1/4" drive
4, 5, 5.5, 6, 7, 8, 9, 10, 11, 12, 13, 14mm
Interior trim, sensors, clips and small brackets.
Impact Socket Sizes: Metric & SAE Drive Size Guide
Impact Socket Size Finder
mm
- Impact socket
- 19mm
- 6-point, Cr-Mo
- Recommended drive
- 3/8"
- square drive
- Nearest SAE
- 3/4"
- 0.750"
A 19mm fastener takes a 19mm impact socket, normally on a 3/8" drive, with 3/4" as the nearest SAE impact size.
Quick sizes
The recommended drive follows the size: 1/4″ up to 14mm, 3/8″ to 19mm, 1/2″ to 32mm, and 3/4″ beyond that. Impact work needs an exact size, never a converted one.
Impact sockets are forged from chrome-molybdenum steel and finished in black oxide, and they are the only sockets that may be used on an impact wrench. The distinction is not about quality or price, it is about how the steel fails. A chrome hand socket is hardened for surface strength and shatters into fragments under the hammering of a gun; an impact socket uses a softer, more ductile alloy that deforms and splits instead. That difference decides whether a failure is an inconvenience or an injury. Sizing itself works exactly as it does for hand sockets, measured across the flats, matched to the fastener head, but with one extra rule: hammering amplifies any mismatch into repeated shock loading on the corners, so impact work needs the correct size rather than the close one that the socket conversion chart might rate as acceptable by hand.
Impact vs Chrome Sockets Compared
Eight properties separate the two, and only one of them is visible from across the workshop.
| Property | Impact socket | Chrome hand socket |
|---|---|---|
| Steel | Chrome-molybdenum (Cr-Mo) | Chrome-vanadium (Cr-V) |
| Hardness | Softer and more ductile | Harder, surface-hardened |
| Failure mode | Deforms and splits | Shatters into fragments |
| Wall thickness | Thick | Thin |
| Finish | Black oxide (phosphate) | Polished chrome plate |
| Retaining feature | Pin hole and O-ring groove | None |
| Safe on an impact gun | Yes | No, never |
| Safe on a hand ratchet | Yes | Yes |
The last two rows are the operational summary: impact sockets are safe on both tools, chrome sockets are safe on only one. That asymmetry runs one way and one way only, and it is worth stating plainly because the reverse assumption is common. Putting an impact socket on a hand ratchet costs you nothing but a little extra weight and wall thickness; putting a chrome socket on a gun risks it coming apart.
Metric Impact Socket Sizes by Drive
Which metric sizes each drive normally carries, and the work that drive is built for.
3/8" drive
8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19mm
The general automotive range, most under-bonnet and brake work.
1/2" drive
10, 12, 13, 14, 15, 16, 17, 18, 19, 21, 22, 24, 27, 30, 32mm
Lug nuts, suspension, crank pulley and axle hardware.
3/4" drive
17, 19, 21, 22, 24, 27, 30, 32, 33, 35, 36, 38, 41, 46, 50mm
Commercial trucks, agricultural and industrial plant.
The ranges overlap deliberately. A 17mm socket exists in 3/8", 1/2" and 3/4" drive, and the three are not interchangeable in use even though the opening is identical. The drive determines how much torque the joint between gun and socket can carry. Choosing a drive is really choosing a torque class, which is laid out in full on the drive size adapter guide.
SAE Impact Socket Sizes by Drive
| Drive | Range | Sizes carried |
|---|---|---|
| 1/4" | 3/16" – 9/16" | 3/16, 7/32, 1/4, 9/32, 5/16, 11/32, 3/8, 7/16, 1/2, 9/16 |
| 3/8" | 5/16" – 3/4" | 5/16, 3/8, 7/16, 1/2, 9/16, 5/8, 11/16, 3/4 |
| 1/2" | 3/8" – 1-1/4" | 3/8, 7/16, 1/2, 9/16, 5/8, 11/16, 3/4, 13/16, 7/8, 15/16, 1, 1-1/16, 1-1/8, 1-1/4 |
| 3/4" | 3/4" – 2" | 3/4, 7/8, 15/16, 1, 1-1/8, 1-1/4, 1-5/16, 1-3/8, 1-1/2, 1-5/8, 1-3/4, 2 |
Comparing this against the metric card grid above shows the two systems cover the same physical span with different step patterns, 1/2" drive runs 10mm to 32mm on one side and 3/8" to 1-1/4" on the other, which are nearly identical ranges expressed in incompatible units. That is the whole reason a conversion chart exists, though for impact work specifically the answer is almost always to buy the correct size rather than convert. Where a converted size genuinely is unavoidable, the SAE to metric chart shows exactly how far off each pairing sits.
Why Chrome Sockets Cannot Go on an Impact Wrench
The failure mode
Chrome vanadium is hardened through to a high surface hardness, which is exactly what makes it hold a polished finish and resist wear by hand. Hard steel is also brittle steel. An impact wrench does not apply smooth torque. It delivers thousands of hammer blows a minute, and each one sends a shock wave through the socket wall. Brittle material under repeated shock does not bend, it fractures, and a thin-walled chrome socket can throw fragments at the speed the anvil was turning.
Why Cr-Mo survives
Chrome molybdenum is deliberately left softer and more ductile, and the walls are made thicker to compensate for the lower hardness. Under the same shock loading it absorbs energy by deforming slightly rather than cracking. When it eventually does fail it splits and stays largely in one piece. The black phosphate finish is not decorative either: plating chrome onto an impact socket would reintroduce a hard brittle surface layer, which is precisely what you are trying to avoid.
The same reasoning extends to accessories. Extensions, universal joints and adapters all come in both grades, and an impact-rated socket on a chrome extension simply moves the brittle component one step down the chain. If a gun is driving it, every part between the anvil and the fastener needs to be impact rated. Reach is the other consideration on a gun, since a short socket often cannot clear a protruding stud. The deep socket length chart covers which jobs need the extra depth.
Reading Impact Socket Markings
Five markings appear on a typical impact socket, and together they identify size, standard and rating.
- Size, e.g. "19mm" or "3/4"
- The opening across the flats. A bare number is metric; a fraction is SAE. This is the only marking that tells you whether the socket fits.
- Black oxide finish
- The visual shorthand for impact-rated. Chrome sockets are polished and bright; impact sockets are matte black because they are phosphate-coated rather than plated.
- Pin hole and O-ring groove
- Larger impact sockets carry a through-hole for a retaining pin and a groove for an O-ring, which stop the socket flying off the anvil under hammering.
- "Cr-Mo" or "Chrome Molybdenum"
- The alloy designation. Cr-Mo is the impact steel; Cr-V (chrome vanadium) is the hand-tool steel and must never go on a gun.
- 6-point vs 12-point
- Impact sockets are overwhelmingly 6-point, because twelve smaller contact points cannot survive repeated hammer blows without rounding the fastener.
If the size stamping has worn off, which happens quickly on the sockets that get used most, measure the opening across two opposing flats with a caliper and match the reading against a size chart. A caliper reading of 0.669" is a 17mm socket; 0.750" is 3/4". That single measurement resolves both the size and the standard at once, and the two-way socket calculator will convert it in whichever direction you need.
More Conversion Tools
The rest of the fastener, thread and drill references on this site, every one of them is one click away from here.
Wrench Size Chart Metric to SAE wrench sizing, separate from socket tolerances Bolt Size Chart Bolt head dimensions matched to the socket that fits them Drill Size Chart Number, letter, fractional and metric drill bit sizes Tap Drill Chart Correct drill size before cutting metric or SAE threads Thread Size Chart Metric, UNC, UNF and Whitworth thread standards compared mm to Inches General millimetre to fractional inch conversion Fraction to Decimal Wrench fraction to decimal inch reference Allen Wrench Sizes Metric and SAE hex key equivalents