Enter a metric socket size in millimetres.
17mm converts to 21/32 inch SAE (0.656"), a difference of +0.013", rated Use with Caution.
Enter a fraction such as 3/4 or 1-1/8, or a decimal inch value.
Shown as socket size minus your exact target, in millimetres. The metric panel compares the other direction, in inches.
3/4" SAE converts to 19.050mm exactly, closest to a 19mm socket, rated Highly Interchangeable.
Metric sockets are sized in millimetres; SAE sockets are sized in fractional inches. This calculator converts between them in both directions, matching each input to the closest available socket size and rating how safely the two can be substituted.
Metric to standard socket conversion matches a millimetre socket size to the closest fractional-inch SAE size and rates whether the difference is small enough to use safely. SAE sockets, the Society of Automotive Engineers standard, are sized in fractional inches; metric sockets are sized in millimetres. The two systems never align, because one inch equals exactly 25.4mm and each standard set its increments independently. Nearly every conversion therefore lands on a decimal with no exact fractional match: 17mm measures 0.669", while the closest SAE socket, 21/32", measures 0.656". The working answer is the closest interchangeable size plus its tolerance: inside ±0.005" the two swap freely, and beyond ±0.015" the socket rounds the bolt head. Going the other way, converting standard sizes back to millimetres uses the same bands, and metric to SAE wrench sizes follow a separate chart with looser tolerances.
All 47 metric socket sizes from 3.18mm to 63mm, each paired with its closest standard SAE socket, the decimal inch value of that socket, the signed difference and an interchangeability rating.
| Metric (mm) | Closest SAE | Decimal (in) | Difference | Rating |
|---|---|---|---|---|
| 3.18mm | 1/8" | 0.125" | 0.000" | Highly Interchangeable |
| 4mm | 5/32" | 0.156" | +0.001" | Highly Interchangeable |
| 5mm | 3/16" | 0.188" | +0.009" | Use with Caution |
| 6mm | 7/32" | 0.219" | +0.017" | Not Recommended |
| 7mm | 1/4" | 0.250" | +0.026" | Not Recommended |
| 8mm | 5/16" | 0.313" | +0.002" | Highly Interchangeable |
| 9mm | 11/32" | 0.344" | +0.011" | Use with Caution |
| 10mm | 3/8" | 0.375" | +0.019" | Not Recommended |
| 11mm | 7/16" | 0.438" | -0.004" | Highly Interchangeable |
| 12mm | 15/32" | 0.469" | +0.004" | Highly Interchangeable |
| 13mm | 1/2" | 0.500" | +0.012" | Use with Caution |
| 14mm | 9/16" | 0.563" | -0.011" | Use with Caution |
| 15mm | 19/32" | 0.594" | -0.003" | Highly Interchangeable |
| 16mm | 5/8" | 0.625" | +0.005" | Highly Interchangeable |
| 17mm | 21/32" | 0.656" | +0.013" | Use with Caution |
| 18mm | 11/16" | 0.688" | +0.021" | Not Recommended |
| 19mm | 3/4" | 0.750" | -0.002" | Highly Interchangeable |
| 20mm | 25/32" | 0.781" | +0.006" | Use with Caution |
| 21mm | 13/16" | 0.813" | +0.014" | Use with Caution |
| 22mm | 7/8" | 0.875" | -0.009" | Use with Caution |
| 23mm | 29/32" | 0.906" | -0.001" | Highly Interchangeable |
| 24mm | 15/16" | 0.938" | +0.007" | Use with Caution |
| 25mm | 1" | 1.000" | -0.016" | Not Recommended |
| 26mm | 1-1/32" | 1.031" | -0.008" | Use with Caution |
| 27mm | 1-1/16" | 1.063" | 0.000" | Highly Interchangeable |
| 28mm | 1-1/8" | 1.125" | -0.023" | Not Recommended |
| 29mm | 1-5/32" | 1.156" | -0.015" | Use with Caution |
| 30mm | 1-3/16" | 1.188" | -0.006" | Use with Caution |
| 31mm | 1-7/32" | 1.219" | +0.002" | Highly Interchangeable |
| 32mm | 1-1/4" | 1.250" | +0.010" | Use with Caution |
| 33mm | 1-19/64" | 1.297" | +0.002" | Highly Interchangeable |
| 34mm | 1-5/16" | 1.313" | +0.026" | Not Recommended |
| 35mm | 1-3/8" | 1.375" | +0.003" | Highly Interchangeable |
| 36mm | 1-7/16" | 1.438" | -0.020" | Not Recommended |
| 38mm | 1-1/2" | 1.500" | -0.004" | Highly Interchangeable |
| 39mm | 1-9/16" | 1.563" | -0.027" | Not Recommended |
| 40mm | 1-9/16" | 1.563" | +0.012" | Use with Caution |
| 41mm | 1-5/8" | 1.625" | -0.011" | Use with Caution |
| 46mm | 1-13/16" | 1.813" | -0.001" | Highly Interchangeable |
| 50mm | 2" | 2.000" | -0.031" | Not Recommended |
| 51mm | 2" | 2.000" | +0.008" | Use with Caution |
| 54mm | 2-1/8" | 2.125" | +0.001" | Highly Interchangeable |
| 55mm | 2-5/32" | 2.156" | +0.009" | Use with Caution |
| 57mm | 2-1/4" | 2.250" | -0.006" | Use with Caution |
| 58mm | 2-9/32" | 2.281" | +0.002" | Highly Interchangeable |
| 60mm | 2-3/8" | 2.375" | -0.013" | Use with Caution |
| 63mm | 2-1/2" | 2.500" | -0.020" | Not Recommended |
The rating column reads as a tolerance band: ✓ Highly Interchangeable means the two sizes differ by ±0.005" or less, ~ Use with Caution means ±0.015" or less, and ✗ Not Recommended means the difference exceeds ±0.015". A positive difference means the SAE socket is smaller than the metric fastener; a negative difference means it is larger and will sit loose on the head.
Three formulas cover every socket conversion: millimetres to inches, inches to millimetres, and a fraction to its decimal value.
Millimetres divided by 25.4 equals inches.
Example: 19 ÷ 25.4 = 0.748"
Result: the closest SAE socket is 3/4" (0.750"), a difference of −0.002", rated Highly Interchangeable.
For sizes outside the socket range, use the general millimetre to fractional inch conversion.
Inches multiplied by 25.4 equals millimetres.
Example: 0.500 × 25.4 = 12.70mm
Result: a 1/2" socket sits closest to a 13mm socket, a difference of +0.30mm, rated Use with Caution.
Every fractional size worked out in this direction sits on our SAE to metric socket chart.
A fraction becomes a decimal when the numerator is divided by the denominator.
Example: 21 ÷ 32 = 0.656"
Result: 21/32" equals 0.656", which is 16.67mm, the nearest SAE socket to a 17mm fastener.
Common wrench fractions are pre-solved on our decimal inch reference.
Each of the 47 metric socket sizes below has its own direct answer and its own anchor link, so #17mm or #28mm jumps straight to that size. If you are starting from a fractional inch size instead, the same values are listed in reverse there.
3.18mm is closest to 1/8" (0.125"), a difference of 0.000", rated Highly Interchangeable.
Highly Interchangeable4mm is closest to 5/32" (0.156"), a difference of +0.001", rated Highly Interchangeable.
Interior trim screws and small electrical terminals.
Highly Interchangeable5mm is closest to 3/16" (0.188"), a difference of +0.009", rated Use with Caution.
Hose clamps and instrument-panel fasteners.
Use with Caution6mm is closest to 7/32" (0.219"), a difference of +0.017", rated Not Recommended.
Battery terminal clamps and small brackets.
Not Recommended7mm is closest to 1/4" (0.250"), a difference of +0.026", rated Not Recommended.
Valve cover and small-engine cowling bolts.
Not Recommended8mm is closest to 5/16" (0.313"), a difference of +0.002", rated Highly Interchangeable.
Body panels, splash shields and small brackets.
Highly Interchangeable9mm is closest to 11/32" (0.344"), a difference of +0.011", rated Use with Caution.
Use with Caution10mm is closest to 3/8" (0.375"), a difference of +0.019", rated Not Recommended.
The single most common bolt head on modern vehicles.
Not Recommended11mm is closest to 7/16" (0.438"), a difference of -0.004", rated Highly Interchangeable.
Battery terminals and alternator brackets.
Highly Interchangeable12mm is closest to 15/32" (0.469"), a difference of +0.004", rated Highly Interchangeable.
Suspension links and accessory pulleys.
Highly Interchangeable13mm is closest to 1/2" (0.500"), a difference of +0.012", rated Use with Caution.
Spark plug hardware, brake caliper and starter bolts.
Use with Caution14mm is closest to 9/16" (0.563"), a difference of -0.011", rated Use with Caution.
Oil drain plugs on most Japanese and domestic engines.
Use with Caution15mm is closest to 19/32" (0.594"), a difference of -0.003", rated Highly Interchangeable.
Alternator and tensioner pivot bolts.
Highly Interchangeable16mm is closest to 5/8" (0.625"), a difference of +0.005", rated Highly Interchangeable.
Ball joint pinch bolts and control-arm hardware.
Highly Interchangeable17mm is closest to 21/32" (0.656"), a difference of +0.013", rated Use with Caution.
Lug nuts, caliper brackets and axle hardware.
Use with Caution18mm is closest to 11/16" (0.688"), a difference of +0.021", rated Not Recommended.
Crank pulley and camshaft sprocket bolts.
Not Recommended19mm is closest to 3/4" (0.750"), a difference of -0.002", rated Highly Interchangeable.
Lug nuts and axle nuts on most passenger cars.
Highly Interchangeable20mm is closest to 25/32" (0.781"), a difference of +0.006", rated Use with Caution.
Use with Caution21mm is closest to 13/16" (0.813"), a difference of +0.014", rated Use with Caution.
Lug nuts on many domestic trucks and SUVs.
Use with Caution22mm is closest to 7/8" (0.875"), a difference of -0.009", rated Use with Caution.
Oxygen sensors and larger suspension hardware.
Use with Caution23mm is closest to 29/32" (0.906"), a difference of -0.001", rated Highly Interchangeable.
Highly Interchangeable24mm is closest to 15/16" (0.938"), a difference of +0.007", rated Use with Caution.
Hub nuts and heavy suspension bolts.
Use with Caution25mm is closest to 1" (1.000"), a difference of -0.016", rated Not Recommended.
Not Recommended26mm is closest to 1-1/32" (1.031"), a difference of -0.008", rated Use with Caution.
Use with Caution27mm is closest to 1-1/16" (1.063"), a difference of 0.000", rated Highly Interchangeable.
Crankshaft pulley bolts on many V6 engines.
Highly Interchangeable28mm is closest to 1-1/8" (1.125"), a difference of -0.023", rated Not Recommended.
Not Recommended29mm is closest to 1-5/32" (1.156"), a difference of -0.015", rated Use with Caution.
Use with Caution30mm is closest to 1-3/16" (1.188"), a difference of -0.006", rated Use with Caution.
Front hub and spindle nuts.
Use with Caution31mm is closest to 1-7/32" (1.219"), a difference of +0.002", rated Highly Interchangeable.
Highly Interchangeable32mm is closest to 1-1/4" (1.250"), a difference of +0.010", rated Use with Caution.
Axle nuts on light trucks and vans.
Use with Caution33mm is closest to 1-19/64" (1.297"), a difference of +0.002", rated Highly Interchangeable.
Highly Interchangeable34mm is closest to 1-5/16" (1.313"), a difference of +0.026", rated Not Recommended.
Not Recommended35mm is closest to 1-3/8" (1.375"), a difference of +0.003", rated Highly Interchangeable.
Highly Interchangeable36mm is closest to 1-7/16" (1.438"), a difference of -0.020", rated Not Recommended.
Heavy axle nuts and trailer hub hardware.
Not Recommended38mm is closest to 1-1/2" (1.500"), a difference of -0.004", rated Highly Interchangeable.
Highly Interchangeable39mm is closest to 1-9/16" (1.563"), a difference of -0.027", rated Not Recommended.
Not Recommended40mm is closest to 1-9/16" (1.563"), a difference of +0.012", rated Use with Caution.
Use with Caution41mm is closest to 1-5/8" (1.625"), a difference of -0.011", rated Use with Caution.
Commercial-truck hub and kingpin nuts.
Use with Caution46mm is closest to 1-13/16" (1.813"), a difference of -0.001", rated Highly Interchangeable.
Heavy equipment spindle hardware.
Highly Interchangeable50mm is closest to 2" (2.000"), a difference of -0.031", rated Not Recommended.
Industrial and agricultural drive hardware.
Not Recommended51mm is closest to 2" (2.000"), a difference of +0.008", rated Use with Caution.
Use with Caution54mm is closest to 2-1/8" (2.125"), a difference of +0.001", rated Highly Interchangeable.
Highly Interchangeable55mm is closest to 2-5/32" (2.156"), a difference of +0.009", rated Use with Caution.
Use with Caution57mm is closest to 2-1/4" (2.250"), a difference of -0.006", rated Use with Caution.
Use with Caution58mm is closest to 2-9/32" (2.281"), a difference of +0.002", rated Highly Interchangeable.
Highly Interchangeable60mm is closest to 2-3/8" (2.375"), a difference of -0.013", rated Use with Caution.
Use with Caution63mm is closest to 2-1/2" (2.500"), a difference of -0.020", rated Not Recommended.
Large-bore hydraulic and press fittings.
Not RecommendedSeven metric sizes account for most fasteners a mechanic or technician meets in a day; the table pairs each with its SAE counterpart and the job it usually belongs to.
| Metric | SAE | Common Use |
|---|---|---|
| 8mm | 5/16" | Small fasteners, splash shields and body panels |
| 10mm | 3/8" | The most common bolt head on a modern vehicle |
| 13mm | 1/2" | Spark plug hardware and brake caliper bolts |
| 14mm | 9/16" | Oil drain plugs on most engines |
| 17mm | 21/32" | Lug nuts and caliper bracket bolts |
| 19mm | 3/4" | Lug nuts and axle nuts |
| 22mm | 7/8" | Oxygen sensors and large suspension hardware |
Socket size follows the bolt head, not the thread diameter. An M8 bolt takes a 13mm socket, not an 8mm one. Our bolt head size reference maps thread sizes to the socket that actually fits them.
Three tolerance bands decide whether a substitution is safe, risky or guaranteed to damage the fastener.
Highly Interchangeable
±0.005" or less
Swap freely, including on torque-critical hardware, provided you use a 6-point socket.
Use with Caution
±0.006" to ±0.015"
Acceptable for hand-tool work at low torque on steel fasteners; never on a torque-spec joint.
Not Recommended
More than ±0.015"
Use the correct-standard socket. At this gap the tool loads the corners of the head, not the flats.
Grip pattern. A 6-point socket tolerates roughly three times the mismatch a 12-point does, because its contact sits mid-flat while the 12-point’s twelve smaller points sit near the corners. Convert with 6-point sockets only.
Torque-critical joints. Head bolts, rod bolts, lug nuts and suspension hardware need the ±0.005" band. Any slack lets the socket turn before it bites, so the wrench registers torque the fastener never saw. The same reason impact-rated socket sizing is quoted exactly.
Soft fasteners. Brass, aluminium and plated hardware deform before steel does, so treat their tolerance as one band tighter. The same rule governs hex key equivalents, where soft set screws round almost instantly, and open-end wrench sizing, which loads only two flats.
The tolerance a conversion can survive depends on the socket type; select a type to see how much mismatch it forgives.
Three short lists cover the sizes each kind of work actually calls for, with the SAE equivalent beside each.
Passenger cars and light trucks built after 1980.
Mowers, generators, chainsaws and outboards.
Commercial trucks, agricultural and industrial machinery.
Automotive work adds length as often as it adds diameter. Spark plugs and long studs need the reach listed in our deep socket reach chart. When a fastener rounds badly enough to drill out, the job moves to a drill bit size chart and then to tap drill sizes for re-cutting the thread.
The diagram labels four features. The drive end is the square recess at the top that the ratchet plugs into, sized 1/4", 3/8", 1/2" or 3/4" regardless of the socket opening. The stamped size marking on the barrel is the opening size. A bare number such as 17 means 17mm, while a fraction such as 21/32 means SAE. The socket opening at the bottom is the end that goes over the fastener, and its measurement across the flats is the number quoted in every conversion on this page. The hexagonal profile shown end-on is the internal shape: six flats for a 6-point socket, twelve engagement points for a 12-point. Measuring a bolt head across its flats and matching it against our bolt head dimension tables identifies the size when the stamping has worn off.
The first table shows which standard dominates in each market; the second shows which standard each manufacturer builds to.
| Region | Standard | Notes |
|---|---|---|
| United States | Mixed | Metric on vehicles built after 1980; SAE on older and industrial hardware |
| Europe | Metric | Metric throughout since the 1960s |
| Japan | Metric | Metric exclusively, in every era |
| United Kingdom | Mixed | Metric on modern vehicles; Whitworth and BSF on pre-1970 machinery |
| Canada | Mixed | Follows US practice, with metric on all current vehicles |
| Australia | Metric | Metric since 1974; imported classics remain SAE |
| Manufacturer | Standard | Notes |
|---|---|---|
| Toyota, Honda, Nissan | Metric | 10mm, 12mm, 14mm, 17mm dominate |
| BMW, Mercedes-Benz, Volkswagen | Metric | Heavy use of 13mm, 16mm, 18mm and Torx |
| Ford | Mixed | Metric since the 1990s; SAE on older trucks and 5.0 V8s |
| General Motors | Mixed | Metric bodies with SAE hardware persisting into the 1990s |
| Chrysler, Dodge, Jeep | Mixed | Both standards on the same chassis through the 1980s and 1990s |
| Harley-Davidson | SAE | Fractional inch throughout, including 1/2" and 9/16" fasteners |
Region determines thread standard as much as socket standard: metric coarse across Europe and Japan, UNC and UNF on older US hardware, and Whitworth on pre-1970 British machinery, all compared on our thread standard comparison.
Twelve companion references cover the fastener, thread and drill sizing that sits either side of a socket conversion.