Metric vs Imperial Size Chart
Metric and imperial (SAE) measurements are used side by side in hardware stores, garages, and workshops around the world. This conversion chart shows the most common size equivalents for drill bits, sockets, and wrenches so you can quickly find the closest match when switching between systems.
Common Size Equivalents
| Imperial | Closest Metric | Decimal (in) | Decimal (mm) |
|---|---|---|---|
| 1/16" | 1.5 mm | 0.0625" | 1.5875 mm |
| 5/64" | 2 mm | 0.0781" | 1.9844 mm |
| 3/32" | 2.5 mm | 0.0938" | 2.3813 mm |
| 1/8" | 3 mm | 0.1250" | 3.1750 mm |
| 5/32" | 4 mm | 0.1563" | 3.9688 mm |
| 3/16" | 5 mm | 0.1875" | 4.7625 mm |
| 1/4" | 6 mm | 0.2500" | 6.3500 mm |
| 5/16" | 8 mm | 0.3125" | 7.9375 mm |
| 3/8" | 10 mm | 0.3750" | 9.5250 mm |
| 7/16" | 11 mm | 0.4375" | 11.1125 mm |
| 1/2" | 13 mm | 0.5000" | 12.7000 mm |
| 9/16" | 14 mm | 0.5625" | 14.2875 mm |
| 5/8" | 16 mm | 0.6250" | 15.8750 mm |
| 3/4" | 19 mm | 0.7500" | 19.0500 mm |
| 7/8" | 22 mm | 0.8750" | 22.2250 mm |
| 1" | 25 mm | 1.0000" | 25.4000 mm |
Important Notes
Metric and imperial sizes are close but not exact equivalents. A 10 mm wrench is slightly smaller than a 3/8-inch wrench (9.525 mm), which means they should not be used interchangeably to avoid rounding bolt heads.
When converting, multiply inches by 25.4 to get millimeters, or divide millimeters by 25.4 to get inches.
In automotive work, Japanese, Korean, and European vehicles use metric fasteners, while older American vehicles use SAE (imperial). Many modern American vehicles use a mix of both.
When purchasing drill bits or socket sets, investing in both metric and SAE sets ensures you always have the right size and avoids damaging fasteners.
Quick Conversion Formulas
Inches to Millimeters
Multiply inches by 25.4
1 inch = 25.4 mm
Millimeters to Inches
Divide millimeters by 25.4
1 mm = 0.03937 inches
Fraction to Decimal
Divide the numerator by the denominator
3/8 = 3 / 8 = 0.375"
Close Enough?
Metric and SAE sizes are close but not identical. Never substitute one for the other on sockets and wrenches. A slightly loose fit will round off bolt heads. For drill bits, a 0.1 mm difference is usually acceptable.