Wrench and Socket Size Chart
Every fractional SAE wrench size converts to an exact millimetre value, which is then rounded to the nearest whole-millimetre metric size for comparison. 1/2" SAE equals 12.7 mm exactly, closest to a 13 mm metric wrench.
SAE and Metric, Side by Side
Hardware manufactured to imperial standards uses SAE fractional bolt heads; hardware manufactured to metric standards uses whole-millimetre bolt heads — and most shops and vehicles end up with a mix of both.
Because 25.4 mm doesn't divide evenly into convenient metric steps, no SAE size lines up exactly with a metric one. The closest matches shown below are useful for a quick field substitute, but a bolt head is always best turned with a wrench from its own system to avoid rounding the corners.
SAE to Nearest Metric Size
Two steps find the closest metric wrench to any SAE size.
MM is the exact millimetre size. Nearest is that value rounded to the closest whole millimetre — the metric wrench size most likely to fit closely.
- INThe SAE size in decimal inches, such as 0.5 for 1/2".
- MMThe exact millimetre equivalent: IN × 25.4.
- NearestMM rounded to the nearest whole millimetre.
Worked calculation — 1/2" SAE
- 0.5 × 25.4 = 12.7Exact millimetre value
- round(12.7) = 13Nearest whole millimetre
1/2" SAE ≈ 13 mm metric (off by 0.3 mm)
SAE to Metric Wrench Sizes
| SAE | Decimal | Exact MM | Nearest metric | Difference |
|---|---|---|---|---|
| 1/4" | 0.25" | 6.35 mm | 6 mm | -0.35 mm |
| 5/16" | 0.3125" | 7.9375 mm | 8 mm | 0.0625 mm |
| 3/8" | 0.375" | 9.525 mm | 10 mm | 0.475 mm |
| 7/16" | 0.4375" | 11.1125 mm | 11 mm | -0.1125 mm |
| 1/2" | 0.5" | 12.7 mm | 13 mm | 0.3 mm |
| 9/16" | 0.5625" | 14.2875 mm | 14 mm | -0.2875 mm |
| 5/8" | 0.625" | 15.875 mm | 16 mm | 0.125 mm |
| 11/16" | 0.6875" | 17.4625 mm | 17 mm | -0.4625 mm |
| 3/4" | 0.75" | 19.05 mm | 19 mm | -0.05 mm |
| 13/16" | 0.8125" | 20.6375 mm | 21 mm | 0.3625 mm |
| 7/8" | 0.875" | 22.225 mm | 22 mm | -0.225 mm |
| 15/16" | 0.9375" | 23.8125 mm | 24 mm | 0.1875 mm |
| 1" | 1" | 25.4 mm | 25 mm | -0.4 mm |
Frequently Asked Questions
What metric wrench is closest to a 1/2 inch SAE wrench?
A 13 mm wrench is closest to 1/2" SAE (12.7 mm exactly) — a difference of just 0.3 mm.
Are SAE and metric wrenches interchangeable?
Not reliably. Even when the nearest metric size is close, the difference — often a few hundredths of a millimetre up to half a millimetre — can be enough to round off a bolt head or slip under load, especially with repeated use. Use the correct system whenever the tool is available.
What size is a 9/16 wrench in mm?
9/16" equals 14.2875 mm exactly, closest to a 14 mm metric wrench.
Why isn't there an exact metric equivalent for most SAE sizes?
Because an inch (25.4 mm) isn't a round multiple of any convenient metric spacing, so SAE fractional sizes almost never land exactly on a whole millimetre value — only approximate substitutes exist.
How do you tell SAE and metric sockets apart?
Most SAE wrenches and sockets are stamped with a fraction, like 1/2" or 9/16", while metric ones are stamped with "mm" or just a plain number, like 13 or 14. Checking the stamped marking is the most reliable way, since the two can look identical in size at a glance.
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