Standard drill bit sizes look simple until a drawing combines metric, fractional, number, and letter systems. A small conversion error can reduce thread engagement, create the wrong clearance, push a finished hole outside tolerance, or disrupt assembly. An unsuitable diameter may also require reaming, boring, or additional tool changes during CNC machining.
This guide covers standard drill bit size charts, conversion methods, drilling DFM limits, and common sizing mistakes. You can use it to compare available diameters, select a practical drill size, decide when finishing is needed, and evaluate whether a hole can be drilled efficiently.
What Are Standard Drill Bit Sizes in CNC Machining?
Standard drill bit sizes are commonly available nominal cutting diameters used for CNC drilling and other holemaking operations. They give engineers, machinists, and programmers a consistent way to match drawing dimensions with available tools. Instead of specifying a unique drill for every hole, a drawing can use a recognized diameter that is easier to source, replace, and include in a CNC tool library.
The four main sizing systems are metric, fractional-inch, number, and letter sizes. Metric drills use millimeters, fractional drills use inch fractions, number drills range from #80 to #1, and letter drills range from A to Z. Each system identifies available diameters in a different format. For example, a 6 mm drill uses a metric size, while a 1/4-inch drill uses a fractional-inch size and equals 6.35 mm.
Standard Drill Bit Size Charts
Standard drill bit size charts group available diameters by the system used to identify them. The tables below cover metric, fractional-inch, number, and letter sizes in the same format, helping you compare nominal drill diameters before checking conversions or planning the machining process.
Metric Drill Bit Sizes
Metric drill bit sizes state the nominal cutting diameter directly in millimeters. They appear widely on international CNC drawings and are available in both whole-millimeter and smaller incremental sizes. The table below lists commonly used metric diameters.
| Drill Size (mm) | Diameter (in) |
| 1.0 | 0.0394 |
| 1.5 | 0.0591 |
| 2.0 | 0.0787 |
| 2.5 | 0.0984 |
| 3.0 | 0.1181 |
| 3.5 | 0.1378 |
| 4.0 | 0.1575 |
| 4.5 | 0.1772 |
| 5.0 | 0.1969 |
| 5.5 | 0.2165 |
| 6.0 | 0.2362 |
| 6.5 | 0.2559 |
| 7.0 | 0.2756 |
| 8.0 | 0.3150 |
| 8.5 | 0.3346 |
| 9.0 | 0.3543 |
| 10.0 | 0.3937 |
| 11.0 | 0.4331 |
| 12.0 | 0.4724 |
| 13.0 | 0.5118 |
Fractional Inch Drill Bit Sizes
Fractional-inch drill sizes express the nominal diameter as a fraction of one inch. Because fractions with different denominators can be difficult to compare at a glance, the decimal-inch and millimeter columns show the actual size of each drill more clearly. This chart focuses on fractional sizes commonly encountered in CNC machining.
| Drill Size | Diameter (in) | Diameter (mm) |
| 1/16 in | 0.0625 | 1.5875 |
| 5/64 in | 0.0781 | 1.9844 |
| 3/32 in | 0.0938 | 2.3813 |
| 7/64 in | 0.1094 | 2.7781 |
| 1/8 in | 0.1250 | 3.1750 |
| 9/64 in | 0.1406 | 3.5719 |
| 5/32 in | 0.1563 | 3.9688 |
| 3/16 in | 0.1875 | 4.7625 |
| 13/64 in | 0.2031 | 5.1594 |
| 7/32 in | 0.2188 | 5.5563 |
| 15/64 in | 0.2344 | 5.9531 |
| 1/4 in | 0.2500 | 6.3500 |
| 9/32 in | 0.2813 | 7.1438 |
| 5/16 in | 0.3125 | 7.9375 |
| 3/8 in | 0.3750 | 9.5250 |
| 7/16 in | 0.4375 | 11.1125 |
| 1/2 in | 0.5000 | 12.7000 |
Number Drill Bit Sizes
Number drill bit sizes, also called wire gauge drill sizes, cover small inch-based diameters identified from #80 to #1. The numbering runs in reverse, so the drill diameter increases as the number becomes smaller. These sizes are often used for small holes, thread preparation, and diameters that fall between common fractional-inch drills. The table shows commonly referenced number drill sizes.
| Drill Size | Diameter (in) | Diameter (mm) |
| #60 | 0.0400 | 1.0160 |
| #56 | 0.0465 | 1.1811 |
| #53 | 0.0595 | 1.5113 |
| #50 | 0.0700 | 1.7780 |
| #47 | 0.0785 | 1.9939 |
| #43 | 0.0890 | 2.2606 |
| #40 | 0.0980 | 2.4892 |
| #36 | 0.1065 | 2.7051 |
| #31 | 0.1200 | 3.0480 |
| #29 | 0.1360 | 3.4544 |
| #27 | 0.1440 | 3.6576 |
| #22 | 0.1570 | 3.9878 |
| #21 | 0.1590 | 4.0386 |
| #19 | 0.1660 | 4.2164 |
| #17 | 0.1730 | 4.3942 |
| #14 | 0.1820 | 4.6228 |
| #10 | 0.1935 | 4.9149 |
| #9 | 0.1960 | 4.9784 |
| #7 | 0.2010 | 5.1054 |
| #3 | 0.2130 | 5.4102 |
| #1 | 0.2280 | 5.7912 |
Letter Drill Bit Sizes
Letter drill sizes run from A to Z and provide additional diameters between many common fractional-inch sizes. Their decimal-inch and metric equivalents make it easier to compare each letter size with nearby standard drill options.
| Drill Size | Diameter (in) | Diameter (mm) |
| A | 0.2340 | 5.9436 |
| B | 0.2380 | 6.0452 |
| C | 0.2420 | 6.1468 |
| D | 0.2460 | 6.2484 |
| E | 0.2500 | 6.3500 |
| F | 0.2570 | 6.5278 |
| G | 0.2610 | 6.6294 |
| H | 0.2660 | 6.7564 |
| I | 0.2720 | 6.9088 |
| J | 0.2770 | 7.0358 |
| K | 0.2810 | 7.1374 |
| L | 0.2900 | 7.3660 |
| M | 0.2950 | 7.4930 |
| N | 0.3020 | 7.6708 |
| O | 0.3160 | 8.0264 |
| P | 0.3230 | 8.2042 |
| Q | 0.3320 | 8.4328 |
| R | 0.3390 | 8.6106 |
| S | 0.3480 | 8.8392 |
| T | 0.3580 | 9.0932 |
| U | 0.3680 | 9.3472 |
| V | 0.3770 | 9.5758 |
| W | 0.3860 | 9.8044 |
| X | 0.3970 | 10.0838 |
| Y | 0.4040 | 10.2616 |
| Z | 0.4130 | 10.4902 |
Standard Drill Bit Size Conversion Table
Standard drill bit size conversions become easier to use when nearby fractional, number, and letter designations appear beside the same metric diameter. The table below lists common metric sizes with their closest standard equivalents across the three inch-based systems.
| Metric Size | Diameter (in) | Closest Fractional Size | Closest Number Size | Closest Letter Size |
| 1.5 mm | 0.0591 | 1/16 in — 1.5875 mm (+0.0875 mm) | #53 — 1.5113 mm (+0.0113 mm) | — |
| 2.0 mm | 0.0787 | 5/64 in — 1.9844 mm (−0.0156 mm) | #47 — 1.9939 mm (−0.0061 mm) | — |
| 3.0 mm | 0.1181 | 1/8 in — 3.1750 mm (+0.1750 mm) | #31 — 3.0480 mm (+0.0480 mm) | — |
| 4.0 mm | 0.1575 | 5/32 in — 3.9688 mm (−0.0312 mm) | #22 — 3.9878 mm (−0.0122 mm) | — |
| 5.0 mm | 0.1969 | 13/64 in — 5.1594 mm (+0.1594 mm) | #9 — 4.9784 mm (−0.0216 mm) | — |
| 5.5 mm | 0.2165 | 7/32 in — 5.5563 mm (+0.0563 mm) | #3 — 5.4102 mm (−0.0898 mm) | — |
| 5.8 mm | 0.2283 | 15/64 in — 5.9531 mm (+0.1531 mm) | #1 — 5.7912 mm (−0.0088 mm) | A — 5.9436 mm (+0.1436 mm) |
| 6.0 mm | 0.2362 | 15/64 in — 5.9531 mm (−0.0469 mm) | — | B — 6.0452 mm (+0.0452 mm) |
| 6.5 mm | 0.2559 | 1/4 in — 6.3500 mm (−0.1500 mm) | — | F — 6.5278 mm (+0.0278 mm) |
| 7.0 mm | 0.2756 | 9/32 in — 7.1438 mm (+0.1438 mm) | — | J — 7.0358 mm (+0.0358 mm) |
| 8.0 mm | 0.3150 | 5/16 in — 7.9375 mm (−0.0625 mm) | — | O — 8.0264 mm (+0.0264 mm) |
| 8.5 mm | 0.3346 | 21/64 in — 8.3344 mm (−0.1656 mm) | — | Q — 8.4328 mm (−0.0672 mm) |
| 9.0 mm | 0.3543 | 23/64 in — 9.1281 mm (+0.1281 mm) | — | T — 9.0932 mm (+0.0932 mm) |
| 10.0 mm | 0.3937 | 25/64 in — 9.9219 mm (−0.0781 mm) | — | X — 10.0838 mm (+0.0838 mm) |
How Can Standard Drill Bit Sizes Improve CNC Manufacturing Efficiency?
Standard drill sizes improve CNC manufacturing efficiency by reducing special tooling, supporting repeatable production, and simplifying process preparation. A machining team can assign known tools, holders, parameters, and inspection methods to common diameters instead of developing a separate process for every hole specification.
Reduce Tooling and Setup Costs
Standard drill bit sizes reduce CNC manufacturing costs because they are more likely to match drills already held in the shop’s tool inventory. The production team can avoid purchasing a custom drill, preparing a dedicated holder, or adding reaming, boring, or interpolation solely to produce an unnecessary non-standard diameter.
They also shorten machine setup. Each additional tool requires presetting, offset entry, programming, trial cutting, and first-piece inspection. Using an existing standard drill reduces these setup steps, limits tool changes, and keeps more machine time available for actual cutting, especially on short runs and parts with repeated hole features.
Improve CNC Production Repeatability
Standard drill bit sizes make proven CNC drilling processes easier to repeat across production batches. Once the team has approved a standard diameter, it can reuse the same tool specification, holder, offsets, cutting parameters, drilling cycle, coolant method, and inspection plan on repeat orders. This reduces the need to rebuild or revalidate the process each time the part returns to production.
Replacement tools are also easier to control when the diameter follows a common standard. The operator can install another drill with the same diameter, geometry, coating, and flute length, then confirm the offset and first-hole result before continuing the batch. This consistency reduces setup adjustments, limits variation between production runs, and helps the machine return to stable cutting conditions faster.
Simplify Process Planning and Tool Management
A defined range of standard drill bit sizes gives CNC programmers a practical base for process planning. They can connect recurring hole diameters with established tool numbers, holders, flute lengths, cutting parameters, and inspection steps. These records can then be reused in CAM programs and setup sheets, reducing repeated data entry and shortening preparation for new or returning orders.
Tool management also becomes more efficient when the shop concentrates on commonly used diameters. Planners can monitor inventory, prepare backup drills, set replacement limits, and reorder tools that support several parts instead of maintaining many low-use special sizes. The official ISO 13399-1 cutting tool data standard provides a general information model for representing and exchanging cutting-tool data. This supports consistent tool information across digital planning and management systems.
CNC Drilling DFM Rules for Standard Drill Bit Sizes
CNC drilling DFM rules help engineers design holes that standard drills can machine directly, safely, and with fewer extra operations. After the diameter is confirmed, the next priorities are hole depth, tool access, and surrounding geometry. Poor design in any of these areas can increase cycle time, reduce stability, or force the shop to add special tooling and secondary machining.
Control the Depth-to-Diameter Ratio
Keep the hole depth within a practical range for the selected standard drill bit size. A 24 mm-deep hole made with an 8 mm drill has a 3:1 depth-to-diameter ratio, while the same depth with a 3 mm drill reaches 8:1. The second feature is harder to machine because the smaller drill has less rigidity and must carry chips through a longer cutting path.
As the ratio increases, chip evacuation, coolant delivery, heat control, and hole straightness become more difficult. The process may require peck drilling, a longer drill, through-tool coolant, or dedicated deep-hole tooling. Avoid unnecessary depth when the part function does not require it, because a shorter hole allows a more stable standard drilling process.
Provide Adequate Tool Access
Place each hole where the drill, holder, and spindle can approach without interference. High sidewalls, narrow pockets, nearby bosses, and deep cavities may block the tool assembly even when the drill diameter fits the feature.
Restricted access often forces the machinist to use a longer drill or an extended holder. That added overhang reduces rigidity and can increase deflection, vibration, and setup complexity. Leave enough clearance around the hole so the shop can use a shorter standard tool and a direct drilling approach.
Avoid Difficult Hole Geometry
Standard drills perform most consistently when the hole starts on a flat surface and follows a continuous cutting path. Angled or curved entry surfaces load the two cutting edges unevenly, while intersecting holes interrupt the cut after the drill enters the material. These geometries can cause drill wandering, irregular hole entrances, heavy burrs, or reduced positional accuracy.
Place hole entrances on flat faces whenever possible, and avoid unnecessary intersections or holes positioned too close to an edge. A conventional twist drill leaves a conical bottom in a blind hole, so a flat-bottom requirement needs an additional milling or boring operation. Keep the hole geometry compatible with direct drilling unless the part function requires a more complex feature.
How to Choose the Right Drill Bit Size for CNC Machining?
The correct drill bit size comes from the required finished hole, its function, and the process needed to reach the specified tolerance. Engineers should not select a drill only because its nominal diameter matches the drawing. They also need to consider whether drilling alone can produce the final result or whether the hole needs additional finishing.
Start with the Finished Hole Size
Begin with the finished diameter shown on the drawing rather than the drill bit size available in the tool cabinet. Confirm the nominal hole size, upper and lower limits, required depth, and whether the feature is a through hole or blind hole. These details define the result that the machining process must deliver.
For example, a drawing may call for a Ø10 mm hole with a general tolerance or a Ø10 H7 locating hole with much tighter limits. The general-tolerance hole may be produced in one drilling operation, while the H7 feature requires closer process control and inspection. The finished-hole requirement should control the tool choice, not the other way around.
Match the Size to Hole Function
The required drill bit size depends on the finished hole’s purpose. A tapped hole must retain enough material for thread formation, while a clearance hole needs additional space for the fastener to pass through. Therefore, two holes associated with the same fastener can require very different drill diameters.
The following examples show this difference for several common metric sizes:
| Nominal Size | Coarse Thread Pitch | Tap Drill Size | Normal Clearance Hole |
| M4 | 0.7 mm | 3.3 mm | 4.5 mm |
| M6 | 1.0 mm | 5.0 mm | 6.6 mm |
| M8 | 1.25 mm | 6.8 mm | 9.0 mm |
| M10 | 1.5 mm | 8.5 mm | 11.0 mm |
Other hole functions use different sizing criteria. A locating hole follows the required dowel fit, while a press-fit hole depends on the permitted interference between the hole and mating pin. Passage holes should provide enough space for the cable, tube, fluid path, or component that moves through them. The hole function must define the finished diameter before you select a standard drill size.
Allow for Tolerance and Finishing
A drill’s nominal diameter does not always equal the measured hole diameter. Tool runout, edge wear, machine rigidity, feed rate, and cutting temperature can cause the hole to finish slightly larger or smaller than expected. Workpiece material also affects the result because aluminum, stainless steel, and engineering plastics respond differently during cutting.
Aluminum may form built-up edge or exit burrs, while stainless steel can increase cutting force and tool deflection. Engineering plastics such as POM, nylon, and PTFE may expand under cutting heat or recover slightly after the drill passes, which can change the measured hole diameter. For tighter tolerances, start with a smaller drill and leave controlled stock for reaming, boring, or circular interpolation. Select the pre-drill size from the required finishing allowance rather than treating the drill as the final sizing tool.
Compare Standard and Custom Sizes
A standard drill is usually the simpler option when its actual diameter can meet the finished-hole requirements. However, the nearest available size should not replace a specified diameter only because it is easier to source.
Use a standard drill when:
- Its nominal diameter falls within the permitted hole range.
- The hole provides general clearance, access, or fluid passage.
- The dimensional difference does not affect assembly or performance.
- Drilling can produce the required result without additional size control.
Keep the specified custom diameter when:
- The hole controls alignment, sealing, flow, or load transfer.
- The feature creates a locating, sliding, or press fit.
- The nearest standard drill falls outside the permitted limits.
- Changing the diameter would affect thread engagement or assembly clearance.
- The initial hole requires a standard drill followed by controlled finishing.
What Are Common CNC Drilling Problems Related to Drill Sizes?
Drill-size problems usually appear as dimensional errors, weak threads, poor fastener fit, or incorrect substitutions between sizing systems. These issues often begin with the wrong nominal diameter, while tool wear, spindle runout, material behavior, and unstable cutting conditions can push the finished hole further from the drawing requirement.
Oversized or Undersized Holes
Oversized or undersized holes occur when the selected drill bit size or the actual drilling result does not match the specified hole diameter. Common causes include excessive drill runout, worn or uneven cutting edges, an incorrect tool diameter, material deformation, and unstable cutting parameters. These factors can shift the finished hole outside the drawing tolerance even when the program uses the intended nominal size.
The machining team should compare the drawing requirement, programmed tool number, nominal drill diameter, and first-piece inspection result. If the deviation grows during production, the team should check drill wear and tool runout before restarting the cycle. Early measurement prevents one incorrect drill size or a deteriorating tool from producing repeated nonconforming holes.
Incorrect Tap Drill Sizes
Choosing an undersized tap drill leaves too much material for the tap to remove or form. Cutting torque rises, chips have less room to move, and the tap faces a greater risk of chipping or breaking. This becomes especially important in blind holes, where trapped chips can stop the tap before it reaches the required thread depth.
At the other extreme, an oversized tap drill reduces thread height and engagement. The thread may still accept a screw, but it may fail a functional gauge or provide less load capacity than required. Engineers should select the pre-drill diameter from the thread size, pitch, tap type, material, and required thread percentage rather than applying one simplified formula to every internal thread.
Clearance Hole Size Mismatch
A clearance hole that matches the bolt’s nominal diameter usually leaves little or no room for assembly variation. Position tolerance, part misalignment, burrs, paint, anodizing, or plating can make the fastener difficult to insert even when each individual feature appears close to the drawing.
An excessively large clearance hole creates a different problem. It may reduce positional control, allow joint movement, or make the fastener head and washer carry the load unevenly. The selected diameter should reflect the fastener size and the intended close, normal, or loose clearance condition, not just the nearest drill available in the tool cabinet.
Using the Wrong Size Conversion
Metric, fractional-inch, number, and letter drills often have nearby values, but nearby does not mean equal. For example, 1/4 inch equals 6.35 mm, while a 6 mm drill is smaller by 0.35 mm. That difference can change bolt clearance, tap engagement, or the amount of stock left for finishing.
Conversion errors also occur when engineers compare fractions visually instead of converting them to decimal values. A 7/32-inch drill equals 0.21875 inch, while a 1/4-inch drill equals 0.25000 inch. Convert each candidate to the same unit, compare the actual difference, and then confirm that the selected size remains within the drawing requirement.
Conclusion
Standard drill bit sizes give engineers a clear reference for comparing metric, fractional-inch, number, and letter systems. However, the chart alone cannot determine the final process. Hole function, dimensional tolerance, depth, tool access, and surrounding geometry still decide whether direct drilling is suitable or whether the feature needs additional machining.
DZ Making reviews hole size, tolerance, depth, material, tool access, and inspection requirements before production. We select the appropriate combination of CNC drilling, reaming, boring, or interpolation milling to control finished dimensions and avoid unnecessary operations. Contact us with your drawings, material specifications, tolerances, and order quantity for a manufacturability review and CNC machining quotation.
FAQs
1. Which is larger, a 7/32 or 1/4 drill bit?
A 1/4-inch drill bit is larger. A 7/32-inch drill measures 0.21875 inch, while a 1/4-inch drill measures 0.25000 inch. The difference between them is 1/32 inch, or approximately 0.794 mm.
2. What imperial drill bit is closest to 8.5 mm?
An 8.5 mm diameter equals approximately 0.3346 inch. The closest letter-size drill is Q at 0.3320 inch, or 8.4328 mm. Among common fractional sizes, 21/64 inch at 8.3344 mm is the closest, although it is 0.1656 mm smaller than 8.5 mm.
3. Is a 5/8 drill bit larger than a 5/16 drill bit?
Yes. A 5/8-inch drill measures 0.625 inch, or 15.875 mm, while a 5/16-inch drill measures 0.3125 inch, or 7.9375 mm. A 5/8-inch drill is exactly twice the diameter of a 5/16-inch drill.
4. What are the most common drill bit sizes?
Common sizes depend on the drawing system, fastener standard, thread specification, and application. Frequently used metric drills include 3, 4, 5, 6, 8, 10, and 12 mm. Common fractional sizes include 1/8, 3/16, 1/4, 5/16, 3/8, and 1/2 inch. CNC shops may also stock number and letter drills for tap holes and dimensions that fall between common fractional increments.
5. Are drill diameter and shank diameter always the same?
No. Straight-shank drills often have a shank diameter equal to the cutting diameter, but reduced-shank drills use a smaller shank so a standard chuck or holder can grip a larger drill. Step drills, indexable drills, and other specialized tools may also have different cutting and shank diameters. The machinist must check the actual shank size before selecting a holder.
6. When should I use reaming instead of drilling?
Use reaming when the hole requires tighter diameter control, improved roundness, a smoother surface, or a consistent fit with a pin or shaft. The process normally starts with an undersized drilled hole and leaves a controlled amount of material for the reamer. Reaming improves hole size and finish, but it cannot reliably correct major positional errors or severe drill wandering.
