Metric to SAE Converter
Match metric and SAE wrench and socket sizes. Enter a size in millimeters or inches and get the nearest SAE fraction or metric equivalent, plus the exact decimal.
Example: with Wrench size 10 · Entered in Millimeters (mm) → Nearest size: 3/8 in (SAE).
- Exact conversion0.3937 in exact (10 mm)
- Fit check0.48 mm off, verify fit
Computed by the calculator below using its default values. Change any input to see your own numbers.
One inch is exactly 25.4 mm. SAE wrenches are sized in inch fractions (1/4, 5/16, 3/8...). The nearest-size logic converts your value and picks the closest common wrench.
Reading across metric and SAE
Metric wrenches are labeled by the bolt head width in millimeters. SAE wrenches — the US customary standard — are labeled in fractions of an inch. Because one inch is exactly 25.4 mm, you can lay the two systems on the same ruler and see which sizes land close together.
The catch is that they rarely land exactly together. A 10 mm bolt head is 0.394 inch, closest to a 3/8 inch (0.375 inch) SAE wrench, but that wrench is about 0.5 mm larger — loose enough that forcing it can round off the corners of a tight fastener. This tool flags how far off the nearest size is so you know when a swap is safe and when to reach for the right wrench.
How it’s calculated
Conversions use 1 in = 25.4 mm exactly. For a mm input, the value is divided by 25.4 and matched to the nearest SAE fraction from 1/4 to 1 in. For an inch input, the value is multiplied by 25.4 and matched to the nearest common metric size (6-24 mm). The gap is reported in mm.
Matches to common wrench and socket sizes only. A close match is not a guaranteed fit — worn or tight fasteners still need the exact size.
Metric to SAE wrench chart
| Metric | Nearest SAE | Decimal in |
|---|---|---|
| 6 mm | 1/4 in | 0.236 |
| 8 mm | 5/16 in | 0.315 |
| 10 mm | 3/8 in | 0.394 |
| 13 mm | 1/2 in | 0.512 |
| 16 mm | 5/8 in | 0.630 |
| 19 mm | 3/4 in | 0.748 |
Computed as mm ÷ 25.4, matched to the nearest common SAE wrench fraction; verify fit before use.
Common mistakes
- Forcing a close SAE wrench onto a metric fastener — a 0.5 mm gap can strip the corners of a tight bolt.
- Assuming 13 mm and 1/2 inch are identical; 1/2 inch is 12.7 mm, about 0.3 mm smaller.
- Rounding 1 inch to 25 mm instead of the exact 25.4 mm, which shifts every match.
Frequently asked questions
What SAE size is closest to 10 mm?
10 mm is 0.394 inch, closest to a 3/8 inch (0.375 inch) SAE wrench. The 3/8 is about 0.5 mm larger, so use it with care on tight bolts.
How do metric and SAE sizes convert?
Multiply an inch fraction by 25.4 to get millimeters, or divide millimeters by 25.4 to get inches. One inch is exactly 25.4 mm.
Is 3/4 inch the same as 19 mm?
Very nearly. 3/4 inch is 19.05 mm, so a 19 mm and a 3/4 inch wrench are close enough to swap in most cases.
Why do my metric and SAE wrenches feel interchangeable sometimes?
A few pairs land within a few hundredths of a millimeter, like 19 mm and 3/4 inch. Most pairs are close but not exact, which is why matched-set damage happens on tight fasteners.