End Mill Size Chart: Standard Metric and Inch Dimensions

Choosing an end mill by diameter alone can create problems before machining even starts.

A 6 mm cutter is not the same size as a 1/4-inch cutter. Two end mills with the same cutting diameter may also have different shank diameters, flute lengths or overall lengths.

This end mill size chart compares common inch and metric diameters and explains the dimensions that matter when selecting a tool.

For solid carbide end mills, always check the complete specification:

• Cutting diameter
• Length of cut
• Shank diameter
• Overall length
• Neck dimensions when extra reach is required

Common catalog dimensions vary by manufacturer and tool series, so use the charts below as a practical reference rather than assuming every dimension is universal.

End Mill Size Chart: Inch to Metric Conversion

Inch end mills are commonly specified by fractional diameter.

The decimal and metric values below describe the same nominal cutter diameter in different units.

These conversions are useful when comparing catalogs, CAM tool libraries or drawings created in different unit systems.

They do not mean that the nearest metric and inch cutters are interchangeable.

For example, a 1/4-inch end mill is exactly 6.35 mm, not 6 mm.

If you are selecting tools for CNC milling rather than only comparing sizes, see our solid carbide end mills for stock and application-specific options.

What Are Common Metric End Mill Sizes?

Metric end mills are specified directly by cutting diameter in millimeters.

Common catalog sizes include 1, 2, 3, 4, 6, 8, 10 and 12 mm, with intermediate and larger diameters available depending on the tool series.

The final column is only a quick comparison.

A 10 mm cutter, for example, is larger than a 3/8-inch cutter. That difference matters when machining a finished pocket, slot or internal profile.

JimmyTool’s current flat carbide end mill series includes metric cutting diameters from 1 mm through 20 mm with different shank and length combinations.

Is a Metric End Mill the Same as the Nearest Inch Size?

Usually not.

Metric and fractional tools may look close on a chart, but relatively small dimensional differences can change the finished feature.

6 mm vs 1/4 Inch

A 6 mm cutter measures:

6.000 mm = 0.2362″

A 1/4-inch cutter measures:

6.350 mm = 0.2500″

The difference is:

0.350 mm, or about 0.0138″

That is too large to ignore when machining a precision slot or pocket.

10 mm vs 3/8 Inch

A 10 mm cutter measures:

10.000 mm = 0.3937″

A 3/8-inch cutter measures:

9.525 mm = 0.3750″

The difference is:

0.475 mm, or about 0.0187″

A nearest-inch or nearest-metric comparison is therefore useful for identifying a similar tool, but not for deciding whether the tools are interchangeable.

Use the actual cutter diameter when programming:

• Finished slots
• Pocket dimensions
• Internal radii
• Profile offsets
• Finishing allowances

What Dimensions Define an End Mill?

When a catalog lists an end mill size, the cutting diameter is only one dimension.

The four dimensions most buyers should check are cutting diameter, shank diameter, length of cut and overall length.

Cutting Diameter — D

The cutting diameter is the diameter of the cutting portion of the tool.

It affects:

• Minimum feature size
• Slot width
• Pocket accessibility
• Internal corner radius
• Tool rigidity

A larger cutter is generally more rigid than a smaller cutter, so the goal is not simply to choose the smallest tool that fits.

For many operations, it is better to use the largest diameter that can machine the required geometry without interfering with the part.

Shank Diameter — d

The shank is the non-cutting section held by the collet, hydraulic chuck, shrink-fit holder or other toolholder.

The shank diameter determines holder compatibility.

It is important to understand that:

cutting diameter does not always equal shank diameter.

This is especially common with small-diameter tools.

For example, JimmyTool’s current 4-flute flat end mill series lists 1, 1.5 and 2 mm cutting diameters with 4 mm shanks. A 3 mm cutter is available with either a 3 or 4 mm shank in the listed series.

A larger shank can improve clamping compatibility and rigidity while retaining a smaller cutting diameter.

Length of Cut — LOC

Length of cut is the axial length of the usable cutting edge.

It may also be called:

• Flute length
• Cutting length
• Blade length in some catalogs

The LOC needs to reach the required cutting depth.

However, longer is not automatically better.

An unnecessarily long cutting section reduces tool stiffness and can increase:

• Deflection
• Chatter
• Wall taper
• Poor surface finish
• Edge chipping

Choose the shortest LOC that can perform the required cut.

Overall Length — OAL

Overall length is the complete tool length from the cutting tip to the end of the shank.

OAL affects reach and machine clearance.

It does not tell you how deeply the tool can cut.

For example, a 75 mm overall-length end mill with a 25 mm LOC does not have 75 mm of usable cutting edge.

Always check LOC separately.

Neck Diameter and Neck Length

Long-reach tools may include a reduced neck between the cutting edge and full-size shank.

This geometry provides clearance for:

• Deep pockets
• Mold ribs
• Steep walls
• Fixtures or part features near the cutter

A reduced neck often provides better rigidity than simply using an unusually long flute length.

Are End Mill Sizes Actually Standardized?

There are dimensional standards for certain end mill and milling cutter families, but the phrase “standard end mill size” should be used carefully.

ASME B94.19 includes definitions, sizes and tolerances for one-piece high-speed-steel milling cutters and end mills. The current ASME listing identifies B94.19-1997 (S2024) as remaining in effect.

ISO 1641-1:2016 specifies general dimensions for several cylindrical-shank end mill and slot-drill series. However, ISO explicitly states that the standard does not apply to solid hard-metal end mills and slot drills. The standard was confirmed in 2024 and remains current.

For modern solid carbide end mills, this means you should distinguish between:

common stock sizes and universal dimensions.

A 6 mm cutting diameter may be common across many carbide tool manufacturers, but its:

• LOC
• Shank diameter
• OAL
• Neck length

can differ between product series.

Always check the complete dimensional drawing before replacing one manufacturer’s tool with another.

What Are Typical Solid Carbide End Mill Dimensions?

The following table shows selected dimensions from JimmyTool’s current 4-flute flat carbide end mill series.

These are product-series dimensions, not universal standards.

This table illustrates why buyers should specify more than diameter.

If someone requests only a “5 mm end mill,” the cutting diameter is clear, but the required shank size, cutting length and overall length may still need confirmation.

How Do You Choose the Right End Mill Diameter?

Start with the smallest feature the cutter must enter.

This may be:

• A slot
• A pocket
• A groove
• An internal corner
• A narrow clearance area

Then choose the largest practical cutter that can machine that feature.

Why larger?

A larger-diameter end mill generally provides greater rigidity and can tolerate more cutting load than a smaller tool of the same design.

Once the diameter is selected, the next step is to confirm the correct cutting parameters. Our carbide end mill speeds and feeds chart explains how cutter diameter affects RPM and chip load.

What Size End Mill Should You Use for a Slot?

For a simple slot, the cutter must physically fit within the target width.

That does not mean a precision 6 mm slot should always be machined with a 6 mm end mill in one full-width pass.

The actual slot width can be influenced by:

• Cutter diameter tolerance
• Tool runout
• Tool deflection
• Machine condition
• Tool wear

For tighter slot tolerances, a smaller end mill can be used to rough the feature and finish each wall separately.

This gives the CNC programmer more control over the final width.

For example, instead of relying on a Ø6 mm cutter to produce a finished 6.000 mm slot directly, a smaller cutter can leave finishing allowance for both walls.

Full-slotting also places more load on the cutter than side milling, so diameter selection and cutting parameters need to be considered together.

What Size End Mill Should You Use for an Internal Corner?

A rotating end mill cannot produce a perfectly sharp internal corner.

The cutter radius sets the theoretical minimum internal radius.

For example:

A 6 mm diameter end mill has a 3 mm radius.

It therefore cannot produce an internal corner smaller than R3.

In practice, choosing a cutter exactly equal to the theoretical maximum diameter can also create very high engagement as the tool enters the corner.

Using a somewhat smaller cutter gives the CAM toolpath more room to maintain smoother engagement.

So when selecting an end mill for an internal radius:

Do not only ask whether the cutter physically fits. Check how the tool will engage the corner during the actual toolpath.

How Long Should the Flute Length Be?

Use enough LOC to cover the required cutting depth, but avoid excessive flute length.

If the required wall depth is 18 mm, a 20 or 22 mm LOC may make more sense than a 40 mm cutting length when both can reach the feature.

The shorter tool generally offers better stiffness.

This becomes particularly important when machining:

• Hardened steel
• Stainless steel
• Titanium
• Thin walls
• Deep mold cavities

When a feature is deep but the cutting edge only needs to machine a short section, a long-neck or reduced-neck end mill may be the better solution.

Stub, Standard or Long-Reach End Mill?

Length style should match the actual access requirement.

The most rigid option that can reach the feature is usually the better starting point.

Do not select a long-reach cutter simply because it provides extra clearance “just in case.”

Unnecessary reach can reduce process stability.

When Do You Need a Custom End Mill Size?

Stock end mill sizes cover many common machining applications.

Custom dimensions become useful when the required combination of diameter, reach and shank cannot be found in a catalog.

Typical examples include:

• Non-standard cutting diameter
• Special precision slot width
• Larger shank with a smaller cutter diameter
• Shorter LOC for better rigidity
• Extended reduced neck
• Special overall length
• Tight diameter tolerance
• Custom corner radius

For example, a customer may need:

Ø6.2 mm cutting diameter × 18 mm LOC × Ø8 mm shank × 75 mm OAL

Even though every individual dimension looks ordinary, the complete combination may not be available as a stock tool.

A custom carbide end mill can be designed around the required feature instead of forcing the part or toolpath to fit a catalog dimension.

Need a Size That Is Not in the Chart?

Send JimmyTool:

• Cutting diameter
• Flute length
• Shank diameter
• Overall length
• Workpiece material
• Required quantity

We can check whether a stock size fits the application or review a custom end mill specification.

What Information Should You Send When Ordering an End Mill?

A clear dimensional specification reduces back-and-forth communication and makes it easier to determine whether a standard tool is suitable.

For a stock tool, provide:

• Tool type
• Cutting diameter
• LOC
• Shank diameter
• OAL if reach is important
• Flute count
• Corner style
• Workpiece material
• Quantity

For a custom tool, also provide any required:

• Neck diameter and neck length
• Diameter tolerance
• Corner radius
• Coating
• Tool drawing
• Part drawing or application sketch

JimmyTool’s inquiry page supports drawing and specification-file uploads for standard and custom tooling requests.

Frequently Asked Questions

What are the most common end mill sizes?

Common inch diameters include:

1/8″, 1/4″, 3/8″ and 1/2″.

Common metric diameters include:

3, 4, 6, 8, 10 and 12 mm.

However, available diameters and length combinations vary between manufacturers and product series.

Is a 6 mm end mill the same as a 1/4-inch end mill?

No.

A 6 mm end mill is 0.2362 inch in diameter.

A 1/4-inch end mill is 0.2500 inch, or 6.35 mm.

The 0.35 mm difference matters in precision slots, pockets and profiles.

Is shank diameter always the same as cutter diameter?

No.

Small and special-purpose end mills often use a shank larger than the cutting diameter.

Always check both dimensions before selecting a collet or toolholder.

What does LOC mean on an end mill?

LOC means Length of Cut.

It describes the axial length of the cutting edge or flute that can engage the workpiece.

What does OAL mean?

OAL means Overall Length.

It is the complete physical length of the end mill.

OAL determines reach but should not be confused with cutting depth.

Should I choose metric or inch end mills?

Use the unit system that best matches:

• The part drawing
• Required feature dimensions
• Existing tool library
• Available toolholders
• Production tolerances

Do not replace a metric cutter with the nearest fractional size without checking the dimensional difference.

Conclusion

An end mill size is more than its cutting diameter.

The diameter determines which features the tool can enter. The length of cut determines usable cutting depth. The shank diameter must match the holder, while overall length determines how far the tool can reach.

Metric and inch sizes that appear close on a conversion chart are not necessarily interchangeable.

For standard CNC machining, select the largest practical diameter and the shortest practical length that can complete the feature.

When a stock combination cannot provide the required diameter, reach, rigidity or tolerance, a custom tool may be the better option.

Need a Standard or Custom Carbide End Mill?

Send JimmyTool your required:

• Cutting diameter
• Flute length
• Shank diameter
• Overall length
• Workpiece material
• Quantity

We can help match your application with a standard solid carbide end mill or review a custom end mill when a catalog size does not fit.