Metric to SAE Wrench and Socket Size Calculator
Enter a metric size in millimeters to find the nearest standard SAE fractional-inch wrench or socket size, plus the exact decimal conversion. Switch to SAE-to-metric mode to go the other direction. The conversion uses the NIST-standardised constant of exactly 25.4 mm per inch.
What is the difference between metric and SAE sizes?
SAE stands for Society of Automotive Engineers. In the context of wrenches and sockets, SAE (also called standard or imperial) sizes are expressed as fractions of an inch, such as 1/4", 3/8", or 1/2". Metric sizes are expressed in whole or half millimeters, such as 8 mm, 10 mm, or 13 mm. The two systems are not directly interchangeable because they are based on different base units: 1 inch equals exactly 25.4 mm, so common fractions of an inch rarely align perfectly with common whole-millimeter steps. Most modern vehicles sold globally use metric hardware, while older American vehicles and many agricultural or industrial machines use SAE fasteners. Carrying a set in both systems, or a reversible set, is the safest approach for working across both.
How to convert metric mm to SAE inches
Divide the metric size in millimeters by 25.4 to get the exact decimal inch equivalent. For example, 10 mm / 25.4 = 0.39370 inches. Then find the standard SAE fraction closest to that decimal. The SAE set most common for automotive work uses fractions with denominators that are powers of 2 (halves, quarters, eighths, sixteenths, thirty-seconds). In this example, 0.39370 in is very close to 3/8" = 0.37500 in, a gap of only 0.019 in (0.48 mm), which is acceptable for light work but not ideal. The nearest size with a smaller gap is 13/32" = 0.40625 in, a gap of 0.012 in on the other side. This calculator tries every standard SAE size and returns the one with the smallest absolute gap.
When is it safe to substitute an SAE wrench on a metric fastener?
A general rule used by mechanics is that a gap under 0.010 inches (about 0.25 mm) between the SAE size and the exact metric equivalent is acceptable for most applications. A gap up to 0.020 inches (0.51 mm) is workable at low torque but carries a noticeable risk of rounding the fastener corners. Beyond 0.020 inches the wrench can slip under normal tightening torque and deform the hex flats, making the fastener very difficult to remove. Even a "safe" substitute should be used only in a pinch: the correct metric wrench or socket always fits better, grips all six flats evenly, and reduces rounding to near zero. Impact-driver work in particular should always use the correct size.
Reverse conversion - SAE to exact metric
Going the other direction, multiply the SAE decimal by 25.4. For 3/8 inch: 3/8 = 0.375 in; 0.375 x 25.4 = 9.525 mm exactly. Because metric wrenches are only made in whole or half-mm increments (10 mm is the nearest), a 3/8 SAE wrench will leave a 0.475 mm gap on a 10 mm fastener. This calculator shows the exact mm equivalent in SAE-to-metric mode so you can judge how close the nearest metric size will be.
Common metric to nearest SAE wrench sizes
| Metric (mm) | SAE fraction | SAE decimal (in) | Exact metric in inches | Gap (in) | Safe to sub? |
|---|---|---|---|---|---|
| 4 mm | 5/32" | 0.1563 | 0.1575 | +0.0012 | Yes |
| 5 mm | 3/16" | 0.1875 | 0.1969 | +0.0094 | Yes |
| 6 mm | 1/4" | 0.2500 | 0.2362 | -0.0138 | Caution |
| 7 mm | 9/32" | 0.2813 | 0.2756 | -0.0057 | Yes |
| 8 mm | 5/16" | 0.3125 | 0.3150 | +0.0025 | Yes |
| 9 mm | 11/32" | 0.3438 | 0.3543 | +0.0106 | Caution |
| 10 mm | 3/8" | 0.3750 | 0.3937 | +0.0187 | Caution |
| 11 mm | 7/16" | 0.4375 | 0.4331 | -0.0044 | Yes |
| 12 mm | 15/32" | 0.4688 | 0.4724 | +0.0037 | Yes |
| 13 mm | 1/2" | 0.5000 | 0.5118 | +0.0118 | Caution |
| 14 mm | 9/16" | 0.5625 | 0.5512 | -0.0113 | Caution |
| 15 mm | 19/32" | 0.5938 | 0.5906 | -0.0032 | Yes |
| 16 mm | 5/8" | 0.6250 | 0.6299 | +0.0049 | Yes |
| 17 mm | 11/16" | 0.6875 | 0.6693 | -0.0182 | Caution |
| 18 mm | 23/32" | 0.7188 | 0.7087 | -0.0101 | Caution |
| 19 mm | 3/4" | 0.7500 | 0.7480 | -0.0020 | Yes |
| 21 mm | 13/16" | 0.8125 | 0.8268 | +0.0143 | Caution |
| 22 mm | 7/8" | 0.8750 | 0.8661 | -0.0089 | Yes |
| 24 mm | 15/16" | 0.9375 | 0.9449 | +0.0074 | Yes |
| 25 mm | 1" | 1.0000 | 0.9843 | -0.0157 | Caution |
Nearest standard SAE fractional-inch size for each common metric wrench/socket. Gap under 0.010 in is generally safe to substitute; larger gaps risk rounding the fastener head.
Frequently asked questions
What is the SAE equivalent of a 10 mm wrench?
10 mm / 25.4 = 0.3937 inches. The nearest standard SAE size is 3/8" (0.3750 in), which is 0.0187 in (0.48 mm) smaller than 10 mm. For light fastening work this is usually acceptable, but a 3/8" wrench grips a 10 mm bolt loosely and can round the corners under high torque. Use the correct 10 mm wrench whenever possible.
What SAE size is closest to 13 mm?
13 mm / 25.4 = 0.5118 in. The nearest SAE size is 1/2" (0.5000 in), a gap of 0.012 in (0.30 mm). This is one of the closer metric-SAE pairs. Many mechanics use a 1/2" on 13 mm fasteners in a pinch, but 13 mm is the right tool for 13 mm hardware.
Is 3/8 the same as 10 mm?
They are close but not identical. 3/8 inch = 9.525 mm; 10 mm = 0.3937 in. The gap is about 0.475 mm or 0.019 in. Both wrenches will engage a hex fastener of either size, but the fit is loose enough to risk rounding under heavy torque. They are not the same size.
What is the metric equivalent of 1/2 inch?
1/2 inch x 25.4 = 12.7 mm exactly. No standard metric wrench is made at 12.7 mm, so the nearest metric sizes are 12 mm (0.7 mm smaller) and 13 mm (0.3 mm larger). A 13 mm metric wrench fits a 1/2" bolt head more closely than a 12 mm does.
Why do some metric sizes have no good SAE substitute?
Because 25.4 mm per inch does not divide evenly by common denominators. For example, 18 mm = 0.7087 in, which falls between 11/16" (0.6875 in, gap 0.021 in) and 23/32" (0.7188 in, gap 0.010 in). 23/32" is not included in most basic SAE socket sets, so many mechanics are stuck with 11/16" and its 0.5 mm gap - marginal for high-torque work. This is why carrying both metric and SAE sets is strongly recommended.
Does the conversion formula change for bolt threads vs. wrench size?
Yes. This calculator converts wrench-across-flat (AF) sizes, which is the dimension the wrench grips. Thread diameter (the M-number on a metric bolt, such as M10) is a different, smaller measurement. A M10 bolt (10 mm thread diameter) typically needs a 17 mm wrench, not a 10 mm one. Always check the across-flat spec, not the thread designation, when selecting a socket or wrench.