Workshop reference
One bolt size chart for metric bolts from M2 to M48 and inch bolts from #4 to 1 1/2 inch. Each size has its thread pitch, wrench and spanner size, tap drill and clearance hole, and there is a screw size chart for machine screw numbers #0 to #12 in inches and mm.
Quick answers
What size wrench for a 1/2 inch bolt? 3/4 inch (19.05 mm) across the flats for a standard hex bolt or nut. Heavy hex structural bolts take 7/8 inch.
What spanner or wrench fits M10? 16 mm to current ISO standards. Older DIN M10 bolts and nuts take 17 mm.
What does M8 x 1.25 mean? An ISO metric thread, 8 mm across, with a 1.25 mm pitch (the distance between thread crests). That is the coarse pitch for M8, so a plain M8 is the same thing.
What size is a #10 screw in mm? 0.190 in, which is 4.83 mm. It comes as 24 threads per inch coarse or 32 fine.
Tap drill and clearance hole for M8? 6.8 mm to tap, and 9 mm for a normal clearance hole.
Four things sit in every row: the thread, the wrench, the tap drill and the clearance hole. They answer four different questions, so it helps to know which one you are after.
M means ISO metric. The number after it is the outside diameter of the thread in millimeters. The second number is the pitch, the distance between thread crests, also in millimeters. Leave the pitch off and you get the coarse one, so M10 means M10 x 1.5. A fine thread is written out in full, like M10 x 1.25.
The first number is the diameter. The second is threads per inch. UNC is the coarse series and UNF the fine one, so 1/2 inch comes as 13 UNC or 20 UNF. Numbered screws follow the same pattern, like #10-24 UNC.
This is the width across the flats of the hex. It is the same on the bolt head and on the matching nut, so one wrench, spanner or socket size covers both.
A tap drill is smaller than the bolt, so the thread has metal to cut into. A clearance hole is bigger, so the bolt passes through. The three clearance fits are close, for accurate location, normal, for general assembly, and loose, for poor alignment, heat movement or big fabrications.
All 23 sizes, in millimeters. The bold columns are the ISO spanner size, the tap drill and the normal clearance hole. Highlighted rows are the four sizes where the spanner changed when ISO replaced DIN. A dash means the source chart has no figure for that size.
| Size | Coarse pitch (mm) | Fine pitch (mm) | Spanner, ISO (mm) | Spanner, old DIN (mm) | Tap drill (mm) | Clearance, close (mm) | Clearance, normal (mm) | Clearance, loose (mm) |
|---|---|---|---|---|---|---|---|---|
| M2 | 0.4 | 0.25 | - | - | 1.6 | 2.2 | 2.4 | 2.6 |
| M2.5 | 0.45 | 0.35 | - | - | 2.05 | 2.7 | 2.9 | 3.1 |
| M3 | 0.5 | 0.35 | 5.5 | 5.5 | 2.5 | 3.2 | 3.4 | 3.6 |
| M4 | 0.7 | 0.5 | 7 | 7 | 3.3 | 4.3 | 4.5 | 4.8 |
| M5 | 0.8 | 0.5 | 8 | 8 | 4.2 | 5.3 | 5.5 | 5.8 |
| M6 | 1 | 0.75 | 10 | 10 | 5 | 6.4 | 6.6 | 7 |
| M8 | 1.25 | 1 | 13 | 13 | 6.8 | 8.4 | 9 | 10 |
| M10 | 1.5 | 1.25 | 16 | 17 | 8.5 | 10.5 | 11 | 12 |
| M12 | 1.75 | 1.25 | 18 | 19 | 10.2 | 13 | 13.5 | 14.5 |
| M14 | 2 | 1.5 | 21 | 22 | 12 | 15 | 15.5 | 16.5 |
| M16 | 2 | 1.5 | 24 | 24 | 14 | 17 | 17.5 | 18.5 |
| M18 | 2.5 | 1.5 | 27 | 27 | - | - | - | - |
| M20 | 2.5 | 1.5 | 30 | 30 | 17.5 | 21 | 22 | 24 |
| M22 | 2.5 | 1.5 | 34 | 32 | 19.5 | 23 | 24 | 26 |
| M24 | 3 | 2 | 36 | 36 | 21 | 25 | 26 | 28 |
| M27 | 3 | 2 | 41 | 41 | 24 | 28 | 30 | 32 |
| M30 | 3.5 | 2 | 46 | 46 | 26.5 | 31 | 33 | 35 |
| M33 | 3.5 | 2 | 50 | 50 | 29.5 | 34 | 36 | 38 |
| M36 | 4 | 3 | 55 | 55 | 32 | 37 | 39 | 42 |
| M39 | 4 | 3 | 60 | 60 | 35 | 40 | 42 | 45 |
| M42 | 4.5 | 3 | 65 | 65 | 37.5 | 43 | 45 | 48 |
| M45 | 4.5 | 3 | 70 | 70 | 40.5 | 46 | 48 | 52 |
| M48 | 5 | 3 | 75 | 75 | 43 | 50 | 52 | 56 |
Spanner is the width across flats of the head and nut to ISO 4014, 4017 and 4032, with the superseded DIN 931, 933 and 934 size beside it. Tap drill is for about 75 percent thread, which is the major diameter minus the pitch. Clearance holes are ISO 273, whose fine, medium and coarse series are shown here as close, normal and loose. The fine pitch column shows the main fine pitch. Other standard fine pitches are M8 0.75; M10 1 and 0.75; M12 1.5 and 1; M14 1.25 and 1; M16 1; M18 2 and 1; M20 2 and 1; M22 2 and 1; M24 1.5 and 1. M2 and M2.5 have no spanner figure in the source chart, and M18 has no tap drill or clearance hole. The tap drill chart has the M18 tap drill.
Most metric hex sizes stayed the same when ISO standards replaced the DIN ones. Exactly four did not: M10, M12 and M14 got smaller, and M22 got bigger. New stock follows ISO, but legacy DIN bolts and nuts may still need the old wrench. Check which standard is stamped on the head.
| Size | Spanner, ISO now (mm) | Spanner, old DIN (mm) | What changed |
|---|---|---|---|
| M10 | 16 | 17 | ISO is smaller |
| M12 | 18 | 19 | ISO is smaller |
| M14 | 21 | 22 | ISO is smaller |
| M22 | 34 | 32 | ISO is larger |
Inch sizes from #4 to 3/4 inch. Wrench size is the finished hex across flats for a standard hex bolt or nut, with the millimeter equivalent beside it. Tap drills are the number, letter or fractional drill with its decimal inch size, for about 75 percent thread. Clearance holes are the drill with its decimal inch size in parentheses.
| Size | UNC (threads per inch) | UNF (threads per inch) | Wrench across flats | Tap drill, UNC | Tap drill, UNF | Clearance, close | Clearance, normal | Clearance, loose |
|---|---|---|---|---|---|---|---|---|
| #4 | 40 | 48 | - | - | - | #31 (0.120) | #30 (0.128) | #27 (0.144) |
| #6 | 32 | 40 | - | #36 0.1065 | #33 0.1130 | #23 (0.154) | #18 (0.170) | #13 (0.185) |
| #8 | 32 | 36 | 11/32 in (8.73 mm) | #29 0.1360 | #29 0.1360 | #15 (0.180) | #9 (0.196) | #3 (0.213) |
| #10 | 24 | 32 | 3/8 in (9.52 mm) | #25 0.1495 | #21 0.1590 | #5 (0.206) | #2 (0.221) | B (0.238) |
| 1/4 in | 20 | 28 | 7/16 in (11.11 mm) | #7 0.2010 | #3 0.2130 | 17/64 (0.266) | 9/32 (0.281) | 19/64 (0.297) |
| 5/16 in | 18 | 24 | 1/2 in (12.7 mm) | F 0.2570 | I 0.2720 | 21/64 (0.328) | 11/32 (0.344) | 23/64 (0.359) |
| 3/8 in | 16 | 24 | 9/16 in (14.29 mm) | 5/16 0.3125 | Q 0.3320 | 25/64 (0.391) | 13/32 (0.406) | 27/64 (0.422) |
| 7/16 in | 14 | 20 | 5/8 in (15.88 mm) | - | - | 29/64 (0.453) | 15/32 (0.469) | 31/64 (0.484) |
| 1/2 in | 13 | 20 | 3/4 in (19.05 mm) | 27/64 0.4219 | 29/64 0.4531 | 17/32 (0.531) | 9/16 (0.562) | 39/64 (0.609) |
| 9/16 in | 12 | 18 | 13/16 in (20.64 mm) | - | - | 19/32 (0.594) * | 5/8 (0.625) * | 21/32 (0.656) * |
| 5/8 in | 11 | 18 | 15/16 in (23.81 mm) | 17/32 0.5312 | 37/64 0.5781 | 21/32 (0.656) | 11/16 (0.688) | 47/64 (0.734) |
| 3/4 in | 10 | 16 | 1 1/8 in (28.58 mm) | 21/32 0.6562 | 11/16 0.6875 | 25/32 (0.781) | 13/16 (0.812) | 29/32 (0.906) |
* 9/16 inch is not a standard ASME B18.2.8 size, so those three clearance figures are interpolated. Confirm them before you rely on them. Clearance depends on the bolt diameter only, not the thread pitch. A dash means the source chart has no figure for that size. It has no wrench size for #4 and #6, and no tap drill for #4, 7/16 or 9/16 inch. The tap drill chart covers the missing UNC and UNF tap drills, and the drill size chart turns any drill into inches and mm.
| Size | Wrench across flats | Clearance, close | Clearance, normal | Clearance, loose |
|---|---|---|---|---|
| 7/8 in | 1 5/16 in (33.34 mm) | 29/32 (0.906) | 15/16 (0.938) | 1 1/32 (1.031) |
| 1 in | 1 1/2 in (38.1 mm) | 1 1/32 (1.031) | 1 3/32 (1.094) | 1 5/32 (1.156) |
| 1 1/8 in | 1 11/16 in (42.86 mm) | 1 5/32 (1.156) | 1 7/32 (1.219) | 1 5/16 (1.312) |
| 1 1/4 in | 1 7/8 in (47.63 mm) | 1 9/32 (1.281) | 1 11/32 (1.344) | 1 7/16 (1.438) |
| 1 3/8 in | 2 1/16 in (52.39 mm) | - | - | - |
| 1 1/2 in | 2 1/4 in (57.15 mm) | 1 9/16 (1.562) | 1 5/8 (1.625) | 1 47/64 (1.734) |
Above 3/4 inch the source chart gives the wrench size and clearance hole only, and it has no clearance hole for 1 3/8 inch. For threads per inch and tap drills at these sizes, use the tap drill chart.
The wrench sizes above are for standard finished hex bolts and nuts. Heavy hex structural bolts, A325, A490 and F3125, are larger across the flats for the same diameter. Do not use the finished hex sizes for them.
| Bolt size | Finished hex wrench | Heavy hex wrench |
|---|---|---|
| 1/2 in | 3/4 in (19.05 mm) | 7/8 in (22.225 mm) |
| 3/4 in | 1 1/8 in (28.58 mm) | 1 1/4 in (31.75 mm) |
| 1 in | 1 1/2 in (38.1 mm) | 1 5/8 in (41.275 mm) |
Small screws are sized by a number, not a fraction. The diameter in inches is 0.060 plus 0.013 times the screw number, so a #10 is 0.060 + 0.130 = 0.190 in, which is 4.83 mm. The sizes below are the numbers in common use.
| Screw number | Diameter (in) | Diameter (mm) |
|---|---|---|
| #0 | 0.060 | 1.52 |
| #1 | 0.073 | 1.85 |
| #2 | 0.086 | 2.18 |
| #3 | 0.099 | 2.51 |
| #4 | 0.112 | 2.84 |
| #5 | 0.125 | 3.18 |
| #6 | 0.138 | 3.51 |
| #8 | 0.164 | 4.17 |
| #10 | 0.190 | 4.83 |
| #12 | 0.216 | 5.49 |
Diameters come from the formula above. #4, #6, #8 and #10 match the diameters in the thread pitch chart in Fabora Steel Tools. Threads per inch for those four are in the inch table. The tap drill chart has them for #0 to #3, #5 and #12, and the fraction to decimal chart helps when a drawing mixes fractions and decimals.
In the Steel Tools app
The free Reference tab in the Fabora Steel Tools app holds the spanner sizes, thread pitch, tap drill, clearance hole and bolt torque charts, and it works with no signal. Steel Tools runs on iPhone, Android and in any browser.
Bolt size FAQ
Short answers, with the numbers taken straight from the charts above.
A 3/4 inch (19.05 mm) wrench or socket fits a 1/2 inch UNC or UNF hex bolt or nut. That is the finished hex size from ASME B18.2.1 and B18.2.2. Heavy hex structural bolts (A325, A490, F3125) are bigger: a 1/2 inch heavy hex bolt takes a 7/8 inch wrench.
16 mm to the current ISO standards (ISO 4014, 4017 and 4032). Older bolts and nuts made to DIN 931, 933 and 934 take 17 mm. Four sizes changed: M10 (16 mm now, 17 mm before), M12 (18 mm now, 19 mm before), M14 (21 mm now, 22 mm before), M22 (34 mm now, 32 mm before). If a spanner feels loose or tight, check which standard is stamped on the head.
M8 x 1.25 is an ISO metric thread with an 8 mm outside diameter and a 1.25 mm pitch, the distance from one thread crest to the next. 1.25 mm is the coarse pitch for M8, so a bolt just marked M8 is the same thread. The fine pitch is 1 mm, written M8 x 1. An M8 tap drill is 6.8 mm and a normal clearance hole is 9 mm.
A #10 screw is 0.190 in, or 4.83 mm, across the thread. The size comes from 0.060 plus 0.013 times the screw number. #10 comes as 10-24 UNC (24 threads per inch) or 10-32 UNF (32 per inch), and the nut is 3/8 inch across the flats.
Coarse threads have the bigger pitch, so fewer threads over the same length. M10 is 1.5 mm coarse or 1.25 mm fine, and 1/2 inch is 13 threads per inch coarse (UNC) or 20 fine (UNF). Coarse is the default and the one you meet most. Fine is used where a drawing asks for it, and it needs a different tap drill.
It depends which hole you mean. A tap drill is smaller than the bolt so the thread can be cut: 8.5 mm for M10, 6.8 mm for M8, 27/64 inch for 1/2-13 UNC. A clearance hole is bigger than the bolt so it passes through: for M10 that is 10.5 mm close, 11 mm normal or 12 mm loose. Normal is the default for general assembly.
A 9/16 inch drill (0.562 in) gives a normal fit. Close is 17/32 inch (0.531 in) and loose is 39/64 inch (0.609 in), from ASME B18.2.8. Close suits accurate location, normal suits general assembly, and loose suits poor alignment, heat movement or big fabrications.
Yes. Heavy hex structural bolts (A325, A490 and F3125) are larger across the flats than the finished hex sizes in the chart: 1/2 inch takes a 7/8 inch wrench, 3/4 inch takes 1 1/4 and 1 inch takes 1 5/8. Do not use the finished hex sizes for them.
Yes. The width across flats is the same on the bolt head and the matching nut at any given size, in both metric and inch, so one spanner, wrench or socket does both.
Multiply by 25.4. 1/2 inch is 12.7 mm, 3/8 inch is 9.525 mm and a #10 screw is 4.83 mm. Inch and metric threads do not swap, though. 3/8 inch (16 threads per inch) sits close to M10 (1.5 mm pitch) but the two will not fit each other.
Every figure in the metric and inch tables is copied from the verified Reference charts in Fabora Steel Tools. Metric spanner sizes are the across-flats sizes in ISO 4014 and 4017 (hex bolts and screws) and ISO 4032 (hex nuts), with the older DIN 931, 933 and 934 sizes beside them. Inch wrench sizes are the finished hex sizes in ASME B18.2.1, B18.2.2 and B18.6.3, cross-checked against published Bolt Depot and Portland Bolt charts.
Thread pitch is from ISO 261 and 262 for metric and ASME B1.1 for UNC and UNF. Tap drills are for about 75 percent thread: major diameter minus pitch for metric, and the long-standing Machinery's Handbook values for inch. Clearance holes are ISO 273 for metric and ASME B18.2.8 for inch, with 9/16 inch interpolated because that standard does not list it.
The machine screw diameters are worked out from 0.060 + 0.013 x the screw number, and match the four the app chart lists. The heavy hex wrench sizes are from the app chart's notes, with the millimeter values worked out as inch x 25.4. Where the source chart has no figure, this page shows a dash and leaves it blank rather than filling it in from somewhere else. For critical work, check the standard stamped on the part.
Fabora Steel Tools
The free Reference tab in Fabora Steel Tools holds the spanner size, thread pitch, tap drill, clearance hole and bolt torque charts, and it works with no signal in the workshop or on site. Steel Tools runs on iPhone, Android and in any browser.
