End Mill Size Chart: Metrico & Inch Sizes Explained

An end mill size chart helps you compare common metric, fractional inch, and decimal inch cutter diameters before choosing a tool. Cutting diameter is usually the first size to check, but it is not the only dimension that matters.

Diametro del gambo, length of cut, lunghezza complessiva, and tool reach may also affect whether a cutter fits the holder and can reach the required feature.

A common example is 6 mm versus 1/4 inch:

6 mm is not the same as 1/4 inch.
6 mm = approximately 0.2362″
1/4″ = 0.2500″ = exactly 6.35 millimetro

If a drawing, larghezza della fessura, or finished dimension depends on cutter diameter, that 0.35 mm difference may matter.

End Mill Size Chart at a Glance

The end mill size chart below covers commonly referenced metric and inch diameters. It is designed for quick reference rather than as a complete list of every available standard or custom size.

Actual shank diameter, LOC, OAL, neck geometry, and other dimensions vary by tool series.

Common Metric End Mill Sizes

This table is useful when a drawing, CAM program, or purchase specification is given in millimeters.

These are common reference diameters rather than a complete inventory list. Smaller, intermediate, larger, and non-standard dimensions may also be available depending on the tool series.

Common Inch and Fractional End Mill Sizes

Fractional inch sizes remain common on US drawings and tooling specifications. Decimal values are useful when the size must be entered into CAM software or compared with metric dimensions.

This table covers frequently referenced fractional sizes. It does not mean that these are the only inch end mill sizes available.

Small and Micro End Mill Sizes

Small-diameter tools need more attention than a simple diameter conversion.

In questa guida, tools below 1 mm are treated as micro end mills for reference rather than as an absolute industry definition. For these cutters, neck diameter, portata, dimensione del gambo, toolholding, esaurire, and machine stability can become especially important.

In our end mill catalog, micro-diameter and deep-groove tools use additional dimensional fields such as Reach and Neck diameter. This is a good example of why cutting diameter alone may not fully describe a small cutter.

Metric to Inch End Mill Size Chart and Conversion

Use the end mill size chart above to identify the nominal diameter first, then check the exact conversion before choosing a nearby metric or inch tool.

Metric and inch dimensions can be converted accurately because:

1 inch = 25.4 millimetro

The calculation is simple. The more important question is whether a nearby commercial cutter can actually replace the specified size.

How to Read Metric, Fractional, and Decimal Sizes

Take a 1/4″ end mill as an example.

First convert the fraction to decimal inches:

1/4″ = 0.2500″

Then convert inches to millimeters:

0.2500 × 25.4 = 6.35 millimetro

UN 6 mm cutter converts in the opposite direction:

6 ÷ 25.4 = approximately 0.2362″

The numbers are close, but the tools do not have the same cutting diameter.

Nearby Metric and Inch Sizes: Do Not Assume They Are Equivalent

The following pairs are often compared because the dimensions are relatively close. The fractional values shown are common reference sizes from this chart, not the mathematically nearest fractional size among every inch tool available on the market.

*Positive means the fractional cutter is larger than the metric cutter. Negative means it is smaller.

This table shows nominal diameter differences only. Actual machining results are also affected by tool compensation, esaurire, deflection, usura degli utensili, e percorso utensile programmato.

If the cutter diameter changes, update the tool definition and review the part tolerance before machining.

Why 6 mm Is Not the Same as 1/4 Inch

UN 6 mm end mill has a nominal cutting diameter of 6.000 millimetro. A 1/4″ cutter measures 6.350 millimetro.

The difference is 0.350 millimetro.

Imagine a slot that is programmed around a 6 mm cutter. If a 1/4″ tool is installed without updating the tool definition or machining strategy, the larger cutter can remove additional material.

For rough machining, a different available cutter may sometimes be evaluated by changing the toolpath and leaving suitable stock for later operations. For a finished slot, wall, fit, or tolerance-controlled feature, the required dimension must be reviewed first.


What Does End Mill Size Mean?

“End mill size” usually refers first to cutting diameter. A complete tool specification includes several dimensions, though.

Across our end mill catalog, dimensional tables use fields such as Cut D or R, LOC, Gamba, and OAL. Micro-diameter and long-reach series may also include Reach and Neck dimensions.

Diametro di taglio (D)

Cutting diameter is the nominal diameter defined by the cutter’s cutting edges as the tool rotates.

Per esempio, a D 6 end mill has a nominal cutting diameter of 6 millimetro.

This dimension affects slot width, access to small features, internal corner radius, rigidità, and material removal capability. It is normally the first dimension checked against the part drawing.

Diametro del gambo (d)

The shank is the section clamped by the collet or toolholder.

UN 6 mm cutting diameter does not guarantee a 6 mm shank on every tool. Necked, long-reach, miniature, and special cutters may use different cutting and shank diameters.

Check the actual shank specification before selecting the holder.

Length of Cut (LOC)

LOC is the axial length of the cutting-edge section.

The flute length needs to cover the required machining depth with suitable clearance. Choosing much more flute length than necessary may reduce tool rigidity.

LOC should not be confused with the recommended axial depth of cut in one pass. Machining depth depends on the cutter design, materiale del pezzo, percorso utensile, macchina, titolare, e condizioni di taglio.

For more information about flute structure, vedi il nostro end mill flute design and selection guide.

Lunghezza totale (OAL) vs Tool Overhang

OAL is the complete physical length of the cutter.

Sporgenza dell'utensile, sometimes called stickout, is different. It is the length of the tool extending from the holder after setup.

Per esempio, two cutters can have the same OAL, but the operator may set different overhangs for each tool. Keeping unnecessary overhang short improves rigidity and helps reduce deflection and chatter.

Reach and Neck Diameter for Deep or Narrow Features

Some necked, deep-groove, and long-reach tool series include reach and neck dimensions.

Check the product drawing or specification table for the exact meaning, because manufacturers and tool series may label reach-related dimensions differently.

In practical terms, these fields help determine whether a cutter can access a deep or narrow area without the larger shank interfering with nearby walls.

Reach, LOC, OAL, and actual tool overhang are separate dimensions and should not be treated as interchangeable.

How to Read End Mill Dimensions

Consider a specification written as:

D 6 × LOC 15 × Shank 6 × OAL 50

It means:

This example makes the main dimensions easy to identify.

Do not rely only on the order of four numbers, though. Manufacturers may arrange dimensions differently. Read the labeled drawing or specification fields before ordering or programming the cutter.

Can Metric and Inch End Mills Be Interchanged?

An end mill size chart can help identify nearby metric and inch cutters, but a nearby size is not automatically a direct replacement.

Three checks matter most: diametro di taglio, shank compatibility, and part tolerance.

Match Cutting Diameter to the Part Feature

For a slot, cutter diameter is closely related to the required width.

For a pocket, a larger cutter can provide useful rigidity and material removal, but it still has to enter every required area.

Internal corners impose another limit. Because an end mill leaves an internal radius, the cutter must be small enough to create the required corner geometry.

Fine details may require a smaller diameter. Allo stesso tempo, smaller cutters are more sensitive to deflection, esaurire, and unstable setups.

Match the Actual Shank to the Collet or Toolholder

Do not decide holder compatibility only by calling a tool “metric” or “inch.”

Invece, confrontare:

• Actual shank diameter
• Collet or holder nominal size
• Manufacturer-specified clamping range
• Required clamping length
• Tool condition and holder condition

Per esempio, a 1/4″ shank measures 6.35 millimetro. UN 6 mm collet should not be assumed to clamp it safely unless the collet manufacturer specifically allows that diameter.

Check Tolerance Before Replacing a Nearby Size

A small diameter difference can still affect a finished feature.

If a different cutter is being considered, check the drawing tolerance, remaining stock, finishing allowance, tool compensation, e percorso utensile programmato.

For a controlled final dimension, update the tool information rather than treating a nearby metric or inch size as an invisible substitution.

How to Choose the Right End Mill Size

The chart narrows the possible diameters. The part geometry then determines which size can actually be used.

Choose a Diameter That Fits the Feature

For a slot, begin with the target slot width.

For a pocket, choose a cutter that can enter the narrowest required region. A larger tool may be more rigid, while a smaller tool provides better access to detailed geometry.

Internal corner requirements also limit cutter diameter because the tool creates a radius rather than a perfectly sharp inside corner.

The best diameter is not simply the largest tool that fits the machine. It must fit the part.

Use the Shortest Practical LOC

Choose an LOC that covers the required cutting depth with reasonable margin instead of automatically selecting the longest flute option.

A shorter cutting section generally provides a more rigid tool structure.

Deep cavities may require a necked or long-reach design. In tal caso, review the complete geometry rather than simply choosing a standard cutter with very long flutes.

Minimize Unnecessary Tool Overhang

Keep the actual overhang as short as practical.

Extra extension can increase:

• Deflessione dell'utensile
• Chatter
• Variazione dimensionale
• Finitura superficiale scadente
• Tool breakage risk

There is no single overhang-to-diameter rule that applies to every job. Materiale del pezzo, geometria della fresa, holder rigidity, impegno dello strumento, and machine condition all affect the result.

Confirm Shank and Holder Compatibility

Before loading the tool, confirm that the holder can clamp the actual shank correctly.

Once the basic dimensions are confirmed, materiale, conteggio dei flauti, rivestimento, and cutter geometry can be selected separately.

For the complete selection process, Leggere how to choose the right end mill.

After Confirming the Size: Choose the Tool Type

Diameter and length do not define the complete cutter. The end profile also needs to match the machining operation.

This is only a quick next-step reference. For a fuller comparison, Vedere end mill cutter types.

Standard vs Custom End Mill Sizes

Common standard end mill sizes in metric and inch systems cover many CNC applications, but special parts often require dimensions outside standard catalog combinations.

Custom requirements may include:

• Non-standard cutting diameter
• Different shank diameter
• Special LOC
• Extended or shortened OAL
• Necked or long-reach geometry
• Special end profile
• Drawing-defined dimensions

Our catalog includes square, naso a palla, raggio d'angolo, micro-diameter, alluminio, hardened-steel, rame, grafite, and other application-specific end mill series. Their dimensional combinations vary because the required geometry changes with the tool type and application.

For standard and drawing-based tooling, visualizza il nostro standard and custom end mill cutters.

Information for a Custom End Mill Inquiry

If available, send:

• Diametro di taglio (D)
• Diametro del gambo (d)
• Length of Cut (LOC)
• Lunghezza totale (OAL)
• Tool type or end profile
• Materiale del pezzo
• Drawing and tolerance requirement
• Requisito del rivestimento, if specified
• Quantità

You do not need to know every specification before contacting us. Send the drawing or information you already have, and the remaining details can be reviewed during the quotation process.

FAQ About End Mill Sizes

What are the most common end mill sizes?

Common sizes depend on the tooling system, market, macchina, e applicazione. The end mill size chart above includes frequently referenced metric diameters such as 3, 4, 6, 8, 10, E 12 millimetro, as well as fractional sizes such as 1/8″, 1/4″, 3/8″, and 1/2″. Many intermediate, miniature, larger, and custom sizes are also available.

What is the difference between a 6 mm and a 1/4 inch end mill?

UN 6 mm end mill measures 6.000 millimetro. A 1/4″ end mill measures 6.350 millimetro. The nominal diameter difference is 0.350 millimetro, so they should not automatically be treated as the same cutter.

Does end mill size mean cutting diameter or shank diameter?

The quoted size usually refers first to cutting diameter. Shank diameter is a separate specification and can differ from the cutting diameter on some tool designs.

Can I use an inch end mill with a metric-sized collet?

Check the actual shank diameter against the collet manufacturer’s permitted clamping range. Do not rely only on whether the cutter is described as metric or inch.

How do I convert fractional end mill sizes to millimeters?

Convert the fraction to decimal inches and multiply by 25.4. Per esempio, 1/4″ equals 0.2500″, E 0.2500 × 25.4 = 6.35 millimetro.

How do I choose flute length and overall length?

Select enough LOC and reach to access the required feature while keeping the cutter and actual overhang as short as practical. Extra length should be added only when the part geometry requires it.

Is a larger end mill always better?

NO. A larger cutter can offer useful rigidity and material-removal capacity, but it may not fit narrow slots, small pockets, dettagli raffinati, or tight internal corners. The part feature determines the usable diameter.

Can you manufacture a non-standard end mill size?

Non-standard cutting diameters, stinchi, LOC, OAL, neck geometry, end profiles, rivestimenti, and other drawing-defined requirements can be evaluated according to the application.

Need a Standard or Custom End Mill Size?

If you already know the required diameter and dimensions, send us the specification for review.

For a non-standard cutter, you can also send a drawing, elenco taglie, sample information, o requisito di lavorazione. We will review the available information and confirm the dimensions needed for quotation.

Explore standard and custom end mill cutters

E-mail: sales@cutterbest.com