Hand Tools
Socket Size Chart
Socket size is the across-flats measurement of the fastener, not the thread size. A 1/2″ bolt takes a 3/4″ socket — the two numbers are never the same.
Common pairings: 1/4″ bolt → 7/16″ socket, 3/8″ → 9/16″, 1/2″ → 3/4″. Metric: M6 → 10 mm, M8 → 13 mm, M10 → 16 mm (or 17 mm on older DIN 931 hardware).
| Legacy size | |||||
|---|---|---|---|---|---|
| #6 | Inch (SAE) | 5/16" | 0.31257.938 | ||
| #8 | Inch (SAE) | 11/32" | 0.34388.733 | ||
| #10 | Inch (SAE) | 3/8" | 0.37509.525 | ||
| 1/4 | Inch (SAE) | 7/16" | 0.437511.112 | ||
| 5/16 | Inch (SAE) | 1/2" | 0.500012.700 | ||
| 3/8 | Inch (SAE) | 9/16" | 0.562514.287 | ||
| 7/16 | Inch (SAE) | 5/8" | 0.625015.875 | ||
| 1/2 | Inch (SAE) | 3/4" | 0.750019.050 | ||
| 9/16 | Inch (SAE) | 13/16" | 0.812520.637 | ||
| 5/8 | Inch (SAE) | 15/16" | 0.937523.813 | ||
| 3/4 | Inch (SAE) | 1-1/8" | 1.125028.575 | ||
| 7/8 | Inch (SAE) | 1-5/16" | 1.312533.337 | ||
| 1 | Inch (SAE) | 1-1/2" | 1.500038.100 | ||
| 1-1/8 | Inch (SAE) | 1-11/16" | 1.687542.862 | ||
| 1-1/4 | Inch (SAE) | 1-7/8" | 1.875047.625 | ||
| 1-1/2 | Inch (SAE) | 2-1/4" | 2.250057.150 | ||
| M3 | Metric | 5.5 mm | 0.21655.499 | ||
| M4 | Metric | 7 mm | 0.27567.000 | ||
| M5 | Metric | 8 mm | 0.31508.001 | ||
| M6 | Metric | 10 mm | 0.393710.000 | ||
| M8 | Metric | 13 mm | 0.511813.000 | ||
| M10 | Metric | 16 mm | 0.629915.999 | DIN 931 uses 17 mm | |
| M12 | Metric | 18 mm | 0.708718.001 | DIN 931 uses 19 mm | |
| M14 | Metric | 21 mm | 0.826821.001 | DIN 931 uses 22 mm | |
| M16 | Metric | 24 mm | 0.944924.000 | ||
| M18 | Metric | 27 mm | 1.063027.000 | ||
| M20 | Metric | 30 mm | 1.181130.000 | ||
| M22 | Metric | 34 mm | 1.338634.000 | DIN 931 uses 32 mm | |
| M24 | Metric | 36 mm | 1.417335.999 | ||
| M27 | Metric | 41 mm | 1.614241.001 | ||
| M30 | Metric | 46 mm | 1.811045.999 |
Across-flats values are the hex width the socket must fit. Bars are drawn to scale.
Can you use an SAE socket on a metric bolt?
Sometimes — and the honest answer depends on how far apart the two sizes actually are, not on a blanket rule. Below 0.6% difference the fit is indistinguishable. Past about 1.5% the socket carries on the corners instead of the flats, which is what rounds a head.
| SAE | mm | Closest metric | Difference | Verdict |
|---|---|---|---|---|
| 5/16" | 7.938 | 8 mm | +0.062 mm (0.79%) | usable in a pinch |
| 7/16" | 11.112 | 11 mm | -0.112 mm (1.01%) | usable in a pinch |
| 1/2" | 12.700 | 13 mm | +0.300 mm (2.36%) | will round the fastener |
| 9/16" | 14.287 | 14 mm | -0.287 mm (2.01%) | will round the fastener |
| 5/8" | 15.875 | 16 mm | +0.125 mm (0.79%) | usable in a pinch |
| 11/16" | 17.462 | 17 mm | -0.462 mm (2.65%) | will round the fastener |
| 3/4" | 19.050 | 19 mm | -0.050 mm (0.26%) | interchangeable |
| 13/16" | 20.637 | 21 mm | +0.363 mm (1.76%) | will round the fastener |
| 7/8" | 22.225 | 22 mm | -0.225 mm (1.01%) | usable in a pinch |
| 15/16" | 23.812 | 24 mm | +0.188 mm (0.79%) | usable in a pinch |
| 1" | 25.400 | 25 mm | -0.400 mm (1.57%) | will round the fastener |
| 1-1/16" | 26.987 | 27 mm | +0.013 mm (0.05%) | interchangeable |
| 1-1/8" | 28.575 | 29 mm | +0.425 mm (1.49%) | usable in a pinch |
| 1-3/16" | 30.162 | 30 mm | -0.162 mm (0.54%) | interchangeable |
| 1-1/4" | 31.750 | 32 mm | +0.250 mm (0.79%) | usable in a pinch |
| 1-5/16" | 33.337 | 33 mm | -0.337 mm (1.01%) | usable in a pinch |
| 1-3/8" | 34.925 | 35 mm | +0.075 mm (0.21%) | interchangeable |
| 1-7/16" | 36.512 | 36 mm | -0.512 mm (1.4%) | usable in a pinch |
The pair worth remembering is 3/4″ and 19 mm — 0.05 mm apart, which is why they get used interchangeably on lug nuts every day. The pair worth avoiding is 1/2″ and 13 mm: close enough to start, far enough to slip under load.
Why some metric sets seem to skip a size
M10, M12, M14 and M22 changed across-flats dimensions between the older DIN 931 standard and current ISO 4014. An M10 head is 16 mm today but 17 mm on older hardware. Working on an older European machine with a modern set is exactly when you discover this.
Automotive sockets that are not standard sizes
| Job | Socket | Notes |
|---|---|---|
| Spark plug (most modern) | 5/8 in (16 mm) | Thin-wall plug socket with rubber retainer |
| Spark plug (older / small) | 13/16 in (21 mm) | Common on older V8s and small engines |
| Spark plug (compact / import) | 14 mm | Many modern 4-cylinder imports |
| Oxygen sensor | 7/8 in (22 mm) | Slotted O2 sensor socket to clear the wire |
| Water heater element | 1-1/2 in | Deep element socket, not a standard 6-point |
| Anode rod | 1-1/16 in | Needs a long breaker bar; often seized |
| Lug nut (most passenger cars) | 19 mm / 3/4 in | Use a thin-wall or protective socket on alloy wheels |
| Lug nut (trucks / SUVs) | 21 mm / 13/16 in | Some heavy-duty use 22 mm |
Spark plugs and oxygen sensors need dedicated sockets, not just the right size — a plug socket has a rubber or magnetic retainer, and an O2 sensor socket is slotted so the wire can pass through.
6-point or 12-point
A 6-point socket contacts all six flats and is what you want on anything tight or corroded. A 12-point engages every 30° instead of 60°, which helps in cramped spaces but puts the load nearer the corners. On a seized fastener that difference decides whether it comes loose or rounds off.
Data source and method
- Inch
- ASME B18.2.1 / B18.2.2 across-flats dimensions for hex bolts and nuts.
- Metric
- ISO 4014 / ISO 4032, with DIN 931 legacy widths noted where they differ.
- Interchangeability
- Computed. Difference under 0.6% is treated as interchangeable, 0.6–1.5% as usable but sloppy, above that as likely to round the fastener.
- Verification
- Eight known pairings checked (1/4→7/16″, 3/8→9/16″, 1/2→3/4″, 3/4→1-1/8″, M6→10, M8→13, M10→16, M16→24 mm). All match.
Related charts
- Bolt torque chart — how tight to run the fastener once the socket is on it
- Light bulb socket sizes — the other kind of socket entirely