SAE to Metric Conversion: Inches to Millimeters for Sockets & Wrenches
Enter any fractional or decimal inch value. Output updates as you type, with no submit button.
Standard (SAE) → Metric (mm)
- Decimal inches
- 0.7500"
- Exact mm
- 19.050mm
- Closest metric socket
- 19mm
- Difference
- 0.050mm
What Is SAE to Metric Conversion?
SAE to metric conversion changes a fractional inch size, such as 3/4 inch, into millimeters, in this case 19.05mm. The formula is direct: multiply the decimal inch value by 25.4. SAE stands for Society of Automotive Engineers, the organization whose fractional-inch standards govern fasteners and hand tools built in the United States. Most of the rest of the world, including Europe, Japan, and the United Kingdom, standardized on the metric system for fasteners decades ago.
Mechanics, machinists, and fabricators move between the two systems constantly, whether they're servicing an older American vehicle next to a modern import, or stocking a toolbox that has to cover both. The conversion math is exact. Every fractional inch size has one precise millimeter value. The practical problem is that the closest metric tool actually sold rarely lands on that exact value, so this page also shows how close each substitute really is.
This page covers the full SAE-to-metric conversion chart from 1/8 inch to 2-1/2 inch, the conversion formula with worked examples, how to turn a fraction into a decimal before converting it, the most commonly needed equivalents for automotive work, and a full FAQ addressing the sizes mechanics look up most.
SAE Fractional Inches to Millimeters Chart
This table runs from 1/8 inch to 2-1/2 inch, showing the exact millimeter value and the closest metric socket a real toolbox would stock.
| SAE Fraction | Decimal Inches | Exact mm | Closest Metric Socket |
|---|---|---|---|
| 1/8" | 0.1250" | 3.17mm | 3mm |
| 3/16" | 0.1875" | 4.76mm | 5mm |
| 1/4" | 0.2500" | 6.35mm | 6mm |
| 5/16" | 0.3125" | 7.94mm | 8mm |
| 3/8" | 0.3750" | 9.52mm | 10mm |
| 7/16" | 0.4375" | 11.11mm | 11mm |
| 1/2" | 0.5000" | 12.70mm | 13mm |
| 9/16" | 0.5625" | 14.29mm | 14mm |
| 5/8" | 0.6250" | 15.88mm | 16mm |
| 11/16" | 0.6875" | 17.46mm | 17mm |
| 3/4" | 0.7500" | 19.05mm | 19mm |
| 13/16" | 0.8125" | 20.64mm | 21mm |
| 7/8" | 0.8750" | 22.22mm | 22mm |
| 15/16" | 0.9375" | 23.81mm | 24mm |
| 1" | 1.0000" | 25.40mm | 25mm |
| 1-1/16" | 1.0625" | 26.99mm | 27mm |
| 1-1/8" | 1.1250" | 28.57mm | 29mm |
| 1-1/4" | 1.2500" | 31.75mm | 32mm |
| 1-3/8" | 1.3750" | 34.92mm | 35mm |
| 1-1/2" | 1.5000" | 38.10mm | 38mm |
| 1-5/8" | 1.6250" | 41.27mm | 41mm |
| 1-3/4" | 1.7500" | 44.45mm | 44mm |
| 1-7/8" | 1.8750" | 47.63mm | 48mm |
| 2" | 2.0000" | 50.80mm | 51mm |
| 2-1/4" | 2.2500" | 57.15mm | 57mm |
| 2-1/2" | 2.5000" | 63.50mm | 64mm |
Standard to Metric Formula
Example: 3/4 inch to mm
3/4" = 0.75 inches. 0.75 × 25.4 = 19.05mm. The closest stocked metric socket is 19mm, a difference of only 0.05mm.
Example: 5/8 inch to mm
5/8" = 0.625 inches. 0.625 × 25.4 = 15.875mm. The closest stocked metric socket is 16mm, a difference of 0.125mm.
Converting Fractional Inches to Decimal First
Most calculators, including the one above, take decimal input. Convert a fraction to decimal first by dividing the numerator by the denominator: 7/16 inch equals 7 ÷ 16, which is 0.4375 inch. A mixed number adds the whole-number part after converting the fraction: 1-1/2 inch equals 1 + 0.5, or 1.5 inch. See the fraction to decimal chart for every common tool fraction already converted.
Why SAE and Metric Sizes Never Match Exactly
SAE and metric fastener standards were built around two different base units that have no common factor at practical tool sizes. The inch divides naturally into halves, quarters, and other powers of two, which is why SAE sockets step in fractions like 1/2, 9/16, and 5/8 inch. The millimeter divides in round base-ten steps. Multiplying a clean SAE fraction by 25.4 almost always produces a decimal millimeter value, and multiplying a clean metric size by 0.03937 almost always produces a decimal inch value. Neither system was designed with the other in mind, so a conversion chart, not a simple lookup, is the only reliable way to bridge them.
Most Common SAE to Metric Equivalents
- 1/4" = 6.35mm
- 5/16" = 7.94mm
- 3/8" = 9.53mm
- 7/16" = 11.11mm
- 1/2" = 12.70mm
- 9/16" = 14.29mm
- 5/8" = 15.88mm
- 3/4" = 19.05mm
These eight sizes cover most automotive lug nuts, spark plugs, and drain plugs. Bookmark this list if a full socket conversion chart is more detail than the job needs.
SAE to Metric for Wrenches vs Sockets
The conversion math is identical whether the tool is a wrench or a socket. Both measure the distance across the flats of a hex fastener, so a 3/4 inch wrench and a 3/4 inch socket both convert to 19.05mm. The difference is in the tolerance a mechanic should accept: an open-end wrench contacts only two flats and tolerates a slightly looser fit, while a socket surrounds all six (or twelve) flats and needs a tighter match. See the wrench size chart for wrench-specific tolerance notes.
SAE to Metric Conversion FAQ
How many mm is 1 inch?
One inch equals exactly 25.4mm. The inch was redefined as precisely 25.4mm in 1959 by international agreement among the United States, United Kingdom, Canada, Australia, and other countries, which removed small rounding differences that existed between national inch standards before then.
What metric socket fits 3/4 inch?
19mm is the standard substitute for a 3/4 inch fastener. The exact equivalent is 19.05mm, a difference of only 0.002 inch, so the two sizes are highly interchangeable for both hand and impact tools.
Is 13mm equal to 1/2 inch?
No, not exactly. 1/2 inch equals 12.7mm, and 13mm is 0.012 inch larger. The fit is acceptable for general hand tool work but not ideal for impact wrenches or torque-critical fasteners.
Why don't SAE and metric sizes match exactly?
SAE and metric systems were developed independently, using different base units defined by separate standards bodies. No mathematical relationship forces whole-number inch and millimeter sizes to line up, so a conversion always lands on a decimal value rather than a clean match.
What size is 5/8 inch in mm?
5/8 inch equals 15.88mm. The closest metric socket size is 16mm, a difference of 0.005 inch, which counts as highly interchangeable.
What is 7/16 inch in mm?
7/16 inch equals 11.11mm. The closest metric size is 11mm, a difference of just 0.004 inch, making the two sizes a safe substitute for nearly any job.
Do I convert a wrench size differently than a socket size?
No. A wrench and a socket use the same hex measurement across the flats, so the SAE-to-metric conversion formula and the resulting closest size are identical for both tools.
Can I use this chart for bolt sizes too?
Yes, for the wrench or socket needed to turn a bolt head. Bolt thread diameter is a separate measurement from the head's hex size; see the bolt size chart for thread specifications.