A 17mm wrench is closest to 21/32" (0.656"), a difference of +0.013", rated Not Recommended for wrench use.
Quick sizes
Wrench tolerance is tighter than socket tolerance: ±0.004" swaps freely, ±0.010" is the outer limit for low-torque box-end work, and anything beyond that will round the fastener.
A metric wrench converts to SAE by dividing the millimetre size by 25.4 and matching the result to the nearest fractional-inch wrench, but the tolerance that makes the swap safe is tighter than it is for sockets. A 6-point socket grips all six flats of a fastener. An open-end wrench grips two. The same 0.010" of slack that a socket absorbs across six faces concentrates on two faces in a wrench, and the jaw splays outward under load until it skids onto the corners. That is why a size pairing rated safe on our metric to standard socket conversion chart can still be the wrong call for a wrench, and why this page re-rates every size against a narrower band.
All 47 metric wrench sizes from 3.18mm to 63mm, each paired with the closest SAE wrench, its decimal inch value, the signed difference, and a rating against wrench tolerance rather than socket tolerance.
| Metric (mm) | Closest SAE | Decimal (in) | Difference | Wrench 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" | ✗ Not Recommended |
| 10mm | 3/8" | 0.375" | +0.019" | ✗ Not Recommended |
| 11mm | 7/16" | 0.438" | -0.004" | ~ Use with Caution |
| 12mm | 15/32" | 0.469" | +0.004" | ✓ Highly Interchangeable |
| 13mm | 1/2" | 0.500" | +0.012" | ✗ Not Recommended |
| 14mm | 9/16" | 0.563" | -0.011" | ✗ Not Recommended |
| 15mm | 19/32" | 0.594" | -0.003" | ✓ Highly Interchangeable |
| 16mm | 5/8" | 0.625" | +0.005" | ~ Use with Caution |
| 17mm | 21/32" | 0.656" | +0.013" | ✗ Not Recommended |
| 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" | ✗ Not Recommended |
| 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" | ✗ Not Recommended |
| 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" | ✗ Not Recommended |
| 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" | ~ Use with Caution |
| 39mm | 1-9/16" | 1.563" | -0.027" | ✗ Not Recommended |
| 40mm | 1-9/16" | 1.563" | +0.012" | ✗ Not Recommended |
| 41mm | 1-5/8" | 1.625" | -0.011" | ✗ Not Recommended |
| 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" | ✗ Not Recommended |
| 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 ±0.004" or less and either wrench fits, ~ Use with Caution means ±0.010" or less and the box end will hold at low torque, and ✗ Not Recommended means the gap will round the fastener. Compare that against the socket ratings on the same 47 sizes and you will see several pairings, 13mm and 1/2" among them, that a socket tolerates but a wrench does not. A positive difference means the SAE wrench is smaller than the metric fastener; a negative difference means it is larger and will sit loose.
How much mismatch a wrench forgives depends entirely on how many faces of the fastener it touches.
Two flats
The jaw slides onto the fastener from the side, contacting only two opposing flats. That is the least forgiving grip on the bench: all the load runs through two faces, and any slack lets the jaw splay and skid toward the corners. Open-end wrenches need the tightest match of any hand tool, and are the worst possible choice for a converted metric-to-SAE fit.
Six or twelve points
The closed ring wraps the head completely. A 6-point box end spreads load across all six faces and tolerates roughly the same mismatch as a 6-point socket; a 12-point box end engages every 30 degrees for tight swing arcs but moves its contact closer to the corners. Where a conversion is unavoidable, use the box end, not the open end.
Both, same size
One open end and one box end of identical size on a single tool, the format most sets ship in. The practical workflow is to crack the fastener loose with the box end, where the grip is strongest, then spin it out with the open end once the torque has dropped to almost nothing.
The practical rule that falls out of those three cards: an exact-size open end is fine, a converted fit belongs in a 6-point box end, and nothing marginal belongs on a torque-critical joint at all. If you are working the other direction and starting from an inch size, the SAE to metric converter lists the same pairings in reverse.
Nominal size is identical across manufacturers, a 17mm wrench is 17mm across the flats whoever made it, but jaw geometry and manufacturing clearance differ enough to change how a marginal fit behaves.
| Brand | Typical geometry | What it means for a converted fit |
|---|---|---|
| Craftsman | Conventional 12-point box | Mass-market geometry with generous jaw clearance. Forgiving on worn or slightly oversize fasteners, but that same clearance shows up as play on a converted fit. |
| Snap-on | Flank Drive | The box end contacts the flats away from the corners by design, which is why it holds on rounded heads other wrenches slip off. The most tolerant of a near-miss size, though still not a substitute for the correct standard. |
| GearWrench | Ratcheting box | The ratcheting pawl needs internal clearance, so a ratcheting box end runs slightly looser than a solid one. Treat a converted fit in a ratcheting wrench as one tolerance band worse. |
| Tekton | Conventional 6-point box | Six-point box ends across most of the range, which is the geometry you want for a marginal fit. Where a set offers both, the 6-point box is always the safer choice for a substitution. |
Read that table as a ranking of how much abuse each geometry absorbs, not as a licence to substitute sizes. A flank-drive profile that grips away from the corners genuinely buys margin on a rounded head, and a ratcheting box end genuinely gives some of that margin back. None of them turn a ✗ rating into a ✓ one. Manufacturers publish across-flats tolerances only for their own product lines and the figures move between ranges, so the qualitative ordering above is more useful than any single number.
| System | How the size is measured | Where you meet it |
|---|---|---|
| Metric (mm) | Across flats, in millimetres | Europe, Japan, and all modern vehicles worldwide |
| SAE / AF (inches) | Across flats, in fractional inches | US hardware, pre-1980 domestic vehicles, Harley-Davidson |
| Whitworth (BSW) | Named for the BOLT diameter, not the jaw | Pre-1970 British cars, motorcycles and machine tools |
| BA (British Association) | Numbered 0 to 16, smaller number is larger | Instruments, electrical gear and model engineering |
The Whitworth trap
A Whitworth spanner stamped "1/4 W" does not have a 1/4" jaw. It fits the hexagon on a 1/4" diameter Whitworth bolt, and that hexagon measures 0.445" across the flats, closer to a 7/16" AF spanner than anything else. Every other system on this page names the jaw; Whitworth names the bolt. Grabbing a Whitworth spanner expecting an AF size is the fastest way to round a fastener on a pre-1970 British car.
They are the same tool. British English says spanner, American English says wrench, and both describe a fixed-jaw tool that turns a hexagonal fastener. British catalogues label inch sizes "AF", short for "across flats", precisely to distinguish them from Whitworth sizes that are not. If you are converting a loose measurement rather than a named size, the millimetre to inch conversion chart handles arbitrary values down to a 64th.
Regional history explains most of what you find in a toolbox. European and Japanese vehicles have been metric throughout since the 1960s. American vehicles crossed over around 1980 and left a generation of trucks carrying both standards on one chassis. British machinery ran Whitworth and BA well into the 1970s. A mechanic working across all three eras genuinely needs three sets, and the conversion charts on this site exist to cover the gaps where one set has to stand in for another. The live two-way socket calculator is the fastest way to check a specific pair before you put a tool on a fastener.
Five questions that come up whenever a metric wrench has to stand in for an SAE one, or the reverse.
Sometimes, but the safe band is narrower than it is for sockets. A wrench needs the two sizes within about ±0.004" before it can be swapped freely, against ±0.005" for a 6-point socket. 19mm and 3/4" differ by 0.002" and work in either direction; 10mm and 3/8" differ by 0.019" and will round the head. Always use the box end rather than the open end when the fit is not exact.
10mm is closest to 3/8", but the pair is not usable. 10mm measures 0.394" and 3/8" measures 0.375", a gap of 0.019", nearly five times the safe band for a wrench. The 7/16" wrench at 0.438" is even further out in the opposite direction. 10mm is the single most common bolt head on a modern car and it has no workable SAE substitute, which is the main reason a metric set is not optional.
Yes, both ends of a combination wrench are cut to the same nominal size across the flats, and a 14mm open end and a 14mm box end fit the same fastener. What differs is how much mismatch each end tolerates: the box end wraps all six faces, while the open end loads only two, so the box end survives a marginal fit that would round the head with the open end.
Either the wrench is a converted size rather than an exact one, or the fastener head has already been rounded. Measure the head across the flats with a caliper and compare it against the chart above. If the head measures under its nominal size the corners have been crushed by a previous oversize tool, and the fix is a bolt extractor socket rather than a tighter wrench.
A 1/2" bolt takes a 3/4" wrench. Wrench size follows the head across the flats, never the thread diameter, the 1/2" figure describes the shank. The same logic runs through metric hardware, where an M8 bolt takes a 13mm wrench. Sizing from the thread instead of the head is the most common reason a wrench pulled from the drawer does not fit.
The rest of the fastener, thread and drill references on this site, every one of them is one click away from here.