Fasteners

Thread Size Chart

The geometry behind the designation. Every value is derived from the basic 60° thread form, so you can check any row against the formulas rather than trusting the print.

Tensile stress area is what actually carries load — not the major diameter. A 1/2-13 has a 0.500″ major diameter but only 0.1419 in² of stress area, about 72% of the solid-rod area you might assume.

Thread geometry for UNC, UNF and metric: pitch, pitch diameter, minor diameter and tensile stress area

#0-80	UNC	0.0600	0.01250	0.0519	0.0465	0.00180
#1-64	UNC	0.0730	0.01562	0.0629	0.0561	0.00262
#2-56	UNC	0.0860	0.01786	0.0744	0.0667	0.00370
#3-48	UNC	0.0990	0.02083	0.0855	0.0765	0.00486
#4-40	UNC	0.1120	0.02500	0.0958	0.0849	0.00603
#5-40	UNC	0.1250	0.02500	0.1088	0.0979	0.00796
#6-32	UNC	0.1380	0.03125	0.1177	0.1042	0.00909
#8-32	UNC	0.1640	0.03125	0.1437	0.1302	0.01401
#10-24	UNC	0.1900	0.04167	0.1629	0.1449	0.01753
#12-24	UNC	0.2160	0.04167	0.1889	0.1709	0.02416
1/4-20	UNC	0.2500	0.05000	0.2175	0.1959	0.03182
5/16-18	UNC	0.3125	0.05556	0.2764	0.2524	0.05243
3/8-16	UNC	0.3750	0.06250	0.3344	0.3073	0.07749
7/16-14	UNC	0.4375	0.07143	0.3911	0.3602	0.10631
1/2-13	UNC	0.5000	0.07692	0.4500	0.4167	0.14190
9/16-12	UNC	0.5625	0.08333	0.5084	0.4723	0.18194
5/8-11	UNC	0.6250	0.09091	0.5659	0.5266	0.22600
3/4-10	UNC	0.7500	0.10000	0.6851	0.6418	0.33446
7/8-9	UNC	0.8750	0.11111	0.8028	0.7547	0.46173
1-8	UNC	1.0000	0.12500	0.9188	0.8647	0.60575
1-1/8-7	UNC	1.1250	0.14286	1.0322	0.9704	0.76328
1-1/4-7	UNC	1.2500	0.14286	1.1572	1.0954	0.96911
1-3/8-6	UNC	1.3750	0.16667	1.2668	1.1946	1.15488
1-1/2-6	UNC	1.5000	0.16667	1.3918	1.3196	1.40525
#0-80	UNF	0.0600	0.01250	0.0519	0.0465	0.00180
#1-72	UNF	0.0730	0.01389	0.0640	0.0580	0.00278
#2-64	UNF	0.0860	0.01562	0.0759	0.0691	0.00393
#3-56	UNF	0.0990	0.01786	0.0874	0.0797	0.00523
#4-48	UNF	0.1120	0.02083	0.0985	0.0895	0.00660
#5-44	UNF	0.1250	0.02273	0.1102	0.1004	0.00831
#6-40	UNF	0.1380	0.02500	0.1218	0.1109	0.01014
#8-36	UNF	0.1640	0.02778	0.1460	0.1339	0.01473
#10-32	UNF	0.1900	0.03125	0.1697	0.1562	0.01999
#12-28	UNF	0.2160	0.03571	0.1928	0.1773	0.02579
1/4-28	UNF	0.2500	0.03571	0.2268	0.2113	0.03637
5/16-24	UNF	0.3125	0.04167	0.2854	0.2674	0.05807
3/8-24	UNF	0.3750	0.04167	0.3479	0.3299	0.08783
7/16-20	UNF	0.4375	0.05000	0.4050	0.3834	0.11872
1/2-20	UNF	0.5000	0.05000	0.4675	0.4459	0.15995
9/16-18	UNF	0.5625	0.05556	0.5264	0.5024	0.20298
5/8-18	UNF	0.6250	0.05556	0.5889	0.5649	0.25596
3/4-16	UNF	0.7500	0.06250	0.7094	0.6823	0.37296
7/8-14	UNF	0.8750	0.07143	0.8286	0.7977	0.50947
1-12	UNF	1.0000	0.08333	0.9459	0.9098	0.66304
1-1/8-12	UNF	1.1250	0.08333	1.0709	1.0348	0.85572
1-1/4-12	UNF	1.2500	0.08333	1.1959	1.1598	1.07295
1-3/8-12	UNF	1.3750	0.08333	1.3209	1.2848	1.31471
1-1/2-12	UNF	1.5000	0.08333	1.4459	1.4098	1.58102
M1.6×0.35	Metric Coarse	0.0630	0.01378	0.0540	0.0481	0.00197
M2×0.4	Metric Coarse	0.0787	0.01575	0.0685	0.0617	0.00321
M2.5×0.45	Metric Coarse	0.0984	0.01772	0.0869	0.0792	0.00526
M3×0.5	Metric Coarse	0.1181	0.01969	0.1053	0.0968	0.00780
M3.5×0.6	Metric Coarse	0.1378	0.02362	0.1225	0.1122	0.01050
M4×0.7	Metric Coarse	0.1575	0.02756	0.1396	0.1276	0.01361
M5×0.8	Metric Coarse	0.1969	0.03150	0.1764	0.1628	0.02198
M6×1	Metric Coarse	0.2362	0.03937	0.2106	0.1936	0.03119
M8×1.25	Metric Coarse	0.3150	0.04921	0.2830	0.2617	0.05674
M10×1.5	Metric Coarse	0.3937	0.05906	0.3553	0.3298	0.08988
M12×1.75	Metric Coarse	0.4724	0.06890	0.4277	0.3979	0.13061
M14×2	Metric Coarse	0.5512	0.07874	0.5000	0.4659	0.17893
M16×2	Metric Coarse	0.6299	0.07874	0.5788	0.5447	0.24284
M18×2.5	Metric Coarse	0.7087	0.09843	0.6447	0.6021	0.29833
M20×2.5	Metric Coarse	0.7874	0.09843	0.7235	0.6809	0.37943
M22×2.5	Metric Coarse	0.8661	0.09843	0.8022	0.7596	0.47027
M24×3	Metric Coarse	0.9449	0.11811	0.8682	0.8170	0.54638
M27×3	Metric Coarse	1.0630	0.11811	0.9863	0.9351	0.71208
M30×3.5	Metric Coarse	1.1811	0.13780	1.0916	1.0319	0.86891
M8×1	Metric Fine	0.3150	0.03937	0.2894	0.2723	0.06071
M10×1.25	Metric Fine	0.3937	0.04921	0.3617	0.3404	0.09486
M10×1	Metric Fine	0.3937	0.03937	0.3681	0.3511	0.09997
M12×1.5	Metric Fine	0.4724	0.05906	0.4341	0.4085	0.13660
M12×1.25	Metric Fine	0.4724	0.04921	0.4405	0.4192	0.14271
M14×1.5	Metric Fine	0.5512	0.05906	0.5128	0.4873	0.19305
M16×1.5	Metric Fine	0.6299	0.05906	0.5916	0.5660	0.25923
M18×1.5	Metric Fine	0.7087	0.05906	0.6703	0.6447	0.33516
M20×1.5	Metric Fine	0.7874	0.05906	0.7490	0.7235	0.42083
M22×1.5	Metric Fine	0.8661	0.05906	0.8278	0.8022	0.51624
M24×2	Metric Fine	0.9449	0.07874	0.8937	0.8596	0.59585
M27×2	Metric Fine	1.0630	0.07874	1.0118	0.9778	0.76840
M30×2	Metric Fine	1.1811	0.07874	1.1300	1.0959	0.96287

80 threads · 48 inch, 32 metric

The formulas

H  = 0.866025 × P          theoretical triangle height
d2 = D − 0.649519 × P      pitch diameter
D1 = D − 1.082532 × P      minor diameter (internal)
At = 0.7854 × (D − 0.9743/n)²      tensile stress area, inch [ASME B1.1]
As = 0.7854 × (D − 0.9382 × P)²    tensile stress area, metric [ISO 898-1]

Why tensile area, not major diameter

A threaded rod fails through the thread roots, so its load-bearing section is smaller than the nominal size suggests. Across common sizes the stress area is only 65–77% of the solid-rod area — so sizing a joint off the major diameter overstates capacity by 30% on a 1″-8 and by 54% on a 1/4-20. The smaller the fastener, the bigger the error.

Data source and method

Standards: ASME B1.1 (Unified inch screw threads); ISO 68-1 / 261 / 965 (metric). Nominal diameters and pitches are factual values.
Calculation: All geometry derived from the basic 60° form using the equations above. No published table reproduced.
Verification: Tensile areas checked against 12 published values (1/4-20, 3/8-16, 1/2-13, 5/8-11, 3/4-10, 1-8, M6, M8, M10, M12, M16, M20). Maximum deviation 0.21%.

Related charts

Tap drill size chart — hole sizes for cutting these threads