SAE to Metric Calculator
Two-way conversion between SAE fractional-inch sizes and metric millimeters — plus a wrench/socket interchange chart, SAE bolt diameter reference, and UNC/UNF thread pitch notes.
Scope: this tool is fastener & wrench focused — fractional-inch ↔ millimeter size conversion, wrench interchange fit, and SAE bolt/thread reference. It is not a general-purpose length converter; for that, see the length converter.
SAE ⇄ metric converter
Type a fraction like 3/4, a mixed number like 1 1/2, or a decimal like 0.625.
Type millimeters, e.g. 19. Editing either field updates the other instantly.
Wrench / socket interchange chart
Nearest standard metric equivalent for each common SAE wrench size, and vice versa. Fit is the size difference: Snug < 0.15 mm, Usable < 0.5 mm, Loose ≥ 0.5 mm — a loose substitute can round the fastener, so reach for the correct size when torque matters.
SAE → metric
| SAE size | Exact (mm) | Nearest metric | Difference | Fit |
|---|
Metric → SAE
| Metric | Exact (in) | Nearest SAE | Difference | Fit |
|---|
Chart generated live from the conversion engine (1 in = 25.4 mm exactly); nearest size is the minimum absolute difference against standard wrench sets.
SAE bolt diameter & thread reference
Common SAE bolt diameters in millimeters, with UNC (coarse) and UNF (fine) threads-per-inch and the metric advance per thread (25.4 ÷ TPI).
| SAE bolt | Diameter | UNC (TPI) | UNF (TPI) | mm / thread | Nearest metric bolt |
|---|
Threads are NOT interchangeable. SAE bolts use Unified threads counted in threads-per-inch; metric bolts use threads counted in millimeters per thread, with a different thread profile. A 1/2"-13 bolt will not correctly thread into an M12 × 1.75 nut even though the diameters are close — always match both diameter and thread.
Reading an SAE bolt designation
1/4"-20 UNC = 1/4 in nominal diameter (6.35 mm), 20 threads per inch, Unified National Coarse. Advance per thread: 25.4 ÷ 20 = 1.27 mm.
3/8"-24 UNF = 3/8 in diameter (9.525 mm), 24 threads per inch, Unified National Fine. Advance per thread: 25.4 ÷ 24 ≈ 1.0583 mm.
3/8"-24 UNF = 3/8 in diameter (9.525 mm), 24 threads per inch, Unified National Fine. Advance per thread: 25.4 ÷ 24 ≈ 1.0583 mm.
“Nearest metric bolt” is diameter proximity only (from standard metric bolt sizes) — it does not mean the threads match.
Worked examples
3/4 in → mm: 0.75 × 25.4 = 19.05 mm. Nearest metric wrench: 19 mm (0.05 mm difference — snug).
19 mm → in: 19 ÷ 25.4 = 0.74803 in ≈ 3/4" (0.75 in). This is why 19 mm and 3/4 in sockets are used interchangeably.
1/2 in → mm: 0.5 × 25.4 = 12.7 mm. Nearest metric wrench: 13 mm (0.3 mm oversize — usable, but a 12 mm wrench at 0.7 mm undersize will not fit at all).
Thread pitch, 5/16"-18 UNC: diameter 0.3125 × 25.4 = 7.9375 mm; advance per thread 25.4 ÷ 18 ≈ 1.4111 mm.
5/8 in vs 16 mm: 5/8 in = 0.625 × 25.4 = 15.875 mm, so 16 mm is 0.125 mm larger — a snug substitution.
Frequently asked questions
What is 3/4 inch in millimeters?
Exactly 19.05 mm (0.75 × 25.4). The nearest standard metric wrench is 19 mm — only 0.05 mm smaller — so a 19 mm wrench fits a 3/4 in bolt head snugly.
What metric wrench is closest to 1/2 inch?
13 mm. One-half inch is exactly 12.7 mm, so a 13 mm wrench is 0.3 mm oversize: usable in a pinch, but the correct 1/2 in wrench fits better and is less likely to round the fastener.
Is 19 mm the same as 3/4 inch?
Almost. 3/4 in equals 19.05 mm, so 19 mm is 0.05 mm smaller. It is the closest standard pairing and the two are interchangeable in practice for wrenches and sockets.
What is 19 mm in inches?
19 ÷ 25.4 = 0.74803 in, which is just under 3/4 in (0.75 in). As a fraction that is about 48/64 in, so 3/4 in is the nearest common fractional size.
Can I use a metric wrench on an SAE bolt?
Sometimes. Snug pairs include 5/16 in with 8 mm (0.0625 mm difference) and 3/4 in with 19 mm (0.05 mm difference). Loose pairs like 3/8 in with 10 mm (0.475 mm difference) can slip and round the fastener — check the fit rating in the chart above before substituting.
What do bolt sizes like 1/4"-20 UNC mean?
1/4 in is the nominal diameter (6.35 mm), 20 is the thread count in threads per inch (TPI), and UNC means Unified National Coarse thread. 20 TPI equals 25.4 ÷ 20 = 1.27 mm of advance per thread.
How do you convert TPI to metric thread pitch?
Divide 25.4 by the threads-per-inch value: mm per thread = 25.4 ÷ TPI. Example: a 3/8"-16 bolt has 25.4 ÷ 16 = 1.5875 mm of advance per thread.
Are SAE and metric bolts interchangeable?
No. SAE bolts use UNC/UNF threads measured in threads per inch while metric bolts use threads measured in millimeters per thread, with different thread profiles. A 1/2"-13 bolt will not correctly thread into an M12 × 1.75 nut even though the diameters are close — always match both diameter and thread.
Which is bigger: 5/8 inch or 16 mm?
16 mm is slightly bigger. 5/8 in equals exactly 15.875 mm, so a 16 mm wrench is 0.125 mm oversize on a 5/8 in fastener — a snug, commonly used substitution.
Why won't my 12 mm wrench fit a 1/2 inch bolt?
Because 1/2 in is 12.7 mm, so a 12 mm wrench is 0.7 mm too small and will not go on. Use a 13 mm wrench (0.3 mm oversize) or, better, the correct 1/2 in wrench.
Is this a general length converter?
No. This tool is focused on fasteners and wrenches: fractional-inch to millimeter size conversion, a wrench/socket interchange chart, and SAE bolt and thread reference. For general length conversion, use a dedicated length converter.
What is the formula for inches to millimeters?
Millimeters = inches × 25.4, exact by the international definition of the inch in effect since 1959. To go the other way: inches = millimeters ÷ 25.4.
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