	20	1.270	6.350	5.537	4.724	5.10
5/16-18	18	1.411	7.938	7.034	6.131	6.50
3/8-16	16	1.587	9.525	8.508	7.492	7.90
7/16-14	14	1.814	11.113	9.951	8.790	9.30
1/2-12	12	2.117	12.700	11.345	9.989	10.50
5/8-11	11	2.309	15.875	14.396	12.918	13.50
3/4-10	10	2.540	19.050	17.424	15.797	16.25
7/8-9	9	2.822	22.225	20.418	18.611	19.25
1"-8	8	3.175	25.400	23.367	21.334	22.00

Whitworth fine — BSF (55° angle)

Thread	TPI	Pitch P (mm)	Major d (mm)	Pitch dia d₂ (mm)	Int. minor D₁ (mm)	Tap drill ≈ (mm)
1/4-26	26	0.977	6.350	5.724	5.099	5.30
5/16-22	22	1.155	7.938	7.199	6.459	6.80
3/8-20	20	1.270	9.525	8.712	7.899	8.30
7/16-18	18	1.411	11.113	10.209	9.306	9.70
1/2-16	16	1.587	12.700	11.683	10.667	11.10
5/8-14	14	1.814	15.875	14.713	13.552	14.00
3/4-12	12	2.117	19.050	17.695	16.339	16.75
1"-10	10	2.540	25.400	23.774	22.147	22.75

Whitworth (W) is 55° — not interchangeable with Unified 60° at the same size. Tolerances per BS 84 (close/medium/free).

Fig. 3 — Parallel vs tapered pipe thread. G (parallel) keeps a constant outer diameter for mechanical joints; R/Rc (tapered) has an outer diameter that decreases at a 1/16 taper for watertight/airtight joints.

Parallel pipe thread — G (JIS B0202 / ISO 228)

Thread	TPI	Major (mm)	Pitch dia (mm)	Minor (mm)	Int. tap drill ≈ (mm)
G1/8	28	9.728	9.147	8.566	8.80
G1/4	19	13.157	12.301	11.445	11.80
G3/8	19	16.662	15.806	14.950	15.25
G1/2	14	20.955	19.793	18.631	19.00
G3/4	14	26.441	25.279	24.117	24.50
G1	11	33.249	31.770	30.291	30.75
G1-1/4	11	41.910	40.431	38.952	39.50
G1-1/2	11	47.803	46.324	44.845	45.25
G2	11	59.614	58.135	56.656	57.00

Tapered pipe thread — R (external) / Rc (internal) (JIS B0203 / ISO 7)

Thread	TPI	Major (mm)	Pitch dia (mm)	Minor (mm)	Int. tap drill ≈ (mm)
R/Rc1/8	28	9.728	9.147	8.566	8.40
R/Rc1/4	19	13.157	12.301	11.445	11.20
R/Rc3/8	19	16.662	15.806	14.950	14.75
R/Rc1/2	14	20.955	19.793	18.631	18.25
R/Rc3/4	14	26.441	25.279	24.117	23.75
R/Rc1	11	33.249	31.770	30.291	30.00
R/Rc1-1/4	11	41.910	40.431	38.952	38.75
R/Rc1-1/2	11	47.803	46.324	44.845	44.50
R/Rc2	11	59.614	58.135	56.656	56.00

Pipe threads are 55°. Diameters are basic dimensions at the gauge plane (for tapered R/Rc, measured at the reference plane; taper 1/16). Internal tap drill values are common practical figures — reconfirm with a thread gauge and your tap-tool data.

Through vs blind holes

• Through hole: passes through. A taper (through) tap with a long lead is easy to use
• Blind hole: has a bottom. A spiral tap threads near the bottom and ejects chips upward

Tap types

Type	Suited hole	Traits
Hand tap (taper, plug, bottoming)	Hand work, through	A set of three, staged
Spiral tap	Blind hole	Ejects chips upward
Gun (point) tap	Through hole	Pushes chips forward

Tapping tips

• Keep the correct pilot diameter (too big = incomplete threads, too small = breakage)
• Use cutting oil
• Keep the tap square (tilting causes defects/breakage)
• For blind holes, mind chip evacuation (spiral taps help)

FAQ

Q. How to find the pilot diameter? A. Estimate with "diameter − pitch" (e.g., M6×1.0 → 6−1 = 5mm). Confirm with a chart or drill table.

Q. Why do taps break? A. Pilot too small, clogged chips, tilt, or insufficient cutting oil. Use spiral taps for blind holes to be safe.

Q. Coarse vs fine pitch? A. Fine has a smaller pitch, resists loosening, and suits fine adjustment. Coarse is the most common.

Conclusion

Threading comes down to "the correct pilot diameter × a tap suited to the hole type." Keeping a chart handy gives peace of mind.

Sontis Management centralizes machining information and quotes. Try it free now.