ASME B18.6.3 Machine Screw Nut Dimensions Chart #0–3/8

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ASME B18.6.3 machine screw nut dimensions — #0 to 3/8

ASME B18.6.3 specifies the dimensions of hex machine screw nuts — the small inch-series hex nuts from #0 through 3/8: width across flats, width across corners and thickness, with max and min limits. #4-40, #6-32, #8-32 and #10-24/#10-32 nuts belong to this standard, not to ASME B18.2.2, which starts at 1/4 in.

ASME B18.6.3 machine screw nut dimensions — width across flats, across corners and thickness

Machine screw nut — across flats (A/F), across corners (A/C) and thickness as charted below.

Size	Basic major dia	A/F basic	A/F max	A/F min	Corners max	Corners min	Thickness max	Thickness min
#0	0.0600	5/32 (0.156)	0.156	0.150	0.221	0.206	0.050	0.043
#1	0.0730	5/32	0.156	0.150	0.221	0.206	0.050	0.043
#2	0.0860	3/16 (0.188)	0.188	0.180	0.265	0.247	0.066	0.057
#3	0.0990	3/16	0.188	0.180	0.265	0.247	0.066	0.057
#4	0.1120	1/4 (0.250)	0.250	0.241	0.354	0.331	0.098	0.087
#5	0.1250	5/16 (0.312)	0.312	0.302	0.442	0.415	0.114	0.102
#6	0.1380	5/16	0.312	0.302	0.442	0.415	0.114	0.102
#8	0.1640	11/32 (0.344)	0.344	0.332	0.486	0.456	0.130	0.117
#10	0.1900	3/8 (0.375)	0.375	0.362	0.530	0.497	0.130	0.117
#12	0.2160	7/16 (0.438)	0.438	0.423	0.619	0.581	0.161	0.148
1/4	0.2500	7/16	0.438	0.423	0.619	0.581	0.193	0.178
5/16	0.3125	9/16 (0.562)	0.562	0.545	0.795	0.748	0.225	0.208
3/8	0.3750	5/8 (0.625)	0.625	0.607	0.884	0.833	0.257	0.239

Dimensions in inches to ASME/ANSI B18.6.3. #10-24 and #10-32 share the same #10 nut body. Commercial “small pattern” hex nuts run a smaller across-flats than this table — those follow the supplier datasheet, not B18.6.3.

Governing document: ASME B18.6.3. Values cross-checked cell-for-cell against two concordant published references, including the Engineers Edge hexagon machine screw nut table (verified July 2026).

B18.6.3 vs B18.2.2 — don’t mix the tables

Where the two ranges overlap (1/4, 5/16 and 3/8 in.), B18.6.3 machine screw nuts are thinner than B18.2.2 finished hex nuts, and the across-flats also differ at some sizes — a 5/16 machine screw nut is 9/16 in. A/F where the B18.2.2 hex nut is 1/2 in. The two series are not interchangeable: pick the standard the assembly calls out. For 1/4–3/4 in. full nuts and jam nuts, see the ASME B18.2.2 hex nut chart.

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Product

UNC Brass Machine Screw Nuts

#0–3/8 nuts machined to ASME B18.6.3 in C36000 free-machining brass.

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Imperial & ANSI Brass Parts

The full UNC/UNF/NPT range — nuts, set screws, standoffs, inserts and flare nuts.

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DIN 439 Brass Thin Nut

Metric thin-pattern hex nuts — the closest metric counterpart.

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More reference charts & tools

Chart

ASME B18.2.2 Hex Nut Dimensions

1/4–3/4 in. hex and jam nut chart — the series above this one.

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Thread Size Chart

UNC/UNF, metric, BSP and NPT thread dimensions side by side.

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Tap Drill Chart

Tap-drill sizes for metric, UNC, BSP (G) and NPT threads.

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Frequently asked questions

What does ASME B18.6.3 specify for nuts?

ASME B18.6.3 specifies the dimensions of hex machine screw nuts — the small inch-series nuts from #0 through 3/8: width across flats, width across corners and thickness. It is the standard below 1/4 in., where ASME B18.2.2 hex nuts begin.

Which standard covers #4-40 hex nuts?

ASME B18.6.3 (machine screw nuts) — not ASME B18.2.2, which starts at 1/4 in. A #4-40 machine screw nut has a 1/4 in. width across flats and a 0.098 in. maximum thickness.

Are machine screw nuts thinner than standard hex nuts?

Yes. At the overlapping 1/4, 5/16 and 3/8 in. sizes, B18.6.3 machine screw nuts are thinner than B18.2.2 finished hex nuts, and the across-flats also differ at some sizes — a 5/16 machine screw nut is 9/16 in. A/F versus 1/2 in. for the B18.2.2 hex nut. The two tables must not be mixed.

Do #10-24 and #10-32 nuts have different bodies?

No. Both pitches use the same #10 nut body — 3/8 in. across flats, 0.130 in. maximum thickness. Only the internal thread differs.

What is a small-pattern hex nut?

A commercial variant with a smaller width across flats than the B18.6.3 table. Small-pattern dimensions follow the individual supplier datasheet rather than the standard — if a drawing calls one out, it is machined to the drawing.

Last reviewed: July 2026. This chart is a reference summary — verify against the governing standard and your drawing for procurement-critical work.

ASME B18.6.3 Machine Screw Nut Dimensions

ASME B18.6.3 machine screw nut dimensions — #0 to 3/8

B18.6.3 vs B18.2.2 — don’t mix the tables

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Frequently asked questions

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