TOOLS · US MACHINE SCREW SIZES
US Machine Screw Size Chart
This page is about the screw itself: what diameter a US machine screw size number means, which thread counts go with it, and which metric size is closest. Hole and tap drill sizes are on the tap drill chart, the complete unified inch thread table (UNC, UNF, UNEF, UN and UNS, including diameters above 1 in) is on the thread chart, and metric machine screws from M1.6 to M10 are on their own chart.
US machine screw sizes #0 to #12
| Size | Nominal dia (in) | Nominal dia (mm) | UNC (threads per inch) | UNF (threads per inch) | Other series | Closest metric |
|---|---|---|---|---|---|---|
| #0 | 0.0600 | 1.524 | — | 80 | — | M1.6 |
| #1 | 0.0730 | 1.854 | 64 | 72 | — | M2 |
| #2 | 0.0860 | 2.184 | 56 | 64 | — | M2 |
| #3 | 0.0990 | 2.515 | 48 | 56 | — | M2.5 |
| #4 | 0.1120 | 2.845 | 40 | 48 | — | M3 |
| #5 | 0.1250 | 3.175 | 40 | 44 | — | M3 |
| #6 | 0.1380 | 3.505 | 32 | 40 | — | M4 |
| #8 | 0.1640 | 4.166 | 32 | 36 | — | M4 |
| #10 | 0.1900 | 4.826 | 24 | 32 | — | M5 |
| #12 | 0.2160 | 5.486 | 24 | 28 | UNEF 32 | M5 |
Diameters and thread counts are read from the source table; the millimetre column, the pitch values and the metric comparison are calculated from them.
Machine screws sized the US way are called out either by a size number (#0 up to #12) or by a fractional inch diameter (1/4 in and up). The table below gives the nominal diameter in inch and millimetre for every size number in this range, the thread counts that go with each diameter, and the closest metric machine screw size.
Full PDF
Download the full PDF (#0–1/2 in, all sizes)
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Sizes with their own page
Every size in this range opens a page with its diameters, thread counts, pitch values and the closest metric size.
How the size numbers are calculated
The size number is not a gauge or a millimetre value: it is an index into a series. The series definition is a straight line, and the diameter of every size number can be recomputed from it.
d = 0.060 + 0.013 × n (inch), where n is the size number
So the first size, #0, is 0.060 in, and every step up in size number adds 0.013 in. A series like that is why a number 8 size works out at 0.1640 in rather than at a round figure. The second column of the table below is the same diameter as it stands in the source table, so the two can be compared line by line.
Series formula against the source table, size by size
| Size number | d = 0.060 + 0.013 × n (in)ᶜ | Source table (in) | Basis |
|---|---|---|---|
| #0 | 0.0600 | 0.0600 | matches the table |
| #1 | 0.0730 | 0.0730 | matches the table |
| #2 | 0.0860 | 0.0860 | matches the table |
| #3 | 0.0990 | 0.0990 | matches the table |
| #4 | 0.1120 | 0.1120 | matches the table |
| #5 | 0.1250 | 0.1250 | matches the table |
| #6 | 0.1380 | 0.1380 | matches the table |
| #8 | 0.1640 | 0.1640 | matches the table |
| #10 | 0.1900 | 0.1900 | matches the table |
| #12 | 0.2160 | 0.2160 | matches the table |
The definition line comes from the numbered-size series in FED-STD-H28/2B appendix A. The table values are the diameters of the same sizes as tabulated there; both are shown so the check can be repeated.
Values marked ᶜ are calculated on this page from the diameter and the thread count: the millimetre value, the pitch, the pitch diameter, the minor diameter and the metric comparison.
Fractional sizes above #12
From 1/4 in upwards US machine screws are called out by the fractional inch diameter instead of a size number. The table below covers the fractional sizes that continue the machine screw range.
| Size | Nominal dia (in) | Nominal dia (mm) | UNC (threads per inch) | UNF (threads per inch) | Other series | Closest metric |
|---|---|---|---|---|---|---|
| 1/4 in | 0.2500 | 6.35 | 20 | 28 | UNEF 32 · UNS 36 | M6 |
| 5/16 in | 0.3125 | 7.938 | 18 | 24 | UN 20 · UN 28 · UNEF 32 · UNS 36 | M8 |
| 3/8 in | 0.3750 | 9.525 | 16 | 24 | UN 20 · UN 28 · UNEF 28 · UNEF 32 · UNS 36 | M10 |
| 7/16 in | 0.4375 | 11.112 | 14 | 20 | UN 16 · UN 32 · UNEF 28 | M12 |
| 1/2 in | 0.5000 | 12.7 | 13 | 20 | UN 16 · UN 32 · UNEF 28 · UNS 12 | M12 |
Sizes 1/4 in to 1/2 in are the fractional part of the machine screw range. Larger fractional diameters — 9/16 in and larger — are tabulated on the unified inch thread chart together with the rest of the thread data, and are not repeated here.
The same two thread counts appear on one diameter in the fractional sizes as in the numbered ones, and the same three series names apply.
UNC and UNF thread counts
A US machine screw callout names the diameter or size number first and the thread count second, for example a number 8 screw with 32 threads per inch, in the coarse series. Most diameters in this range carry two thread counts: a coarse one (UNC) and a fine one (UNF). The finer count has a smaller pitch, so a finer thread takes a shallower thread form, needs a smaller hole for tapping, and grips over more threads for the same engagement length.
Which one to specify
The general-purpose series is the coarse one: it is the thread count a callout usually means when no series is written, and it is the more forgiving of the two where the material is soft or the engagement is short. The fine series is chosen when the wall thickness is thin, when the thread has to resist vibration loosening, or when adjustment is needed. The two are not interchangeable: a fine screw does not go into a coarse hole, and forcing one damages both.
What this range contains
Every size number from #0 to #12 is tabulated with its thread counts on this page: 10 size numbers, of which size #0 appears in the fine series only and #12 also carries an extra-fine row. The counts themselves are in the table at the top of this page.
Unified inch thread chart →Machine screw head types and drives
Machine screws are made with several head shapes and several drive recesses. The names and the uses below are given so that a callout or a drawing can be read; the dimensions of these heads are not reproduced here.
Pan head
The general-purpose head: a low rounded top with a flat bearing face, used where the head sits on the surface.
Flat head (countersunk)
Sits flush in a countersunk hole; the head angle has to match the hole angle, and it centres the joint as it is tightened.
Oval head (raised countersunk)
A countersunk head with a rounded top that stands slightly above the surface, used where a decorative profile is wanted.
Truss head
A wide, low head that spreads the load over a larger area, used on thin sheet and on soft materials.
Binding head
A slightly domed head with a small undercut, traditionally used on electrical and instrument work.
Fillister head
A tall, cylindrical head with a flat top, used where a deeper drive recess is needed.
Hex head
A hexagonal head turned with a wrench rather than a screwdriver, used where more tightening torque is needed.
Socket head cap
A cylindrical head with an internal hex recess, tightened with a hex key and able to take high torque in a small space.
Drive recesses
Slotted — The original drive: a single slot, usable with a plain screwdriver, and still found on older equipment.
Cross recess (Phillips type) — The most common cross recess: the driver centres itself, and the recess cams out before the head is overloaded.
Internal hex socket — Takes a hex key, which allows higher torque and a smaller head than an external drive.
Six-lobe (Torx type) — Six lobes with more driving area than a cross recess, used where the head is small or the torque is high.
Square recess — A square recess that resists cam-out and can be driven one-handed where access is difficult.
Combination slotted and cross recess — A recess that accepts either a slotted or a cross-recess driver, used where the tools at hand are unknown.
Head and drive recess dimensions — head diameter, head height, recess width and depth — are tabulated in ASME B18.6.3. The names above are given so that a callout can be read; dimensions for a specific head should be taken from that standard or from the supplier's drawing.
Metric machine screw sizes near these sizes
The last column of the tables above gives the metric machine screw whose nominal diameter is closest to each US size, comparing against the metric sizes tabulated on this site (M1.6 to M10, plus the larger coarse sizes).
This is a diameter proximity comparison, not an interchangeability statement. Metric and unified inch threads are two separate systems: the diameters and the thread counts differ, so a near-diameter metric screw will start in a US threaded hole and then bind and damage the thread. Use the comparison to read which callout a drawing means, not to substitute parts.
Metric machine screws near this size →Drill and tap sizes for machine screws
The hole a machine screw passes through, and the hole that is tapped to receive it, are separate questions from the screw's own dimensions, and they depend on the fit that is wanted.
Tap drill sizes for the unified inch series are on the tap drill chart, per series and per size, with the nearest standard drill. Only some of the sizes in this range have a drill value tabulated there, and the size pages link to those that do.
Drill designations come in four notations — number drills, letter drills, fractional inch drills and millimetre drills — and a hole size can be written in any of them. The three notations that come up with machine screws (number, fractional and metric) are listed side by side on the drill bit size chart, so the drill that a drawing calls for can be found in the set on hand.
Where these numbers come from
The diameters and thread counts on this page are the size and thread data of FED-STD-H28/2B, appendix A, tables II.A.1 and II.A.2 — a US federal standard. Each row of the table can be traced back to its line in the source data file by the line number given on the size pages. Millimetre values are a straight conversion of the inch value; pitch and thread-form values are calculated, not quoted from the table.
What is covered
The size numbers from #0 to #12 are tabulated in full: 10 size numbers and 20 diameter and thread count rows, of which 19 were accepted directly from the source and 1 was recovered after review. The fractional sizes 1/4 in to 1/2 in are also tabulated, 5 sizes and 28 rows.
Rows accepted after review
- #4-48 UNF
1 row in this range could not be read cleanly from the source scan and was recovered from a second, independent source together with the size and thread count arithmetic. It is named below rather than silently included.
What is not covered
Size #14 is not tabulated here. The source used for this chart ends its numbered sizes at #12, so #14 is left out rather than filled in from a supplier chart. The source does contain a further 221 fractional rows from 1/4 in up to the largest size, but only the machine screw part of that range — 1/4 in to 1/2 in — is repeated on this page; sizes from 9/16 in and larger are on the unified inch thread chart. Head and drive recess dimensions are not given here; take them from the current standard or the supplier's drawing.
Sources
- FED-STD-H28/2B — Unified Inch Screw Threads UN and UNR (US federal standard; appendix A size and thread data)
- Yaxiio metric size tables — machine screw sizes used for the metric comparison column
- ASME B18.6.3 — the current standard for machine screw and machine screw nut dimensions, cited for the current designation
Values on this page are taken from the sources listed and computed here as described; the sources' typography and additional columns are not reproduced.
Send the callouts, get the matching machine screws quoted
If your drawing, picking list or enquiry sheet uses US machine screw callouts — or mixes them with metric ones — send the list across and we will read it to size, thread count, head type and drive, and quote the parts against it. We work as a trading partner and solution provider: the enquiry is read as a specification, and what comes back is a quotation with the parts, the surface finish and the documents that go with them.
- Callouts as written, including size number or fractional diameter, thread count and head type
- Quantities per size, and whether a fine thread is needed or the coarse one will do
- Material, surface finish and any part marking or documentation the job requires
- Where the parts go, so the specification can be checked against the working conditions
Checking a tightening value instead?
The torque calculator turns a preload target or a friction state into a tightening torque, and the torque charts give the values tabulated by diameter and grade.
Cite this chart
YAXIIO. US Machine Screw Size Chart. https://www.yaxiio.com/en/tools/machine-screw-size-chart (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
Frequently asked questions
What is a number 8 machine screw in millimetres?
A number 8 machine screw has a nominal diameter of 0.1640 in, which is 4.166 mm. The nearest metric machine screw size is M4. It is tabulated in two thread counts: 32 threads per inch in the coarse series and 36 in the fine series.
Is a US machine screw size the same as a metric one?
No. The two systems use different diameters and different thread counts, even where the number in the callout is close. A near-diameter metric screw can be started in a US threaded hole and then bind, damaging both threads. The metric comparison on this page reads across from one system to the other; it does not make parts interchangeable.
Which thread counts are available for each size?
Two, in most sizes in this range: a coarse series (UNC) and a fine series (UNF). Size #0 exists in the fine series only, and #12 also carries an extra-fine row. The counts for every size are in the table at the top of this page, and the full list of series for the whole inch range is on the unified inch thread chart.
Is size #14 covered?
No. The size and thread data behind this page ends at #12, so #14 is not tabulated here and no values are supplied for it from another source. The same applies to size numbers below #0.
Are #12 and 1/4 in the same screw?
No. The size number series ends at #12, which has a nominal diameter of 0.2160 in; the fractional series starts at 1/4 in, which is 0.2500 in. They are neighbouring diameters in the range, not two names for the same part.
Does this page give drill or torque values?
No. This page gives the screw's own dimensions. Tap drill and hole sizes are on the tap drill chart, drill designations are on the drill bit size chart, and tightening values are on the torque charts.
Related References
Unified inch thread chart
The full thread data for UNC, UNF, UNEF, UN and UNS: threads per inch, pitch, basic pitch diameter and minor diameter, including diameters above 1 in.
Tap drill chart — UNC and UNF
The hole to drill before tapping, by series and size, with the nearest standard drill from the inch drill set.
Screw size chart — metric machine screws
The metric track: head dimensions and drive sizes for metric machine screws M1.6 to M10, with the nearest US machine screw size.
Drill bit size chart
Number, letter, fractional and metric drill designations side by side, so the drill a drawing calls for can be found in the set on hand.
Sizes on this page are nominal (basic) values compiled for design and purchasing reference. They are not tolerances and not inspection limits. Fastener dimensions must be verified against the governing standard and the drawing before production.