SAE to Metric Converter: Inches to Millimetres for Sockets & Wrenches
SAE to Metric Converter
- Exact metric
- 19.050mm
- in × 25.4
- Closest socket
- 19mm
- stocked metric size
- Difference
- -0.05mm
- socket − exact
3/4" converts to 19.050mm exactly, closest to a 19mm socket, a difference of -0.05mm, rated Highly Interchangeable.
Quick sizes
Accepts a simple fraction (3/4), a mixed number (1-1/8), or a decimal (0.75). The exact millimetre value is always given first, then the closest socket a metric set actually contains.
To convert an SAE size to metric, multiply the size in inches by 25.4. One inch is defined as exactly 25.4 millimetres, so the arithmetic is exact and not an approximation, 3/4" is 0.75 × 25.4 = 19.05mm, full stop. What is not exact is the socket you end up reaching for. Metric sockets come in whole millimetres, so 19.05mm becomes a 19mm socket with 0.05mm to spare, while 1/2" at 12.70mm becomes a 13mm socket with 0.30mm of slack. This page runs the conversion in the opposite direction to the main metric-to-SAE socket chart, which is the one you want when the fastener is metric and the toolbox is not.
Full SAE to Metric Conversion Chart
Every stocked SAE socket and wrench size from 1/8" to 2-1/2", converted to its exact millimetre value and matched to the closest size a metric set actually holds.
| SAE size | Decimal (in) | Exact metric | Closest socket | Difference |
|---|---|---|---|---|
| 1/8" | 0.1250 | 3.17mm | 3.18mm | +0.01mm |
| 5/32" | 0.1563 | 3.97mm | 4mm | +0.03mm |
| 3/16" | 0.1875 | 4.76mm | 5mm | +0.24mm |
| 7/32" | 0.2188 | 5.56mm | 6mm | +0.44mm |
| 1/4" | 0.2500 | 6.35mm | 6mm | −0.35mm |
| 9/32" | 0.2813 | 7.14mm | 7mm | −0.14mm |
| 5/16" | 0.3125 | 7.94mm | 8mm | +0.06mm |
| 11/32" | 0.3438 | 8.73mm | 9mm | +0.27mm |
| 3/8" | 0.3750 | 9.52mm | 10mm | +0.48mm |
| 13/32" | 0.4063 | 10.32mm | 10mm | −0.32mm |
| 7/16" | 0.4375 | 11.11mm | 11mm | −0.11mm |
| 15/32" | 0.4688 | 11.91mm | 12mm | +0.09mm |
| 1/2" | 0.5000 | 12.70mm | 13mm | +0.30mm |
| 17/32" | 0.5313 | 13.49mm | 13mm | −0.49mm |
| 9/16" | 0.5625 | 14.29mm | 14mm | −0.29mm |
| 19/32" | 0.5938 | 15.08mm | 15mm | −0.08mm |
| 5/8" | 0.6250 | 15.88mm | 16mm | +0.13mm |
| 21/32" | 0.6563 | 16.67mm | 17mm | +0.33mm |
| 11/16" | 0.6875 | 17.46mm | 17mm | −0.46mm |
| 23/32" | 0.7188 | 18.26mm | 18mm | −0.26mm |
| 3/4" | 0.7500 | 19.05mm | 19mm | −0.05mm |
| 25/32" | 0.7813 | 19.84mm | 20mm | +0.16mm |
| 13/16" | 0.8125 | 20.64mm | 21mm | +0.36mm |
| 27/32" | 0.8438 | 21.43mm | 21mm | −0.43mm |
| 7/8" | 0.8750 | 22.22mm | 22mm | −0.22mm |
| 29/32" | 0.9063 | 23.02mm | 23mm | −0.02mm |
| 15/16" | 0.9375 | 23.81mm | 24mm | +0.19mm |
| 31/32" | 0.9688 | 24.61mm | 25mm | +0.39mm |
| 1" | 1.0000 | 25.40mm | 25mm | −0.40mm |
| 1-1/32" | 1.0313 | 26.19mm | 26mm | −0.19mm |
| 1-1/16" | 1.0625 | 26.99mm | 27mm | +0.01mm |
| 1-3/32" | 1.0938 | 27.78mm | 28mm | +0.22mm |
| 1-1/8" | 1.1250 | 28.57mm | 29mm | +0.43mm |
| 1-5/32" | 1.1563 | 29.37mm | 29mm | −0.37mm |
| 1-3/16" | 1.1875 | 30.16mm | 30mm | −0.16mm |
| 1-7/32" | 1.2188 | 30.96mm | 31mm | +0.04mm |
| 1-1/4" | 1.2500 | 31.75mm | 32mm | +0.25mm |
| 1-19/64" | 1.2969 | 32.94mm | 33mm | +0.06mm |
| 1-5/16" | 1.3125 | 33.34mm | 33mm | −0.34mm |
| 1-3/8" | 1.3750 | 34.92mm | 35mm | +0.08mm |
| 1-7/16" | 1.4375 | 36.51mm | 36mm | −0.51mm |
| 1-1/2" | 1.5000 | 38.10mm | 38mm | −0.10mm |
| 1-9/16" | 1.5625 | 39.69mm | 40mm | +0.31mm |
| 1-5/8" | 1.6250 | 41.27mm | 41mm | −0.27mm |
| 1-11/16" | 1.6875 | 42.86mm | 41mm | −1.86mm |
| 1-3/4" | 1.7500 | 44.45mm | 46mm | +1.55mm |
| 1-13/16" | 1.8125 | 46.04mm | 46mm | −0.04mm |
| 1-7/8" | 1.8750 | 47.63mm | 46mm | −1.63mm |
| 1-15/16" | 1.9375 | 49.21mm | 50mm | +0.79mm |
| 2" | 2.0000 | 50.80mm | 51mm | +0.20mm |
| 2-1/16" | 2.0625 | 52.39mm | 51mm | −1.39mm |
| 2-1/8" | 2.1250 | 53.97mm | 54mm | +0.03mm |
| 2-5/32" | 2.1563 | 54.77mm | 55mm | +0.23mm |
| 2-3/16" | 2.1875 | 55.56mm | 55mm | −0.56mm |
| 2-1/4" | 2.2500 | 57.15mm | 57mm | −0.15mm |
| 2-9/32" | 2.2813 | 57.94mm | 58mm | +0.06mm |
| 2-5/16" | 2.3125 | 58.74mm | 58mm | −0.74mm |
| 2-3/8" | 2.3750 | 60.32mm | 60mm | −0.32mm |
| 2-7/16" | 2.4375 | 61.91mm | 63mm | +1.09mm |
| 2-1/2" | 2.5000 | 63.50mm | 63mm | −0.50mm |
The Exact metric column is the true conversion and the Closest socket column is what you can actually pick up. The gap between them is the difference: a negative value means the metric socket is smaller than the SAE fastener, a positive value means it is larger and will sit loose on the head. Scanning the difference column shows how uneven the pairing is, 3/4" lands within 0.05mm of 19mm, while 11/16" misses 17mm by nearly ten times that.
The Fraction-First Workflow
Doing this by hand is three steps, and the order matters, converting the fraction before you multiply is what keeps the answer exact.
- 1
Turn the fraction into a decimal
Divide the numerator by the denominator. 9/16 becomes 9 ÷ 16 = 0.5625". A mixed number is added on top: 1-1/8" is 1 + (1 ÷ 8) = 1.125". This step is pure arithmetic and it is where almost every manual conversion goes wrong, because people reach for the nearest remembered decimal instead of dividing.
- 2
Multiply the decimal by 25.4
One inch is defined as exactly 25.4mm, so the multiplication is exact rather than approximate. 0.5625" × 25.4 = 14.29mm. Nothing is rounded yet, carrying the full decimal through to the end is what keeps the final match honest.
- 3
Match to a socket that exists
14.29mm is not a socket you can buy. The nearest stocked metric size is 14mm, 0.29mm under the exact figure. That last step is the one this page automates: knowing the exact millimetre value matters far less than knowing which real socket to reach for and how far off it sits.
Step one is the step worth automating on its own, and our fraction to decimal chart has every wrench fraction from 1/64" upward already worked out. For values that are not socket sizes at all, a measured gap, a shim thickness, a drawing dimension, the general millimetre to inch conversion runs the same arithmetic without snapping the answer to a tool size.
Most Common SAE to Metric Equivalents
Twelve pairings cover almost everything a mechanic meets. The last column is the practical verdict, not just the arithmetic.
| SAE | Exact metric | Closest socket | Difference | What it is for |
|---|---|---|---|---|
| 1/4" | 6.35mm | 6mm | −0.35mm | Small brackets and trim |
| 5/16" | 7.94mm | 8mm | +0.06mm | Body panels and splash shields |
| 3/8" | 9.53mm | 10mm | +0.47mm | The classic near-miss, do not substitute |
| 7/16" | 11.11mm | 11mm | −0.11mm | Battery terminals and alternator brackets |
| 1/2" | 12.70mm | 13mm | +0.30mm | Spark plug and caliper hardware |
| 9/16" | 14.29mm | 14mm | −0.29mm | Oil drain plugs |
| 5/8" | 15.88mm | 16mm | +0.12mm | Ball joint pinch bolts |
| 11/16" | 17.46mm | 17mm | −0.46mm | Lug nuts, close, but loose enough to round |
| 3/4" | 19.05mm | 19mm | −0.05mm | Lug nuts and axle nuts. The best pairing on the chart |
| 13/16" | 20.64mm | 21mm | +0.36mm | Truck and SUV lug nuts |
| 7/8" | 22.23mm | 22mm | −0.23mm | Oxygen sensors |
| 15/16" | 23.81mm | 24mm | +0.19mm | Hub nuts |
Two rows in that table deserve singling out. 3/8" to 10mm is the pairing people reach for most often and it is the worst on the list, 0.47mm of slack, easily enough to shear the corners off a 10mm head. 11/16" to 17mm looks tempting on lug nuts and is nearly as bad at 0.46mm. Against those, 3/4" to 19mm at 0.05mm is in a different league entirely. When you need to check a pairing that is not on this list, the two-way socket calculator rates any combination in both directions, and wrench sizes carry their own tighter tolerance table because an open-end jaw forgives far less than a socket does.
Frequently Asked Questions
The questions that come up most when working from an inch size toward a metric tool.
How do I convert SAE to metric?
Multiply the size in inches by 25.4. A 3/4" socket is 0.75 × 25.4 = 19.05mm, which is why a 19mm socket fits a 3/4" fastener almost perfectly. If the size is a fraction, divide the numerator by the denominator first to get the decimal, then multiply. The conversion is exact because one inch is defined as exactly 25.4 millimetres, not rounded to it.
What is 1/2 inch in mm?
1/2 inch is exactly 12.7mm. The closest socket in a metric set is 13mm, which sits 0.30mm oversize, enough that it rates Use with Caution rather than a free swap. It will turn a 1/2" fastener at low torque with a 6-point socket, but on anything torqued to a specification you want the correct 1/2" socket.
Is 19mm the same as 3/4 inch?
Almost exactly. 3/4" is 19.05mm and a 19mm socket is 19mm, a difference of 0.05mm or 0.002 inches. That is the closest pairing anywhere in the common socket range and the two are freely interchangeable in both directions, including on wheel lug nuts. It is the one substitution nobody argues about.
Why is there no exact metric equivalent for most SAE sizes?
Because the two systems chose their increments independently. SAE steps in 1/32" and 1/16" jumps, metric steps in whole millimetres of about 0.039", and 25.4 divides evenly into no common SAE denominator. The sizes interleave rather than line up, so nearly every conversion lands between two available sockets and the useful answer is which one is closer and by how much.
Do I convert from the bolt size or the head size?
Always the head size, measured across the flats. A 1/2" bolt describes the shank diameter and takes a 3/4" socket; an M8 bolt takes a 13mm socket. Converting the thread figure instead of the head figure produces a socket several sizes too small, which is the most common reason a converted size appears not to fit.
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.