{"id":9750,"date":"2026-03-16T16:34:35","date_gmt":"2026-03-16T08:34:35","guid":{"rendered":"https:\/\/princefastener.com\/?p=9750"},"modified":"2026-03-18T09:49:29","modified_gmt":"2026-03-18T01:49:29","slug":"screw-size-chart-metric-imperial-conversion-guide","status":"publish","type":"post","link":"https:\/\/princefastener.com\/ru\/screw-size-chart-metric-imperial-conversion-guide\/","title":{"rendered":"Screw Size Chart Guide for Metric and Imperial Standards"},"content":{"rendered":"
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This guide gives you a clear, printable screw size chart<\/strong> for both metric and imperial standards \u2014 with every conversion table, thread pitch reference, and socket screw dimension you need in one place. No more toggling between six browser tabs to match an M6 to a #12 or figure out whether your 1\/4″-20 bolt is UNC or UNF.<\/p>

If you purchase or specify fasteners in the United States, understanding both metric and imperial sizing is not optional \u2014 it is a daily operational requirement. Roughly 97 % of the world’s manufacturing base runs on ISO metric threads, yet the majority of legacy American machinery, automotive platforms built before 2000, and residential construction still relies on unified inch-series fasteners. A maintenance technician at a Midwest grain elevator told \u041f\u0440\u0438\u043d\u0446 \u0417\u0430\u0441\u0442\u0435\u0436\u043a\u0430<\/a> that cross-threading a 5\/16″ nut onto an M8 stud cost his crew a four-hour shutdown and a $1,200 replacement coupling \u2014 simply because the two sizes “looked close enough.”<\/p>

That scenario is more common than it should be. An M8 bolt measures 8.00 mm (0.3150″) across the major diameter; a 5\/16″ bolt measures 7.94 mm (0.3125″). The difference is just 0.06 mm \u2014 invisible to the naked eye, but enough to strip threads under load. This guide resolves that confusion with side-by-side charts, practical conversion tips, and clear explanations of thread pitch versus thread count. Use the tables for quick, accurate selection and keep them bookmarked for every future order.<\/p>

\"Assorted<\/p>

<\/p>

Screw Size Chart Overview<\/h2>

<\/p>

Metric Screw Size Chart<\/h3>

Metric screws follow the ISO 261 \/ ISO 262 standard. The naming convention is straightforward: the letter “M” followed by the nominal major diameter in millimetres, then “\u00d7” and the pitch (if it differs from the default coarse pitch), then “\u00d7” and the length. For example, M8 \u00d7 1.25 \u00d7 40 means an 8 mm diameter screw with a 1.25 mm coarse pitch thread and a 40 mm length. When no pitch is stated (e.g., “M8 \u00d7 40”), the coarse pitch is implied.<\/p>

The table below covers the metric sizes most frequently ordered through Prince Fastener’s machine screw catalogue<\/a> and encountered in industrial, automotive, and electronics applications. Diameter and pitch data conform to ISO 261<\/a>.<\/p>
Metric Size<\/th>Major Diameter (mm)<\/th>Major Diameter (inch)<\/th>Coarse Pitch (mm)<\/th>Fine Pitch (mm)<\/th>Common Lengths (mm)<\/th><\/tr><\/thead>
M1.6<\/strong><\/td>1.60<\/td>0.0630<\/td>0.35<\/td>0.20<\/td>3 \u2013 16<\/td><\/tr>
M2<\/strong><\/td>2.00<\/td>0.0787<\/td>0.40<\/td>0.25<\/td>3 \u2013 20<\/td><\/tr>
M2.5<\/strong><\/td>2.50<\/td>0.0984<\/td>0.45<\/td>0.35<\/td>4 \u2013 25<\/td><\/tr>
M3<\/strong><\/td>3.00<\/td>0.1181<\/td>0.50<\/td>0.35<\/td>5 \u2013 30<\/td><\/tr>
M4<\/strong><\/td>4.00<\/td>0.1575<\/td>0.70<\/td>0.50<\/td>6 \u2013 50<\/td><\/tr>
M5<\/strong><\/td>5.00<\/td>0.1969<\/td>0.80<\/td>0.50<\/td>8 \u2013 80<\/td><\/tr>
M6<\/strong><\/td>6.00<\/td>0.2362<\/td>1.00<\/td>0.75<\/td>10 \u2013 100<\/td><\/tr>
M8<\/strong><\/td>8.00<\/td>0.3150<\/td>1.25<\/td>1.00<\/td>12 \u2013 150<\/td><\/tr>
M10<\/strong><\/td>10.00<\/td>0.3937<\/td>1.50<\/td>1.25<\/td>16 \u2013 200<\/td><\/tr>
M12<\/strong><\/td>12.00<\/td>0.4724<\/td>1.75<\/td>1.50<\/td>20 \u2013 250<\/td><\/tr>
M14<\/strong><\/td>14.00<\/td>0.5512<\/td>2.00<\/td>1.50<\/td>25 \u2013 300<\/td><\/tr>
M16<\/strong><\/td>16.00<\/td>0.6299<\/td>2.00<\/td>1.50<\/td>25 \u2013 300<\/td><\/tr>
M20<\/strong><\/td>20.00<\/td>0.7874<\/td>2.50<\/td>1.50<\/td>30 \u2013 400<\/td><\/tr>
M24<\/strong><\/td>24.00<\/td>0.9449<\/td>3.00<\/td>2.00<\/td>40 \u2013 500<\/td><\/tr><\/tbody><\/table>

Data source: ISO 261 \/ ISO 262. Prince Fastener stocks all sizes above in carbon steel, stainless steel 304\/316, and brass.<\/em><\/p>

Imperial Screw Size Chart<\/h3>

Imperial (inch-series) screws use two different naming systems depending on diameter. Screws below 1\/4″ are identified by a gauge number (#0 through #14), where each gauge step adds 0.013″ to the major diameter starting from 0.060″ at #0. The formula is: Diameter = 0.060 + (gauge number \u00d7 0.013). Screws at 1\/4″ and above are called out directly by their fractional diameter.<\/p>

Thread engagement is specified by threads per inch (TPI) under the Unified Thread Standard (UTS), which defines UNC (Unified National Coarse), UNF (Unified National Fine), and UNEF (Unified National Extra Fine) series. The table below covers the sizes most commonly handled by Prince Fastener’s self-tapping and machine screw lines<\/a>.<\/p>
\u0420\u0430\u0437\u043c\u0435\u0440 \u0432\u0438\u043d\u0442\u0430<\/th>Major Diameter (inch)<\/th>Major Diameter (mm)<\/th>UNC (TPI)<\/th>UNF (TPI)<\/th>Nearest Fraction<\/th><\/tr><\/thead>
#0<\/strong><\/td>0.060<\/td>1.52<\/td>\u2014<\/td>80<\/td>1\/16″<\/td><\/tr>
#1<\/strong><\/td>0.073<\/td>1.85<\/td>64<\/td>72<\/td>5\/64″<\/td><\/tr>
#2<\/strong><\/td>0.086<\/td>2.18<\/td>56<\/td>64<\/td>3\/32″<\/td><\/tr>
#3<\/strong><\/td>0.099<\/td>2.51<\/td>48<\/td>56<\/td>7\/64″<\/td><\/tr>
#4<\/strong><\/td>0.112<\/td>2.84<\/td>40<\/td>48<\/td>7\/64″<\/td><\/tr>
#5<\/strong><\/td>0.125<\/td>3.18<\/td>40<\/td>44<\/td>1\/8″<\/td><\/tr>
#6<\/strong><\/td>0.138<\/td>3.51<\/td>32<\/td>40<\/td>9\/64″<\/td><\/tr>
#8<\/strong><\/td>0.164<\/td>4.17<\/td>32<\/td>36<\/td>5\/32″<\/td><\/tr>
#10<\/strong><\/td>0.190<\/td>4.83<\/td>24<\/td>32<\/td>3\/16″<\/td><\/tr>
#12<\/strong><\/td>0.216<\/td>5.49<\/td>24<\/td>28<\/td>7\/32″<\/td><\/tr>
#14<\/strong><\/td>0.250<\/td>6.35<\/td>20<\/td>28<\/td>1\/4\u2033<\/td><\/tr>
1\/4\u2033<\/strong><\/td>0.250<\/td>6.35<\/td>20<\/td>28<\/td>\u2014<\/td><\/tr>
5\/16\u2033<\/strong><\/td>0.3125<\/td>7.94<\/td>18<\/td>24<\/td>\u2014<\/td><\/tr>
3\/8\u2033<\/strong><\/td>0.375<\/td>9.53<\/td>16<\/td>24<\/td>\u2014<\/td><\/tr>
7\/16\u2033<\/strong><\/td>0.4375<\/td>11.11<\/td>14<\/td>20<\/td>\u2014<\/td><\/tr>
1\/2\u2033<\/strong><\/td>0.500<\/td>12.70<\/td>13<\/td>20<\/td>\u2014<\/td><\/tr>
5\/8\u2033<\/strong><\/td>0.625<\/td>15.88<\/td>11<\/td>18<\/td>\u2014<\/td><\/tr>
3\/4\u2033<\/strong><\/td>0.750<\/td>19.05<\/td>10<\/td>16<\/td>\u2014<\/td><\/tr>
1\u2033<\/strong><\/td>1.000<\/td>25.40<\/td>8<\/td>12<\/td>\u2014<\/td><\/tr><\/tbody><\/table>

Data source: ASME B1.1 Unified Inch Screw Threads (UN and UNR Thread Form).<\/em><\/p>

Socket Screw Sizes Reference<\/h3>

Socket head cap screws (SHCS) use an internal hexagonal (Allen) drive and are specified by DIN 912 \/ ISO 4762 in metric or ASME B18.3 in imperial. The critical installation dimension beyond diameter and length is the hex key (Allen wrench) size required. Selecting the wrong key rounds the socket, making removal destructive. The reference below covers the most common sizes stocked by Prince Fastener.<\/p>
Metric Screw<\/th>Hex Key (mm)<\/th>Head Dia. (mm)<\/th>Head Height (mm)<\/th>Imperial Equivalent<\/th>Hex Key (inch)<\/th><\/tr><\/thead>
M3<\/td>2.5<\/td>5.5<\/td>3.0<\/td>#4 \/ #5<\/td>5\/64″<\/td><\/tr>
M4<\/td>3.0<\/td>7.0<\/td>4.0<\/td>#8<\/td>7\/64″<\/td><\/tr>
M5<\/td>4.0<\/td>8.5<\/td>5.0<\/td>#10<\/td>1\/8″<\/td><\/tr>
M6<\/td>5.0<\/td>10.0<\/td>6.0<\/td>1\/4\u2033<\/td>3\/16″<\/td><\/tr>
M8<\/td>6.0<\/td>13.0<\/td>8.0<\/td>5\/16\u2033<\/td>1\/4\u2033<\/td><\/tr>
M10<\/td>8.0<\/td>16.0<\/td>10.0<\/td>3\/8\u2033<\/td>5\/16\u2033<\/td><\/tr>
M12<\/td>10.0<\/td>18.0<\/td>12.0<\/td>1\/2\u2033<\/td>3\/8\u2033<\/td><\/tr>
M16<\/td>14.0<\/td>24.0<\/td>16.0<\/td>5\/8\u2033<\/td>1\/2\u2033<\/td><\/tr>
M20<\/td>17.0<\/td>30.0<\/td>20.0<\/td>3\/4\u2033<\/td>5\/8\u2033<\/td><\/tr><\/tbody><\/table>

Socket head cap screw dimensions per DIN 912 \/ ISO 4762 and ASME B18.3.<\/em><\/p>

\"Digital<\/p>

<\/p>

Metric vs Imperial Fasteners Explained<\/h2>

<\/p>

Metric Screw Basics<\/h3>

The metric system for screws, defined by ISO 261, uses millimetres for every dimension. A metric screw callout includes three values: diameter, pitch, and length \u2014 all in mm. Thread pitch is the distance from one thread crest to the next, measured along the axis. An M8 \u00d7 1.25 screw has peaks spaced exactly 1.25 mm apart. When no pitch is written, the ISO coarse pitch is assumed \u2014 so “M8” always means M8 \u00d7 1.25 unless stated otherwise.<\/p>

Metric threads dominate global manufacturing. The ISO metric thread is the most widely produced screw thread standard worldwide, used as the default in the EU, China, Japan, South Korea, India, and virtually all of Southeast Asia and South America. Prince Fastener’s Shanghai production line runs metric tooling on 78 % of its output by volume \u2014 a ratio that has climbed steadily from 65 % a decade ago as export markets outside the U.S. have expanded.<\/p>

Imperial Screw Basics<\/h3>

The imperial (inch) system \u2014 formally the Unified Thread Standard (UTS) governed by ASME B1.1<\/a> \u2014 expresses diameter in inches (fractional or gauge number) and thread density as threads per inch (TPI). A 1\/4″-20 UNC screw has a 0.250″ major diameter with 20 threads packed into each inch of threaded length. The higher the TPI number, the finer (closer together) the threads.<\/p>

Imperial fasteners remain the primary standard in the United States, and they persist in legacy automotive, aerospace, and infrastructure applications in Canada, the UK (particularly older British Standard Whitworth equipment), and Liberia. Any machine shop servicing American-manufactured equipment \u2014 from John Deere harvesters to Boeing airframe jigs \u2014 needs inch-series tooling alongside metric.<\/p>

\u041a\u043b\u044e\u0447\u0435\u0432\u044b\u0435 \u0440\u0430\u0437\u043b\u0438\u0447\u0438\u044f<\/h3>

The differences extend beyond measurement units. Metric threads define pitch as a distance (mm between crests); imperial threads define pitch inversely as a count (threads per inch). Metric thread profiles use a 60\u00b0 included angle identical to UTS, but the tolerance classes, pitch sequences, and designation conventions differ completely. A metric M10 \u00d7 1.5 and an imperial 3\/8″-16 UNC look nearly identical \u2014 10.00 mm vs. 9.53 mm diameter \u2014 yet their threads will not engage. Forcing them together strips both.<\/p>

Wrench sizes also diverge. An M10 hex bolt head measures 16 mm across flats; a 3\/8″ hex bolt head measures 9\/16″ (14.29 mm). Grabbing the wrong wrench rounds the flats and turns a two-minute task into a twenty-minute extraction with a damaged bolt head. Keeping metric and imperial tool sets visibly separated \u2014 colour-coded drawer liners are a simple solution \u2014 prevents this on the shop floor.<\/p>

<\/p>

Global Thread Standard Usage by Region<\/h3>

Global Fastener Thread Standard Usage (%)<\/p>

\"Global<\/p><\/div>

Estimated from ISO adoption data, ASME\/ANSI publications, and Prince Fastener export records across 45 countries.<\/em><\/p>

<\/p>

Metric to Imperial Conversion Guide<\/h2>

<\/p>

Conversion Chart<\/h3>

The table below maps every common metric screw diameter to its nearest imperial equivalent. “Nearest” means the closest standard inch-series size \u2014 not an exact match. The difference column shows the actual dimensional gap. Any gap larger than 0.15 mm means the two sizes are functionally non-interchangeable and should never be substituted.<\/p>
Metric Size<\/th>\u0414\u0438\u0430\u043c\u0435\u0442\u0440 (\u043c\u043c)<\/th>Nearest Imperial<\/th>\u0414\u0438\u0430\u043c\u0435\u0442\u0440 (\u043c\u043c)<\/th>Difference (mm)<\/th>Interchangeable?<\/th><\/tr><\/thead>
M2<\/td>2.00<\/td>#2 (0.086″)<\/td>2.18<\/td>+0.18<\/td>\u041d\u0435\u0442<\/td><\/tr>
M2.5<\/td>2.50<\/td>#3 (0.099″)<\/td>2.51<\/td>+0.01<\/td>Close*<\/td><\/tr>
M3<\/td>3.00<\/td>#4 (0.112″)<\/td>2.84<\/td>-0.16<\/td>\u041d\u0435\u0442<\/td><\/tr>
M3.5<\/td>3.50<\/td>#6 (0.138″)<\/td>3.51<\/td>+0.01<\/td>Close*<\/td><\/tr>
M4<\/td>4.00<\/td>#8 (0.164″)<\/td>4.17<\/td>+0.17<\/td>\u041d\u0435\u0442<\/td><\/tr>
M5<\/td>5.00<\/td>#10 (0.190″)<\/td>4.83<\/td>-0.17<\/td>\u041d\u0435\u0442<\/td><\/tr>
M6<\/td>6.00<\/td>1\/4″ (0.250″)<\/td>6.35<\/td>+0.35<\/td>\u041d\u0435\u0442<\/td><\/tr>
M8<\/td>8.00<\/td>5\/16″ (0.3125″)<\/td>7.94<\/td>-0.06<\/td>Close*<\/td><\/tr>
M10<\/td>10.00<\/td>3\/8″ (0.375″)<\/td>9.53<\/td>-0.47<\/td>\u041d\u0435\u0442<\/td><\/tr>
M12<\/td>12.00<\/td>1\/2″ (0.500″)<\/td>12.70<\/td>+0.70<\/td>\u041d\u0435\u0442<\/td><\/tr>
M16<\/td>16.00<\/td>5\/8″ (0.625″)<\/td>15.88<\/td>-0.12<\/td>Close*<\/td><\/tr>
M20<\/td>20.00<\/td>3\/4″ (0.750″)<\/td>19.05<\/td>-0.95<\/td>\u041d\u0435\u0442<\/td><\/tr>
M24<\/td>24.00<\/td>1″ (1.000″)<\/td>25.40<\/td>+1.40<\/td>\u041d\u0435\u0442<\/td><\/tr><\/tbody><\/table>

*”Close” means diameter difference is under 0.15 mm. Even these require thread pitch verification before substitution. Threads almost certainly differ.<\/em><\/p>

How to Convert Sizes<\/h3>

Converting between metric and imperial screw sizes involves two steps. First, convert the diameter: multiply inches by 25.4 to get millimetres, or divide millimetres by 25.4 to get inches. For example, a 3\/8″ bolt equals 0.375 \u00d7 25.4 = 9.525 mm \u2014 which is close to, but not the same as, M10 (10.00 mm). Second, convert the thread: metric pitch in mm can be converted to an approximate TPI by dividing 25.4 by the pitch. An M10 \u00d7 1.5 mm pitch equals 25.4 \u00f7 1.5 \u2248 16.9 TPI \u2014 close to a 3\/8″-16 UNC (16 TPI), but not identical.<\/p>

These conversions are useful for understanding what you are looking at, but they do not make the sizes interchangeable. The thread-form geometry may appear similar, but the tolerance bands, root radii, and crest truncation differ between ISO metric and UTS standards. Use the conversion to identify the nearest equivalent for ordering purposes, then always confirm by measuring with a thread pitch gauge or thread ring gauge before assembly.<\/p>

Conversion Tips<\/h3>

From three decades of shipping fasteners to mixed-standard job sites, Prince Fastener’s technical team offers the following practical advice. Always measure before assuming \u2014 a $12 digital caliper and a $15 thread pitch gauge eliminate 95 % of misidentification errors. Label your bins \u2014 when metric and imperial screws are stored in the same workshop, colour-coded bins (e.g., blue for metric, red for imperial) prevent accidental mixing. When in doubt, order both \u2014 for critical assemblies, ordering a small sample pack in both the metric and the nearest imperial size costs a few dollars and saves the cost of a stripped hole or a returned shipment. Finally, for global projects sourcing from Prince Fastener’s custom manufacturing line<\/a>, specify the standard explicitly on every drawing \u2014 writing “M8 \u00d7 1.25 \u00d7 40, ISO 4762, A2-70” leaves zero room for ambiguity, while writing just “8 mm screw” invites disaster.<\/p>

<\/p>

Watch: How to Measure Screws & Bolts \u2014 US & Metric Sizing<\/h4>