TOOLS · US HEX NUT SIZES
3/8-16 UNC nut dimensions
This table answers one question: how wide, how thick and how large across the corners is a US hex nut body? Width across flats, width across corners and nut thickness for the UNC sizes in the source are below. The metric nut family (ISO 4032 / GB/T 6170, M6 to M36) is a different dimensional system, with different across-flats steps and its own thickness convention: a US nut and the nearest M-size nut are not interchangeable, and a similar nominal diameter does not make the two assemble. Metric nut dimensions and weights (ISO 4032)
Answers first
Across flats s MAX
0.5625in
Across flats s MIN: 0.5510 in
Across corners e MAX
0.6500in
Across corners e MIN: 0.6280 in
Thickness m MAX
0.3370in
Thickness m MIN: 0.3200 in
Across flats s NOM
9/16in
Thickness m NOM: 21/64 in
Across corners e = 2s/√3
0.6495in
calculated
Threads per inch
16
Dash numbers A / B / C
MS51967-7 / -8 / -9
Symbols: s across flats, e across corners, m thickness. MAX and MIN are the source's own limits; values marked calculated are computed on this page.
3/8-16 UNC is a US hex nut: 16 threads per inch in the UNC-2B series, 9/16 in across flats nominal, with an across-corner dimension of 0.6500 in printed and 0.6495 in calculated, and a nominal thickness of 21/64 in. Everything below is either read from the source table or computed from those values.
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3/8-16 UNC in full
One line per dimension, with the basis of each value stated so that printed values can be told apart from the ones calculated on this page.
| Parameter | Value | Basis |
|---|---|---|
| Across flats s, nominal | 9/16 in | read from MS51967D Table I |
| Across flats s, maximum | 0.5625 | read from MS51967D Table I |
| Across flats s, minimum | 0.5510 | read from MS51967D Table I |
| Across corners e, maximum | 0.6500 | read from MS51967D Table I |
| Across corners e, minimum | 0.6280 | read from MS51967D Table I |
| Across corners e, calculated from the nominal across flats | 0.6495ᶜ | calculated on this page |
| Thickness m, nominal | 21/64 in | read from MS51967D Table I |
| Thickness m, maximum | 0.3370 | read from MS51967D Table I |
| Thickness m, minimum | 0.3200 | read from MS51967D Table I |
| Threads per inch | 16 | read from MS51967D Table I |
| Dash numbers for grades A / B / C | MS51967-7 / -8 / -9 | read from MS51967D Table I |
Read from the source, row by row: nominal size, thread count, across flats NOM / MAX / MIN, across corners MAX / MIN, thickness NOM / MAX / MIN and the dash numbers. Calculated on this page: the across-corners geometry column and every millimetre value. Calculated values are marked in the tables and are reproducible from the formulas above.
The same nominal diameter in the other US charts
The body dimensions above are one half of the question; the thread and the screw of the same nominal diameter are on the pages below, each following its own chart's list of sizes with their own page.
Across Flats vs Across Corners: What the Standard Actually Prints
A hexagon is a geometry, not a table: once the width across flats is fixed, the distance from corner to opposite corner follows from it. This table keeps both, side by side, so the printed across-corners column can be checked against the geometry instead of being taken on trust.
- Across corners e = 2s / √3 = s / cos 30°, that is 1.154701 times the across flats s, with s taken as the nominal (NOM) value.
- Checked over every row in this table, the printed across-corners MAX differs from that geometry by at most 0.0005 in — three-decimal printing, not a different dimension.
- The printed across-corners MIN divided by the printed across-flats MIN stays between 1.13778 and 1.14130 across the whole table, that is about 1.14 throughout.
- The across-flats MIN column sits between 0.0095 and 0.1500 in below the MAX column, and every across-flats value printed here is a whole number of sixteenths of an inch.
- Because the sizes are sixteenths, the step from one nut to the next is not constant: it runs from 1 sixteenth of an inch in the small sizes to 6 sixteenths as the diameter grows.
The calculated column is not a replacement for the printed one. Where the two disagree, the source value is what a drawing or a purchase order means; the calculated column exists to show that both columns describe the same hexagon.
UNC-2B: the Only Thread Series in This Standard
Every row in this table is a UNC-2B nut, class 2B internal thread: that is the scope of the source standard, and it is the only series tabulated here. This chart does not list UNF or UNEF nut rows, and it does not claim that the body dimensions above carry over to those series — the across flats of a fine-thread nut of the same nominal diameter is a conclusion from a different standard, which is not this source. For the thread dimensions themselves (threads per inch, pitch, pitch diameter), the unified inch thread chart is the matching page.
Unified inch thread chart (UNC, UNF, UNEF, UN and UNS) →Material Grades A / B / C and the MS51967 Dash Numbers
The source prints one row of body dimensions per nut size, and three dash numbers against it: grade A, grade B and grade C. The three grades share the same across flats, across corners and thickness in this table, so a dash number identifies the material grade rather than a different body size. The dash numbers run consecutively down the table, three per size, from 1 to 60.
Grade A
The lightest of the three grades: the choice where the nut has to be as small and light as the joint allows, and where the joint is not highly stressed.
Grade A: dash numbers MS51967-1 to MS51967-58 across the table. MS51967-1 – MS51967-58
Grade B
The middle grade, and the usual general-purpose choice for a carbon steel nut in ordinary structural and mechanical work.
Grade B: dash numbers MS51967-2 to MS51967-59 across the table. MS51967-2 – MS51967-59
Grade C
The heaviest grade: for joints with higher load or tighter preload demands, where the extra material in the nut is worth its cost.
Grade C: dash numbers MS51967-3 to MS51967-60 across the table. MS51967-3 – MS51967-60
Grade and finish are stated qualitatively in the source (carbon steel, cadmium plated, per the purchase specification). This page carries the dash numbers only; it makes no further numeric promise about plating, material properties or strength.
US vs Metric (ISO 4032): why a US nut is not an M-size nut
The two systems differ in the shape of the size ladder, not only in the units. That is why a conversion table is not enough and why this page does not print a closest M-size column: the across flats is chosen by a different rule in each system, and the thickness is defined by a different convention, so two nuts with a similar nominal diameter can still take different wrenches and give different thread engagement.
Here the across flats climbs in sixteenths of an inch, with the step widening as the diameter grows (from 1 sixteenth to 6 sixteenths) — a ladder that does not line up with the metric across-flats steps.
The thickness in this table runs from 7/32 in to 2-37/64 in for sizes of 7/16 in to 4-1/2 in across flats, that is between 0.500 and 0.583 of the across flats — a ratio taken from this source alone.
Read a US nut as a US nut: order it by the callout and the dash number, and check the wrench opening separately. Where a drawing mixes the two systems, the metric side is covered by the metric nut chart — used as a reference for the difference in steps and conventions, not as a statement of interchangeability.
Metric hex nut dimensions and weights (ISO 4032 / GB/T 6170) →How the Calculated Columns Are Derived
Two things on this page are not printed in the source: the across-corners geometry column and the millimetre values. Both are computed here from values that are printed.
Read from the source and calculated here
- Across corners e = 2s / √3 = 1.154701 × s, with s the nominal (NOM) across flats
- Millimetres = inches × 25.4
√3 is the geometry of a regular hexagon seen corner to corner; 25.4 is the definition of the inch. Neither introduces a value that is not already in the across-flats and thickness columns.
Read from the source, row by row: nominal size, thread count, across flats NOM / MAX / MIN, across corners MAX / MIN, thickness NOM / MAX / MIN and the dash numbers. Calculated on this page: the across-corners geometry column and every millimetre value. Calculated values are marked in the tables and are reproducible from the formulas above.
A value marked calculated is produced by the formulas on this page and is not a printed table value: across corners e = 2s / √3, taken from the nominal across flats, and millimetres = inches × 25.4. The source prints its own across-corners MAX and MIN as specification values; those two printed columns are shown beside the calculated one and are not replaced by it.
Across flats, across corners and thickness on this page are quoted as the source prints them, with the limits the source gives: the NOM, MAX and MIN columns are the standard's own values, and each row can be traced back to the page and line of the source it was read from. Tolerances, limits of size and class of fit to specify are set by the standard and by the drawing you are working to, not by this page.
Nut Data Source and Row-Level Provenance
Every row is one nut size taken from MS51967D (Nut, Plain, Hexagon — Carbon Steel, Cadmium Plated, UNC-2B), table I of that US military procurement standard. This page takes the body dimensions, the thread count and the dash numbers from that table and computes the across-corners geometry column and the millimetre values itself. In the table above, each row carries the page and line of the source it was read from, so a single value can be traced back to the row it came from.
What this table covers
All 20 nut sizes listed in the source table, from 1/4 in to 3 in across flats on pages 1 and 2 of it, for the UNC-2B thread series. That is one row per nominal diameter, with across flats, across corners, thickness, thread count and the dash numbers for grades A, B and C.
How each row is traced
Rows are numbered in the order of the source table: the first ten sizes come from page 1 and the remaining ten from page 2, one line each. The page and line number travel with the row in the markup of this page, so a value can be checked against the source without guessing which row it came from.
Version status of the source
MS51967D is marked Inactive in DLA ASSIST (Notice 6 inactivations, dated 2022-04-15): what this page records is the dimension set of that historical edition, not a currently active one. Before design selection or purchasing, confirm the equivalent standard your supplier is working to. This page does not claim that the publication is still in force, and it does not treat its values as the only usable set.
What this table does not cover
Stated here so that the absence is not read as coverage.
- The thread column in this table is UNC-2B only, because that is the scope of the source standard. No UNF, UNEF, UN or UNS nut sizes are listed, and no conclusion is drawn here that a fine-thread nut of the same nominal diameter has the same body dimensions — that would be a statement of a different standard.
- Heavy hex nuts are not in this table. The source tabulates one nut per nominal diameter, with a single across-flats value for each; the heavy hex series belongs to another set of standards and no value is added here to fill the gap.
- The list is fixed at 20 sizes. In-between diameters that are not printed in the source, and sizes above the largest one here, are not interpolated or estimated. Metric nuts are on the metric nut chart, a different dimensional system.
- The material and finish column of the source is a qualitative description (carbon steel of grades A, B and C, cadmium plated, per the purchase specification), not a dimensional table value. This page carries the dash number that goes with each grade and makes no further numeric promise about plating or material.
- What is given here is the nut body as the specification limits it: MAX and MIN for across flats, across corners and thickness, all of them printed values. It is not a thread tolerance table: the UNC-2B class says which internal thread the table applies to, and this page prints no thread limits of size, because the source does not give them.
Coverage: This is one of the 10 sizes that have a page of their own; the full chart lists all 20 sizes of the source, with across flats, across corners, thickness and the dash numbers for every one of them.
Sources for this size
- ASME B18.2.2 — the current ASME nut standard; the values on this page follow MS51967D Table I
- MS51967D Nut, Plain, Hexagon — Carbon Steel, Cadmium Plated, UNC-2B, 1970-03-30, Rev D, Table I (across flats, across corners, thickness, thread count and the material dash numbers)
- MS51967D Table III — the metric translation table of that same publication, used to cross-check the inch columns of Table I cell by cell (same source, same standard)
Values on this page are compiled from the sources below; the conversion and cross-check columns are worked out here.
Charts that go with this size
- Metric hex nut dimensions and weights →
The metric nut family (ISO 4032 / GB/T 6170): a different dimensional system, for reading drawings and catalogues that switch between the two.
- Unified inch thread chart →
Threads per inch, pitch and thread diameters for the unified inch series, so the thread the nut carries can be checked on its own terms.
- Inch bolt torque chart →
Tightening torque for inch-diameter bolts by grade, so the assembly this nut closes can be tightened to a stated value.
- Bolt wrench and socket opening chart →
The wrench or socket opening that goes with a given across flats, so the nut can be assembled on site.
Working to a US callout? Send it as written
If a drawing, a bill of materials or an enquiry sheet names US hex nuts — a callout, a dash number, or just a width across flats — send the line as written with the quantity, the material and the finish, and where the job is. We read it against the table above, confirm the size and the grade each line means, and come back with the hex nuts, washers and bolts that match, with the documents incoming inspection asks for.
- Callouts as written, with the dash number or the grade where the drawing gives one
- Quantities per size, and whether a nut and a washer are wanted together
- Material, finish and the standard or drawing the parts must meet
- Project location and the delivery window
Cite this chart
YAXIIO. 3/8-16 UNC nut dimensions. https://www.yaxiio.com/en/tools/uss-sae-nut-chart/3-8-16 (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
Frequently asked questions
What is the across flats of 3/8-16 UNC?
Nominal across flats is 9/16 in. The source prints limits on it as well: 0.5625 in maximum and 0.5510 in minimum, which is the width a wrench or socket has to cover.
How far across the corners is 3/8-16 UNC?
The source prints 0.6500 in maximum and 0.6280 in minimum from corner to opposite corner. The geometry gives 0.6495 in from the nominal across flats, so the printed and the calculated values describe the same hexagon to within the printing precision of the table.
How thick is 3/8-16 UNC?
Nominal thickness is 21/64 in, with a maximum of 0.3370 in and a minimum of 0.3200 in. Thickness decides how much of the bolt thread the nut engages, so it is worth quoting with the nut rather than assuming a value.
Which material grade does the dash number of 3/8-16 UNC identify?
This size carries three dash numbers, one for each grade in the source: MS51967-7 / -8 / -9. All three share the body dimensions above; the grade decides the material the nut is supplied in, not a different nut size.
Related References
Metric hex nut dimensions and weights (ISO 4032 / GB/T 6170-2015)
The metric nut family with its own across-flats ladder, thickness and weights — the other system, shown for comparison rather than for substitution.
Unified inch thread chart (UNC, UNF, UNEF, UN and UNS)
Threads per inch, pitch, basic pitch diameter and basic minor diameter for the thread inside the nut, with a page for the most-queried sizes.
Inch bolt torque chart
Tightening torque for inch-diameter bolts, so the US nut you order can be tightened to a stated value.
Bolt wrench and socket opening chart
The wrench or socket that fits a given bolt head, nut or carriage bolt neck, so the fasteners on site can be assembled.
Nut data on this page is compiled for design and purchasing reference. Body dimensions are quoted as the source prints them; tolerances, limits of size and acceptance gauging are governed by the applicable standard and by the drawing you are working to.