Choosing the Right Through-Hole for an M6 Fastener

By Rowan Blake — garage-shop machine builder Last reviewed: August 8, 2026 Source check: For controlled work, verify dimensions against the engineering drawing and the current official standard named by it.

Quick answer: what size is an M6 clearance hole?

Quick answer

Use a 6.6 mm drill for a typical general-purpose M6 clearance hole unless the engineering drawing or governing standard specifies another diameter or fit.

Common workshop choices are:

• 6.4 mm: close fit
• 6.6 mm: normal or medium fit
• 7.0 mm: loose or free fit

These M6 values are reproduced in metric clearance-hole tables such as The Engineer’s Bible chart.

The M6 designation represents a nominal 6.0 mm thread diameter. It does not mean the clearance hole should be 6.0 mm. A clearance hole is deliberately larger than the fastener so its shank and threads can pass through the workpiece without engaging it.

There is no single correct diameter for every M6 assembly. The choice depends on the permitted movement, the accuracy of the hole pattern, the number of parts being aligned, the required ease of assembly, and any limits imposed by the drawing.

Fit	Nominal hole diameter	Nominal radial clearance per side*	Assembly behavior	Typical selection logic
Close	6.4 mm	0.20 mm	Least play and least tolerance for misalignment	Use when accurate hole position and reduced movement justify tighter clearance
Normal or medium	6.6 mm	0.30 mm	Practical balance between positioning and easy insertion	Default for ordinary brackets, plates, covers, and general assembly
Loose or free	7.0 mm	0.50 mm	Easiest insertion and greatest alignment allowance	Use for stacked parts, field assembly, or less accurately aligned holes

The radial values are calculated from a nominal 6.0 mm fastener. The nominal sizes and close, normal, and loose classifications are reproduced in the AmesWeb M6 clearance-hole chart. They are nominal geometric values, not guaranteed clearances around every real fastener.

For a one-off bracket with no controlling drawing or special positioning requirement, 6.6 mm is the practical default. Select 6.4 mm only when reduced play is useful and the parts are accurately aligned; select 7.0 mm when easier insertion or additional alignment allowance is more important.

M6 fit-size table: 6.4 mm, 6.6 mm, and 7.0 mm

The differences between the three common sizes are small on a ruler but meaningful during assembly. They can be compared with the nominal 6.0 mm diameter represented by M6.

Nominal diametral clearance is:

Diametral clearance = hole diameter - nominal fastener diameter

Nominal radial clearance per side is half that result:

Radial clearance per side = hole diameter-6.0 mm ÷ 2

Fit description	Nominal hole	Calculated nominal diametral clearance	Calculated nominal radial clearance per side	Finished-hole range reproduced by secondary charts
Close	6.4 mm	0.4 mm	0.20 mm	6.40–6.55 mm
Normal or medium	6.6 mm	0.6 mm	0.30 mm	6.60–6.82 mm
Loose, free, or coarse	7.0 mm	1.0 mm	0.50 mm	7.00–7.36 mm

The nominal sizes and finished-hole intervals above appear in secondary charts, including the AmesWeb metric clearance-hole table. AmesWeb presents the data as derived from ASME B18.2.8, but it is not the official standard.

For example, the nominal calculation for a 6.4 mm hole is:

6.4-6.0 = 0.4 mm diametral clearance

Dividing that result by two gives:

0.4 ÷ 2 = 0.20 mm nominal radial clearance per side

Applying the same calculation produces 0.30 mm per side for a 6.6 mm hole and 0.50 mm per side for a 7.0 mm hole. These are calculated nominal values based on the listed hole diameter and a nominal 6.0 mm fastener—not measured clearances guaranteed in an assembled joint.

A nominal drill size also differs from an acceptable finished-hole interval. The first identifies a tool or target dimension. The second defines limits for the completed feature. A secondary chart attributed by its publisher to ISO 273, for example, pairs the nominal M6 choices with finished-hole ranges of 6.40–6.55 mm, 6.60–6.82 mm, and 7.00–7.36 mm for close, normal, and loose fits respectively. That third-party chart distinguishes nominal dimensions from the stated finished-hole intervals.

These drawing instructions are therefore not necessarily equivalent:

• “Drill 6.6 mm”
• “M6 normal clearance”
• “Hole diameter 6.60–6.82 mm”
• “Ø6.6”

The first may identify the intended tool. The second invokes a fit concept that must be interpreted under the governing requirements. The third states explicit finished-hole limits. The fourth provides a nominal dimension whose permitted variation may come from the drawing’s general tolerances.

For controlled production, inspection, or critical work, do not assume that a secondary chart supplies the governing tolerance. Check the drawing and the current applicable standard. The reproduced ranges help explain how some charts are structured, but they are not universal process allowances for every material or hole-making method.

How to choose between close, normal, and loose clearance

Fit selection is a tradeoff between reduced movement and easy assembly. Less clearance can reduce the amount that parts initially shift relative to the fastener, but it also leaves less room for imperfectly positioned holes. Greater clearance makes insertion easier and accommodates more alignment variation.

For related workshop context, see this guide to bolt torque and tightening sequence.

Choose a close fit when:

• Hole locations are accurately controlled.
• Reduced assembly play is useful.
• Mating parts are expected to align closely before fastener insertion.
• The actual parts can be inspected or test-assembled.
• The drawing explicitly requires close clearance.

Among the three common M6 choices, 6.4 mm is the minimum-clearance option. It may suit an accurately made cover, fixture component, or machined plate with a well-controlled hole pattern. The cost is reduced forgiveness: small positional errors can consume the available clearance and make assembly difficult. A metric clearance-hole reference describes 6.4, 6.6, and 7.0 mm as fine, medium, and coarse M6 options with progressively greater clearance.

This problem becomes more pronounced with multiple fasteners.

A clearance bolt should not automatically be treated as a precision locating pin.

Choose a normal or medium fit when:

• The joint is an ordinary workshop assembly.
• Reasonable insertion clearance is wanted without an unnecessarily large hole.
• The parts are drilled or fabricated with normal care but are not a precision locating system.
• No drawing, standard, or manufacturer instruction specifies another fit.

For these conditions, 6.6 mm is the practical general-purpose choice. It offers more alignment allowance than a close-fit hole while avoiding the additional freedom of a loose fit.

Choose a loose or free fit when:

• Several plates or components must align at once.
• Hole-position variation is likely.
• The fastener must be inserted during field or awkward assembly.
• Parts were produced in separate operations.
• Additional positioning freedom is intentional.

A 7.0 mm hole provides the greatest nominal allowance of the three common options. It can help with stacked plates, welded components, guards, and separately fabricated brackets. It cannot correct a fundamentally defective pattern, but it can accommodate moderate variation when the design permits loose clearance.

A practical decision sequence is:

1. Read the drawing first. Follow any specified diameter, limits, or fit class.
2. Confirm the feature’s function. Decide whether the fastener passes through or engages internal threads.
3. Identify the priority. Determine whether reduced movement or easier insertion matters more.
4. Select the fit. - Accurately aligned parts with a justified need for less play: consider 6.4 mm. - Routine general assembly: use 6.6 mm. - Easier insertion or greater alignment allowance: consider 7.0 mm.
5. Evaluate the complete pattern. A diameter that works for one hole may be too restrictive across several mating holes.
6. Confirm the finished assembly. Test or inspect the feature when its limits matter.

The M6 values in this sequence are common workshop recommendations rather than permission to override a controlled drawing; the same distinction is made in this clearance-hole and tap-drill guide.

Terminology varies among references. The sets close, normal, and loose; close, medium, and free; and fine, medium, and coarse describe broadly corresponding selection concepts. The labels alone do not establish that dimensions from different standards are identical. A third-party machining guide compares the ISO and ASME terminology conceptually while advising verification of critical dimensions.

Clearance hole versus M6 tap-drill hole

The numbers 5.0 mm and 6.6 mm often appear in the same drill chart because they describe different operations.

Feature	Purpose	Size relative to nominal M6 diameter	Common example
M6 clearance hole	Lets the fastener pass through without thread engagement	Larger than 6.0 mm	6.6 mm for general-purpose clearance
M6 tap-drill hole	Leaves material that a tap cuts into internal threads	Smaller than 6.0 mm	5.0 mm before tapping M6×1.0

TR Fastenings lists 5.0 mm for tapping a standard M6 metric thread and 6.6 mm for M6 clearance, describing both as recommendations in millimetres. See its tapping and clearance-hole table.

A 5.0 mm hole is not an M6 clearance hole.

The 5.0 mm drill is commonly used to prepare a hole for an M6×1.0 internal thread. A tap then cuts the thread into the material remaining around that smaller hole. A clearance hole must instead be larger than the nominal fastener diameter because the screw or bolt is meant to pass through without engaging the workpiece.

Consider a two-part assembly:

• Part A, such as a cover or bracket, receives an M6 clearance hole.
• Part B, such as a thicker mounting block, receives the appropriate tap-drill hole and is then tapped.
• The M6 screw passes through Part A and threads into Part B.

If both parts received a 5.0 mm hole, the screw would not pass freely through Part A. If both received a clearance-size hole, Part B would not provide the intended prepared feature for cutting an M6 internal thread.

Some combined charts also give 5.25 mm as an M6 fine-thread tap-drill recommendation. That remains a tap-drill value, not a clearance diameter, and must be associated with the particular thread specification being produced. Accu’s chart separates 5.00 and 5.25 mm M6 tap-drill recommendations from its single 7.0 mm clearance-hole recommendation. The Accu chart shows those three entries in separate columns.

Whenever a drawing, note, or verbal request says only “M6 hole,” ask:

Must the fastener pass through the hole, or must the hole receive internal threads?

If the fastener must pass through, select a suitable clearance fit. If the hole must receive threads, identify the complete thread specification before choosing the tap-drill preparation.

A counterbore is a third feature. It accommodates a screw head or related component and must not be confused with either the through-hole diameter or the tap-drill diameter. A head-recess dimension is not a substitute for the M6 clearance hole beneath it.

Nominal drill size is not the same as finished-hole size

A drill marked 6.6 mm has a nominal tool diameter. The marking alone does not guarantee that every completed hole will measure exactly 6.600 mm or satisfy a controlled finished-hole requirement.

A chart may provide any of the following:

• A nominal drill diameter
• A nominal hole diameter
• A permitted finished-hole interval
• A fit label without explicit limits
• A single recommended clearance diameter without a fit classification

These forms of information are not interchangeable.

For example, one secondary chart pairs a 6.6 mm nominal normal-fit drill size with a 6.60–6.82 mm finished-hole interval. The same chart provides separate nominal and finished-hole values for close and loose fits. Its M6 entries show the distinction directly.

The functional result can be affected by:

• The hole-making process
• Tool condition and setup
• Workpiece material
• Hole position and angular alignment
• Burrs or damaged edges
• Coatings applied after machining
• The number and arrangement of mating holes
• The actual fastener being installed

These factors do not mean every one-off bracket needs elaborate inspection. They mean verification should match the job.

For ordinary noncritical work, selecting the appropriate drill, deburring the hole, and checking the intended fastener may be sufficient. For a dimensionally controlled component, the drill marking is not adequate evidence of compliance. Measure the finished feature using a method appropriate to the specified limits.

Hole position matters as much as diameter in many assemblies. A close-fit hole can meet its size requirement but still fail to assemble if its center is misplaced relative to the mating feature.

Designers and builders should therefore distinguish among requirements for:

• Nominal drill selection
• Finished-hole size
• Hole position
• Relationships between multiple holes
• Surface condition after coating
• Fit in the final assembled condition

Do not automatically open every hole simply because a bolt binds. Enlarging a hole may ease insertion, but it can also alter positioning, edge distance, washer support, or another controlled feature. Resolve the specification before modifying an inspected or critical part.

What to do when charts disagree

Most of the higher-relevance secondary tables in the evidence reviewed here converge on:

• 6.4 mm close
• 6.6 mm normal or medium
• 7.0 mm loose or free

That agreement supports 6.6 mm as a practical general-purpose recommendation, but it does not establish that every chart or application must use the same dimensions.

One lower-authority commercial DIY guide instead lists 6.2 mm for close fit, while retaining 6.6 mm for normal fit and 7.0 mm for free fit. This is a genuine disagreement and should not be blended into a false consensus. A 6.2 mm hole has only 0.2 mm of calculated nominal diametral clearance relative to a nominal 6.0 mm fastener, making it tighter than the commonly reproduced 6.4 mm close-fit choice. HomeDIYer is the commercial guide publishing the differing 6.2 mm value.

Accu, meanwhile, provides a single 7.0 mm M6 clearance-hole recommendation without dividing it into close, normal, and loose classes. That does not necessarily make its table incorrect; it shows that a one-number chart may favor easier clearance rather than reduced play. Its listed 5.00 and 5.25 mm figures are separately identified as tap-drill recommendations, not through-hole sizes. Accu’s combined chart displays this one-number clearance approach.

Standards attributions also vary among secondary pages. AmesWeb associates its close, normal, and loose table with ASME B18.2.8-1999 and references the 29th and 30th editions of Machinery’s Handbook. That is a secondary presentation, not the current official ASME text.

Another third-party chart attributes similar metric dimensions to ISO 273. Because the evidence consists of secondary reproductions rather than verified current copies of the standards, it is not sufficient to conclude that current ISO and ASME requirements are dimensionally identical or interchangeable.

Use this hierarchy when values conflict:

1. The engineering drawing and the current governing standard it identifies
2. Documented customer, project, manufacturer, or inspection requirements
3. Approved internal specifications or established shop instructions
4. Generic engineering and workshop charts
5. Unattributed or one-number recommendations

The first applicable controlling requirement governs the job. Lower levels are selection aids only when they do not conflict with it.

Also check whether the sources are answering the same question. Apparent disagreement can result from differences in:

• Fit class
• Nominal drill diameter versus finished-hole limits
• Clearance hole versus tap-drill hole
• Through-hole diameter versus counterbore diameter
• Workshop preference versus a formal drawing requirement
• Metric dimensions versus approximate substitutions

Do not average conflicting recommendations. Identify the function, fit, and authority behind each number. If no controlling requirement exists, 6.6 mm remains the best-supported general-purpose choice.

A practical M6 drilling and inspection checklist

1. Confirm the feature type. Determine whether the part needs a clearance hole, tapped hole, counterbore, or combination of features. “M6 hole” is incomplete unless its function is clear.

2. Read the drawing before consulting a generic chart. Look for a specified diameter, fit class, finished-hole limits, positional tolerance, general tolerance note, and governing standard.

3. Confirm the units. Make sure dimensions are in millimetres. Do not treat an approximate inch drill as an exact substitute for a specified metric diameter.

4. Select the fit only if nothing else controls. - Use 6.6 mm for ordinary general-purpose clearance. - Use 6.4 mm when reduced play is justified and alignment is accurate. - Use 7.0 mm when easier insertion or additional alignment allowance is the priority.

These workshop selections match the close, medium, and free M6 values in the Stephens Gaskets metric clearance-hole table.

5. Do not confuse the tap drill with the clearance drill. A 5.0 mm drill is commonly associated with preparation for an M6×1.0 tapped thread, not a general-purpose M6 through-hole. TR Fastenings lists 5.0 mm for standard-thread tapping and 6.6 mm for clearance.

6. Consider the complete assembly. Check the number of parts, their thicknesses, insertion access, and whether the mating holes were made together or in separate operations.

7. Review multi-hole patterns carefully. Close clearance leaves less room for accumulated position variation. Parts that appear acceptable individually may bind when every fastener is inserted.

8. Check the intended fastener. Confirm that it is the specified M6 component and that damaged threads, contamination, or the wrong fastener are not being mistaken for an undersized hole.

9. Account for the finished condition. Burrs, damaged edges, and coatings can obstruct insertion. Evaluate the part in the condition in which it will actually be assembled.

10. Test-fit where appropriate. For ordinary workshop work, trial assembly with the intended fastener can reveal alignment problems that a diameter check alone will miss.

11. Measure when finished-hole limits control. Do not infer compliance solely from the drill marking. Inspect the completed feature using a method suited to the specified limits.

12. Stop at the limit of generic guidance. For inspected, customer-controlled, safety-relevant, or otherwise critical work, verify the current governing document rather than treating a workshop chart as the design authority.

The practical rule is simple: choose 6.6 mm for an ordinary M6 through-hole when nothing else controls, move to 6.4 mm only when reduced play and accurate alignment justify close clearance, and use 7.0 mm when easier insertion or greater alignment allowance matters. Keep 5.0 mm separate because it is commonly associated with preparation for tapping M6×1.0, not with letting an M6 fastener pass through. A workshop-oriented technical guide makes the same clearance-versus-tap distinction.

Frequently asked questions

Is 6.6 mm the standard M6 clearance-hole size?

It is better described as the common general-purpose or normal-fit size, not the only possible size. Common metric charts list 6.4 mm for close fit, 6.6 mm for normal or medium fit, and 7.0 mm for loose or free fit. If no other requirement applies, 6.6 mm is the practical default for an ordinary M6 through-hole.

Will an M6 bolt fit through a 6.0 mm hole?

Do not assume it will. M6 identifies a nominal 6.0 mm thread diameter, while a functional clearance hole must be larger so the fastener can pass without engaging or binding. Common workshop options are 6.4, 6.6, or 7.0 mm depending on the required fit.

What is the difference between a 5.0 mm M6 tap drill and a 6.6 mm clearance drill?

A 5.0 mm tap-drill hole is smaller than the nominal thread diameter so material remains for an M6×1.0 tap to cut into internal threads. A 6.6 mm clearance hole is larger than the nominal fastener diameter so the M6 screw or bolt can pass through the component. These values are listed separately in combined tapping and clearance charts.

Should I use a 6.4 mm or 7.0 mm hole when two parts are difficult to align?

Use 7.0 mm if the problem is modest position variation and the design permits loose clearance. Based on nominal geometry, it provides 0.50 mm radial clearance per side around a nominal 6.0 mm fastener, compared with 0.20 mm per side for a 6.4 mm hole.

A 6.4 mm close-fit hole is more likely to aggravate an alignment problem. Do not enlarge a controlled feature merely to conceal a defective pattern or override the drawing.

Do ISO and ASME use the same M6 clearance-hole dimensions?

Secondary references associate similar fit concepts with ISO 273 and ASME B18.2.8, but that does not prove that the current official standards are identical. Their terminology is also presented differently: fine, medium, and coarse are often compared conceptually with close, normal, and loose.

For informal workshop selection, the familiar 6.4, 6.6, and 7.0 mm choices are useful. For controlled work, consult the standard and edition named by the engineering drawing rather than assuming ISO and ASME requirements are interchangeable.