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SAE to Metric Wrench Converter
Convert SAE (fractional inch) wrench and socket sizes to millimetres and find the nearest metric wrench, or go from metric to SAE. Each match is flagged as interchangeable, loose or not a fit, with a full conversion chart.
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Measure the nut or bolt head between two flat sides with calipers
Enter your values to see the result.
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How to use the SAE to Metric Wrench Converter
- Choose the direction: SAE to metric, metric to SAE, or a measured size.
- Pick the wrench size or type the across-flats measurement.
- Press Convert to see the exact equivalent and the nearest wrench.
- Check the fit flag before using a substitute on a tight fastener.
- Scan the chart for every SAE size and its metric partner.
Quick SAE and metric wrench equivalents
| SAE | Exact mm | Metric partner | Fit |
|---|---|---|---|
| 5/16" | 7.94 | 8 mm | Interchangeable |
| 3/8" | 9.525 | 10 mm | Too loose (9.5 mm is rare) |
| 7/16" | 11.11 | 11 mm | Interchangeable |
| 1/2" | 12.70 | 13 mm | Loose, light duty |
| 9/16" | 14.29 | 14 mm too small / 15 mm too loose | No good partner |
| 5/8" | 15.88 | 16 mm | Interchangeable |
| 11/16" | 17.46 | 17 mm too small / 18 mm too loose | No good partner |
| 3/4" | 19.05 | 19 mm | Interchangeable |
| 13/16" | 20.64 | 21 mm | Loose, light duty |
| 7/8" | 22.23 | 22 mm too small | Use 7/8" on 22 mm only loosely |
| 15/16" | 23.81 | 24 mm | Loose, light duty |
| 1-1/16" | 26.99 | 27 mm | Interchangeable |
Bolt size vs wrench size
The wrench size is the distance across the flats of the head, not the bolt's thread diameter.
| US bolt (hex cap screw) | Wrench | Metric bolt | Wrench (ISO 4014 / DIN 931) |
|---|---|---|---|
| 1/4" | 7/16" | M5 | 8 mm |
| 5/16" | 1/2" | M6 | 10 mm |
| 3/8" | 9/16" | M8 | 13 mm |
| 7/16" | 5/8" | M10 | 16 mm / 17 mm |
| 1/2" | 3/4" | M12 | 18 mm / 19 mm |
| 5/8" | 15/16" | M16 | 24 mm |
| 3/4" | 1-1/8" | M20 | 30 mm |
- How the fit flag works: the tool compares nominal sizes. Within ±0.13 mm is treated as interchangeable, up to 0.4 mm oversize as loose, and anything undersize as not fitting. Real tools have small extra clearance, so a "loose" pair may still feel snug.
- Use six-point sockets for stuck or high-torque fasteners; twelve-point sockets and open-end wrenches round corners more easily.
- Japanese and some European vehicles use 12, 14 and 17 mm heads where ISO specifies 13, 16 and 18 mm, so keep a full metric set.
Frequently asked questions
What metric size is a 1/2 inch wrench?
1/2" is 12.70 mm. The nearest metric wrench is 13 mm, which is 0.3 mm larger: it will turn a 1/2" nut in a pinch but is loose, so avoid it on tight or rusted fasteners. A 1/2" wrench will not fit a 13 mm nut.
Which SAE and metric wrenches are interchangeable?
Pairs within about 0.13 mm work as substitutes: 5/16" and 8 mm, 7/16" and 11 mm, 5/8" and 16 mm, 3/4" and 19 mm, and 1-1/16" and 27 mm. Most other pairs are either loose or too small.
What is a 10 mm socket in SAE?
10 mm is 0.394 in. The closest SAE size is 3/8" (9.53 mm), which is too small to fit, and 13/32" (10.32 mm) is rarely sold. Use a 10 mm socket on 10 mm fasteners.
Why is a loose wrench a problem?
An oversized wrench contacts only the corners of the nut. Under high torque it can round the corners, making the fastener hard to remove. Six-point sockets of the exact size grip the flats and are the safest choice.
How do I tell if a bolt is SAE or metric?
Measure across the flats of the head with calipers. If it reads close to a whole millimetre (13.0, 17.0 mm) it is metric; if it reads close to a 16th or 32nd of an inch (0.5625 in) it is SAE. Bolt head markings also help: metric bolts show property classes like 8.8 or 10.9.
Is SAE the same as standard or imperial?
Yes. In tool sets, SAE, standard, imperial, inch and AF (across flats) all refer to fractional-inch sizes such as 3/8", 1/2" and 9/16".
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.
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