Most modern engines are a mix of SAE and metric fasteners — a Chevy LS is metric throughout but often sits in an SAE-fastened chassis, and a Cummins swap drops a metric engine into an SAE chassis. When the right wrench isn't on the bench, knowing which size from the other system will actually fit (and which will round the bolt) is the difference between finishing the job and a trip to the parts store. Use "Measure with calipers" mode when you've got an unknown bolt and need to ID the wrench size from a tape or caliper reading.
Identifying an unknown wrench size from a measurement
If you don't know what size wrench a bolt takes, measure it. Two rules to get a clean reading:
- Measure across the flats, not the corners. A hex head has six flats and six corners — the flat-to-flat distance is what defines the wrench size. The corner-to-corner distance is about 15% larger and will lead you to the wrong wrench every time.
- Expect 0.1–0.2 mm (4–8 thou) undersize from nominal. A 13 mm bolt head typically measures 12.85–12.95 mm. A 1/2" head typically measures 0.490–0.498". That's the manufacturing tolerance — the wrench size is the bigger, nominal value, not what your calipers read.
Tape measure works for ≥3/8" (10 mm) hardware if you've got a good rule, but calipers are the right tool. Cheap dial or digital calipers from any hardware store get you within 0.05 mm, which is more than enough to ID a wrench size. For really small fasteners (≤8 mm) the wrench-size step is tight enough that a millimeter of measurement error can land you on the wrong size — measure twice and pick the smallest wrench that's not smaller than your reading.
This calculator's "Measure with calipers" mode takes your reading and shows you the smallest wrench from both the SAE and metric sets that will still go over the head (never one that's smaller than your reading), so you can grab whichever you have in hand. When one set fits snugly and the other is clearly loose, that usually tells you which system the bolt came from; when both are close, it says so.
How the conversion works
1 inch = 25.4 mm exactly. Multiply a fractional SAE size by 25.4 to get its mm equivalent. The question isn't the math — it's whether the closest standard wrench in the other system is close enough to actually grip the fastener without slipping.
- SAE wrench sets step in 1/16" or 1/32" — about 1.59 mm or 0.79 mm per size.
- Metric wrench sets step in 1 mm (sometimes 0.5 mm at the small end) — finer resolution above 13 mm.
Because the two systems use different step sizes, some fasteners land between standard wrenches in one system but exactly on a wrench in the other. That's why a 19 mm wrench is a near-perfect 3/4", but 1/2" has no clean metric equivalent (12.7 mm — your closest metric is 13 mm, which is 0.30 mm too big on a 1/2" head; and a 1/2" wrench is 0.30 mm too small to go on a 13 mm head at all).
Which swaps are actually safe?
This calculator classifies each pairing by the size gap and its direction — a substitute wrench that's bigger goes on with play, one that's smaller may not go on at all:
- Same (substitute up to 0.15 mm smaller or 0.20 mm bigger, about 6–8 thou): Functionally identical. Either wrench fits with no risk.
- Close (0.20–0.40 mm bigger): Workable on a hand-tight fastener. Will round the bolt if you torque hard against the gap — the wrench rocks before the fastener turns.
- Loose (0.40–0.80 mm bigger): Don't. The wrench will slip and round the head before the bolt breaks loose.
- Tight (0.15–0.25 mm smaller): May not go on; fits only heads that run undersize. Don't hammer it on.
- Won't fit (more than 0.25 mm smaller) / no equivalent: Use the correct size.
A small undersize is allowed because wrench openings are made a little over nominal and bolt heads a little under. The pairs worth memorizing (within 0.20 mm, either wrench on either head): 5/16" ≈ 8 mm, 7/16" ≈ 11 mm, 5/8" ≈ 16 mm, 3/4" ≈ 19 mm, 15/16" ≈ 24 mm, 1-1/16" ≈ 27 mm, plus the less common 9/32" ≈ 7 mm and 1-1/2" ≈ 38 mm. 1-3/16" and 30 mm are 0.16 mm apart: the 1-3/16" wrench fits a 30 mm head, the 30 mm wrench is tight on a 1-3/16" head. 7/8" vs 22 mm is 0.23 mm apart: a 7/8" wrench on a 22 mm head is close (slightly loose), but a 22 mm wrench on a 7/8" head is tight and may not go on.
Why this matters under torque
A wrench that's 0.30 mm oversize fits over the bolt head with visible play. With light hand torque, you can wiggle it onto the flats and turn the bolt. With real torque, the flats start to deform — the corners of the bolt head get squeezed, the wrench rotates a few degrees before the bolt does, and one of two things happens: the wrench slips off and rounds the head, or the corners shear and you're now looking at extractors.
This is especially bad on aluminum heads, soft brass plugs, or any bolt that was already torqued once before. Use the right size for anything you're tightening to spec. The "close enough" swaps are for breaking loose a bolt you're going to replace anyway.
Six-point vs twelve-point matters too
Even when the size matches, a twelve-point wrench or socket grips less of the bolt head than a six-point — only the corners actually carry load. On a size that's slightly off, a twelve-point will round the corners much faster than a six-point. If you're forced into a "Close" swap, use six-point if you have it.