Sources and scope for this page: ANSI/ASME B94.19-1997 (R2017), covering the standard cutter diameters and shank sizes; and ISO 1641-1:2016, covering the metric end mill diameter series (one more source is tabulated below). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. Full sources, values and limits on this page.
Page last changed 2026-11-08 · based on ANSI/ASME B94.19-1997 (R2017)
Quick Answer
End Mill Diameters and Shank Sizes
| Cutter diameter[1] | Diameter in[2] | Diameter mm[2] | Common flute counts[1] | Typical use[1] | Radius in[2] | Cross-section area in²[2] |
|---|---|---|---|---|---|---|
| 1/32 # | 0.0313 | 0.795 | 2 | Fine detail and slotting in small work | 0.0157 | 0.0008 |
| 1/16 # | 0.0625 | 1.587 | 2 or 4 | Small slots and detail; 2-flute for aluminium | 0.0312 | 0.0031 |
| 3/32 # | 0.0938 | 2.383 | 2 or 4 | Small pockets and profiles | 0.0469 | 0.0069 |
| 1/8 # | 0.1250 | 3.175 | 2, 3 or 4 | The most-used small end mill — aluminium 2 or 3 flute, steel 4 | 0.0625 | 0.0123 |
| 3/16 # | 0.1875 | 4.762 | 2, 3 or 4 | General small work | 0.0938 | 0.0276 |
| 1/4 # | 0.2500 | 6.350 | 2, 3 or 4 | The default general-purpose size | 0.1250 | 0.0491 |
| 5/16 # | 0.3125 | 7.938 | 3 or 4 | General milling | 0.1562 | 0.0767 |
| 3/8 # | 0.3750 | 9.525 | 3 or 4 | Heavier general milling and roughing | 0.1875 | 0.1104 |
| 1/2 # | 0.5000 | 12.700 | 4 or 5 | The workhorse size for roughing and finishing | 0.2500 | 0.1963 |
| 5/8 # | 0.6250 | 15.875 | 4 or 6 | Heavy roughing | 0.3125 | 0.3068 |
| 3/4 # | 0.7500 | 19.050 | 4 or 6 | Heavy roughing and face milling | 0.3750 | 0.4418 |
| 1 # | 1.0000 | 25.400 | 4 to 8 | Large roughing; often indexable rather than solid | 0.5000 | 0.7854 |
| 3 mm # | 0.1181 | 3 | 2, 3 or 4 | Metric standard size | 0.0591 | 7.0686 |
| 4 mm # | 0.1575 | 4 | 2, 3 or 4 | Metric standard size | 0.0787 | 12.5664 |
| 5 mm # | 0.1969 | 5 | 2, 3 or 4 | Metric standard size | 0.0984 | 19.6350 |
| 6 mm # | 0.2362 | 6 | 2, 3 or 4 | Metric standard size | 0.1181 | 28.2743 |
| 8 mm # | 0.3150 | 8 | 3 or 4 | Metric standard size | 0.1575 | 50.2655 |
| 10 mm # | 0.3937 | 10 | 3 or 4 | Metric standard size | 0.1969 | 78.5398 |
| 12 mm # | 0.4724 | 12 | 3 or 4 | Metric standard size | 0.2362 | 113.0973 |
| 16 mm # | 0.6299 | 16 | 4 or 6 | Metric standard size | 0.3150 | 201.0619 |
| 20 mm # | 0.7874 | 20 | 4 or 6 | Metric standard size | 0.3937 | 314.1593 |
| 25 mm # | 0.9843 | 25 | 4 or 6 | Metric standard size | 0.4921 | 490.8739 |
Flute count is a choice, not a specification. Two flutes give the largest chip room and are used for aluminium and for slotting, where the chip has to clear a full-width cut. Four or more flutes give a stronger core, a better finish and higher feed rates in steel, but less chip clearance. Three-flute cutters sit between and are popular for aluminium in production. Variable-helix and variable-index cutters reduce chatter and are worth their cost on difficult setups.
The radius and area columns are computed from the diameter and are useful for chip-load and deflection calculations — a 1/8 in cutter has only 0.0123 in² of cross-section, which is why small end mills deflect so readily and why tool overhang matters so much at small diameters.
Choosing Flute Count by Material
The rule that matters most: fewer flutes for softer, gummier materials; more flutes for harder ones.
Aluminium needs two or three flutes. The material cuts easily but produces a large, soft chip that must be evacuated — with four flutes in a full-width slot the chip has nowhere to go, the cutter recuts it, and the result is built-up edge and a poor finish. Three-flute cutters have become the production standard for aluminium because they combine adequate chip room with a stronger core and a higher feed rate than two flutes.
Steel takes four or more. The chip is smaller and harder, the cutter needs a stronger core to resist deflection, and more flutes mean a higher feed rate for the same chip load per tooth. Five, six and eight-flute cutters are used for finishing and for high-productivity roughing in harder materials.
Slotting is the case that overrides the rule: a full-width slot in any material leaves the chip nowhere to go, so flute count is reduced — or the slot is roughed with a smaller cutter and finished with a full-width pass.
Shank Diameter and Tool Deflection
Deflection, not strength, is what limits an end mill. A cutter is a cantilever, and its deflection goes with the fourth power of diameter and the cube of overhang. Halving the diameter makes it sixteen times as flexible; doubling the overhang makes it eight times as flexible.
That is why small cutters are often made with a larger shank — a 1/4 in cutting diameter on a 3/8 in shank — and why the shank should be gripped as deeply as the work allows. It is also why the practical depth of cut is limited: a cutter can only reach about one diameter deep before deflection makes the cut inaccurate, and long-reach cutters need reduced feeds and depths to compensate.
Where a deep pocket is needed in a small size, the usual approach is a smaller cutter with a necked shank rather than a long fluted length — the neck is relieved to just below the cutting diameter, which reduces deflection compared with a full-length fluted cutter of the same reach.
Frequently Asked Questions
- How many flutes should an end mill have?
- Two or three for aluminium, to give the large soft chip somewhere to go, and four or more for steel, for a stronger core and a higher feed rate. Three-flute cutters are the production compromise for aluminium. For full-width slotting, reduce the flute count regardless of material so the chip can clear.
- Is the shank diameter the same as the cutter diameter?
- Often, but not always. Below about 1/2 in, small cutters are frequently made with a larger shank for rigidity — a 1/4 in cutter on a 3/8 in shank is common. Check both figures when ordering, and grip the shank as deeply as the holder and the work allow.
- What is the most common end mill size?
- 1/2 in for general work, with 1/4 in and 1/8 in as the common small sizes. In metric practice, 6, 8, 10 and 12 mm cover most work. The 1/2 in four-flute carbide cutter is the default for steel and the 1/2 in three-flute for aluminium.
- Why do small end mills break so easily?
- Because they are flexible rather than weak. A 1/8 in cutter has a cross-section of only 0.0123 in², so it deflects readily, and deflection leads to chatter, then to a suddenly increased chip load, then to fracture. Keep overhang short, reduce the feed per tooth, and use a cutter with a necked shank for deep reaches.
- What is a roughing end mill?
- A cutter with a sinusoidal or serrated edge profile that breaks the chip into small pieces, reducing cutting force and allowing much higher material removal rates. It leaves a scalloped finish, so a roughing cutter is normally followed by a finishing pass with a conventional cutter.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | ANSI/ASME B94.19 — Milling Cutters and End Mills | reference | ANSI/ASME B94.19-1997 (R2017) |
| [2] | Value computed from the standard's defining relationship | derived | Computed at build time from the defining formula stated on the page, then verified against every row and anchored by known standard values. |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| ANSI/ASME B94.19 — Milling Cutters and End Mills | ANSI/ASME B94.19-1997 (R2017) | the standard cutter diameters and shank sizes |
| ISO 1641 — End mills and slot drills | ISO 1641-1:2016 | the metric end mill diameter series |
| Tooling manufacturers' technical guides | catalogues as published 2024–2026 | the flute counts and applications |
Verification scope
No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| Diameter in mm, radius and cross-section area | Millimetres = inches × 25.4; radius = diameter ÷ 2; area = π·d²/4. All recomputed at build time for every row. |
Cutting diameters are standard; flute counts are guidance, not a specification, and vary by manufacturer and by the specific cutter geometry. Shank diameter should be confirmed on the specific tool, since small cutters are often supplied on a larger shank than their cutting diameter.
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