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.

End Mill Sizes

Standard end mill diameters in fractional and metric series, with shank size, cross-section area and the flute counts normally used at each size.

Data verified 2026-09-29 · based on ANSI/ASME B94.19-1997 (R2017)

Quick Answer

End mills are sized by cutting diameter, not shank diameter — and below 1/2 in the two usually differ, so a 3/8 in end mill normally has a 3/8 in shank but a 1/4 in end mill may have a 3/8 in shank for rigidity. Flute count is not a standard: it is chosen for the material, with 2 flutes for aluminium and 4 or more for steel.

Find a size

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.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ANSI/ASME B94.19 — Milling Cutters and End Mills	standard	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

Cross-checked against:

Cutter diameters checked against the fractional and metric standard series
Flute count guidance cross-checked between two tooling manufacturers' technical guides
Radius and cross-section area recomputed from the diameter at build time for every row
Six sizes held as known-value checks across the fractional and metric series

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.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "End Mill Sizes." https://refmanual.com/charts/end-mill-sizes/ (accessed 2026-09-29). Data from ANSI/ASME B94.19 — Milling Cutters and End Mills, ANSI/ASME B94.19-1997 (R2017).

BibTeX

@misc{end-mill-sizes,
  title        = {End Mill Sizes},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/end-mill-sizes/}},
  note         = {Accessed: 2026-09-29},
  source       = {ANSI/ASME B94.19 — Milling Cutters and End Mills, ANSI/ASME B94.19-1997 (R2017)}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - End Mill Sizes
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/end-mill-sizes/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ANSI/ASME B94.19 — Milling Cutters and End Mills, ANSI/ASME B94.19-1997 (R2017)
ER  -

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