End Mill Size Chart
The diameters you'll actually find on a shelf or in a catalog, fractional and metric, side by side.
Fractional (in)
| Fraction | Decimal (in) |
|---|---|
| 1/16" | 0.0625 |
| 3/32" | 0.0938 |
| 1/8" | 0.1250 |
| 5/32" | 0.1563 |
| 3/16" | 0.1875 |
| 7/32" | 0.2188 |
| 1/4" | 0.2500 |
| 5/16" | 0.3125 |
| 3/8" | 0.3750 |
| 7/16" | 0.4375 |
| 1/2" | 0.5000 |
| 9/16" | 0.5625 |
| 5/8" | 0.6250 |
| 3/4" | 0.7500 |
| 7/8" | 0.8750 |
| 1" | 1.0000 |
Metric (mm)
| Size | Decimal (in) |
|---|---|
| 1 mm | 0.0394 |
| 1.5 mm | 0.0591 |
| 2 mm | 0.0787 |
| 2.5 mm | 0.0984 |
| 3 mm | 0.1181 |
| 4 mm | 0.1575 |
| 5 mm | 0.1969 |
| 6 mm | 0.2362 |
| 7 mm | 0.2756 |
| 8 mm | 0.3150 |
| 9 mm | 0.3543 |
| 10 mm | 0.3937 |
| 12 mm | 0.4724 |
| 14 mm | 0.5512 |
| 16 mm | 0.6299 |
| 18 mm | 0.7087 |
| 20 mm | 0.7874 |
| 25 mm | 0.9843 |
Decimal inch equivalents are exact conversions (÷ 25.4), shown for comparing against an inch-based fixture or existing tooling.
Why these particular sizes
These are the diameters nearly every tool manufacturer stocks as standard catalog items — anything off this list (a 0.157" or a 13mm end mill, for instance) usually has to be special-ordered or ground custom, at a real cost and lead time penalty. When a design allows any reasonable tool diameter, picking one from this list keeps sourcing simple.
Fractional and metric aren't interchangeable by "closest size"
A 1/4" (0.250") end mill and a 6mm (0.2362") end mill are close but not the same — substituting one for the other shifts a pocket or slot width by a few thousandths, which matters on a tolerance-critical feature even if it looks negligible on paper. Match the tool system called out on the print rather than assuming the nearest catalog size in the other system is equivalent.
Smaller than 1/16" or 1mm gets specialized fast
Micro end mills below these sizes exist, but they come with much lower rigidity, tighter runout requirements, and dramatically less tolerance for a bad feed or speed choice — treat anything under 1/16" (or 1mm) as its own category requiring extra care, not just a smaller version of the sizes above.