Designing a threaded hole or matching a drill bit to a drawing specification often leads to costly delays when standard sizes are misaligned. A single non-standard drill size can add 30% to your CNC machining cost and extend lead times by 5-7 days as shops source custom tooling. We’ve compiled data from over 100,000 manufacturing projects across automotive, medical, and consumer electronics industries to create the most comprehensive drill size reference available. For engineers, machinists, and procurement teams working on CNC projects, this guide eliminates guesswork and ensures your designs use readily available tooling. Here is the complete conversion chart, tap drill size calculator, and actionable manufacturing rules you need to keep your projects on budget and on schedule.
Complete Drill Bit Size Chart
To maintain high precision tolerances and fast lead times, engineers must select hole diameters that align with widely available standard tooling. Below is the comprehensive standard drill size chart, categorized by the four primary sizing systems.
1. Most Common Standard Drill Sizes
| Metric (mm) | Fractional (in) | Number/Letter | Inch Decimal |
| 1.0 | — | #60 | 0.0394 |
| 1.5 | 1/16 | — | 0.0625 |
| 2.0 | — | #47 | 0.0787 |
| 2.5 | — | #39 | 0.0984 |
| 3.0 | — | #31 | 0.1181 |
| 3.175 | 1/8 | — | 0.1250 |
| 4.0 | — | #21 | 0.1575 |
| 5.0 | — | #8 | 0.1969 |
| 6.0 | — | A | 0.2362 |
| 6.35 | 1/4 | E | 0.2500 |
| 8.0 | — | O | 0.3150 |
| 9.525 | 3/8 | — | 0.3750 |
| 10.0 | — | X | 0.3937 |
| 12.7 | 1/2 | — | 0.5000 |
| 16.0 | — | — | 0.6299 |
| 19.05 | 3/4 | — | 0.7500 |
| 25.4 | 1 | — | 1.0000 |
2. Most Common US Thread Tap Drill Sizes (75% Thread Depth)
| Thread Specification | Tap Drill Size | Metric Equivalent (mm) |
| #6-32 | #36 | 2.705 |
| #8-32 | #29 | 3.454 |
| #10-32 | #21 | 4.039 |
| 1/4-20 | #7 | 5.105 |
| 5/16-18 | F | 6.528 |
| 3/8-16 | 5/16 | 7.938 |
| 1/2-13 | 27/64 | 10.716 |
Note: Any size not listed above is considered semi-standard or non-standard and may incur additional tooling costs. Upload your CAD file to RapidDirect’s platform to get an automatic DFM check that flags all non-standard drill sizes before you order.
Key Drill Size System Overview
| System | Range | Common Use Case | Availability | Cost Premium for Non-Standard |
| Metric (mm) | 0.05mm – 38mm | Global standard for most modern engineering designs | Universal | 0% |
| Fractional (in) | 1/64″ – 1 1/2″ | Traditional US manufacturing and woodworking | North America | 10-15% |
| Number (#) | #97 – #1 | Small diameter holes (0.15mm – 5.8mm) for electronics and fine threads | Global | 20-30% |
| Letter (A-Z) | A – Z | Medium diameter holes (5.9mm – 10.5mm) for US standard fasteners | Global | 20-30% |
Four Standard Drill Size Systems Explained
1. Metric Drill Sizes (ISO Standard)
Metric drills are the most widely used system globally, defined by ISO 235-1. Sizes increase in 0.1mm increments up to 10mm, 0.5mm increments from 10mm to 20mm, and 1mm increments above 20mm. RapidDirect’s CNC machining facilities stock all standard metric drill sizes from 0.5mm to 38mm, ensuring no custom tooling delays for standard designs.
Rule of Thumb: For most CNC machining projects, use standard metric drill sizes whenever possible. They are universally available, have the lowest tooling costs, and are compatible with all modern CNC machines.
Pro Tip: When designing metric holes, avoid sizes ending in 0.05mm (e.g., 3.05mm, 4.15mm) unless absolutely necessary. These are considered semi-standard and may not be in stock at all machine shops.
2. Fractional Drill Sizes
Fractional drills are based on 1/64″ increments, commonly used in older US engineering drawings and woodworking. While still used in some industries, fractional sizes are gradually being replaced by metric sizes in global manufacturing.
Pro Tip: When converting fractional inches to millimeters, use the exact conversion (1 inch = 25.4mm) rather than rounding. A 1/4″ drill is exactly 6.35mm, not 6.4mm, which can cause thread fit issues and lead to rejected parts.
3. Number Drill Sizes
Number drills range from #97 (smallest, 0.15mm) to #1 (largest, 5.8mm). They were originally developed for wire gauge sizes and are now primarily used for tapping small US standard threads. Number drills fill the gaps between fractional sizes that are too small for letter drills.
Important Note: Number drill sizes decrease as the number increases. A #1 drill is larger than a #80 drill, which is a common source of confusion for engineers new to US standards.
4. Letter Drill Sizes
Letter drills range from A (5.94mm) to Z (10.49mm). They fill the gaps between fractional sizes and are commonly used for medium-sized US threads. Like number drills, letter sizes are most commonly used in North American manufacturing.
Thread Depth Cost & Performance Comparison Table
(All data based on RapidDirect’s 100,000+ CNC machining projects)
| Thread Depth | Primary Use Case | Tapping Time Increase | Tool Wear Rate | Cost Premium vs Standard | Lead Time Impact | Failure Risk |
| 50% | Soft materials (aluminum, ABS, PP)Non-load-bearing fasteners | -20% | Low | -10-15% | 0 days | Low |
| 75% (INDUSTRY STANDARD) | 99% of all CNC applicationsSteel, brass, stainless steelLoad-bearing fasteners | 0% | Medium | 0% | 0 days | Very Low |
| 100% | Critical aerospace/medical componentsHigh-vibration applicationsExtreme load-bearing requirements | +300% | Very High | +30-50% | +1-3 days | High (tap breakage) |
Quick Decision Rule
Use 75% thread depth for everything unless you have a written engineering requirement for 100%. 75% depth provides 95% of the tensile strength of 100% depth at 1/3 the tapping time and zero additional cost.
Cost Driver Explanation
CNC machining costs are calculated by machine hour. A 100% thread depth requires three full passes of the tap, compared to one pass for 75% depth. This triples the machining time for that operation and significantly increases the risk of tap breakage, which requires manual rework and additional lead time.
Common Engineering Misconception
Many engineers default to 100% thread depth out of habit. In reality, the tensile strength of a threaded connection is almost always limited by the material strength of the part, not the thread depth. For most materials, 75% depth is more than sufficient to achieve the full rated strength of the screw.
RapidDirect’s instant quote engine automatically analyzes all threaded holes in your CAD file and recommends the optimal thread depth for your material and application. It will also show you the exact cost difference between 75% and 100% thread depth before you place your order.
Tap Drill Size Selection Rules
The correct tap drill size creates a hole that allows the tap to cut threads with the proper depth and strength. For 75% thread depth (the industry standard for most applications), use the following formulas:
- Metric threads (ISO): Tap drill size = Major diameter – Pitch
- US Unified threads (UNC/UNF): Tap drill size = Major diameter – (1 / Number of threads per inch)
Example: For a #10-32 thread (major diameter 0.190″, 32 threads per inch), the tap drill size is 0.190 – (1/32) = 0.159″, which corresponds to a #21 drill.
How to Avoid Non-Standard Drill Sizes in Your Designs
Non-standard drill sizes are one of the most common causes of unexpected cost increases and lead time delays in CNC machining. Follow these rules to ensure your designs use readily available tooling:
- Use standard increments: Stick to 0.1mm increments for metric holes and 1/64″ increments for fractional holes. Avoid sizes that fall between these increments.
- Reference standard thread tables: Always use the recommended tap drill size for your chosen thread, rather than rounding to the nearest metric size. Rounding even by 0.1mm can cause thread fit issues or require custom tooling.
- Adjust design tolerances: If your design requires a specific hole diameter, adjust the tolerance to fit a standard drill size rather than specifying a non-standard diameter. For example, instead of specifying a 4.05mm hole with ±0.05mm tolerance, specify a 4.0mm hole with +0.1mm/-0.0mm tolerance.
- Use DFM analysis early: Upload your CAD file to RapidDirect’s platform to get instant DFM feedback on drill size standardization and other manufacturability issues. This allows you to make design changes early in the process when they are least expensive.
- Avoid custom thread sizes: Whenever possible, use standard metric or US thread sizes. Custom thread sizes require custom taps and dies, which can add 50-100% to machining costs and extend lead times by 2-3 weeks.
- Consider reaming for tight tolerances: If you need a hole with a very tight tolerance, use a standard drill size followed by a reamer rather than specifying a non-standard drill size. Reaming provides a more accurate and consistent hole diameter than drilling alone.
Drill Bit Types and Applications
Not all drill bits are created equal. The type of drill bit you specify can have a significant impact on machining cost, lead time, and part quality. Here are the most common drill bit types used in CNC machining:
- High-Speed Steel (HSS) Drills: The most common and least expensive type of drill bit. Suitable for drilling most materials including aluminum, steel, and plastic. HSS drills are available in all standard sizes and are typically in stock at all machine shops.
- Carbide-Tipped Drills: More expensive than HSS drills but last 5-10 times longer. Suitable for high-volume production and drilling hard materials such as stainless steel and titanium. Carbide-tipped drills are available in most standard sizes.
- Solid Carbide Drills: The most expensive type of drill bit but provide the best performance and longest life. Suitable for high-precision applications and drilling very hard materials. Solid carbide drills are available in most standard sizes but may not be in stock at all machine shops.
- Center Drills: Used to create a starting point for drilling operations. Center drills are available in standard sizes and are always in stock at machine shops.
- Spot Drills: Used to create a shallow pilot hole to ensure accurate drilling. Spot drills are available in standard sizes and are always in stock at machine shops.
Pro Tip: For most CNC machining projects, specify HSS drills for low-volume production and carbide-tipped drills for high-volume production. This provides the best balance of cost and performance.
Cost and Lead Time Impact of Non-Standard Drill Sizes
The table below shows the typical cost and lead time impact of using non-standard drill sizes in CNC machining:
| Drill Type | Cost Premium | Lead Time Extension |
| Standard metric drill | 0% | 0 days |
| Standard fractional drill | 10-15% | 0 days |
| Standard number/letter drill | 20-30% | 0-1 days |
| Semi-standard drill (0.05mm increments) | 30-50% | 3-5 days |
| Custom drill size | 50-200% | 7-14 days |
| Custom tap size | 100-300% | 14-21 days |
As you can see, using a single non-standard drill size can double your machining cost and extend lead times by two weeks. For projects with multiple non-standard sizes, the impact can be even more significant.
Key Takeaways for Drill Size Selection in CNC Manufacturing
Using standard drill sizes is one of the most effective ways to reduce CNC machining costs and shorten lead times. This complete drill size chart provides all the conversions you need to ensure your designs use readily available tooling. By following the DFM guidelines outlined in this guide, you can avoid unexpected cost increases and ensure your projects are delivered on time and on budget.
Upload your CAD file to RapidDirect’s platform today to get an instant quote and free DFM analysis. Our system will automatically check all hole sizes against standard drill libraries and flag any potential cost or lead time issues before you place your order.
FAQs
Masonry fasteners depend on specific masonry bits that do not follow standard machining dimensions. Checking a tapcon drill bit size chart reveals that a 1/4″ anchor requires a specialized 3/16″ masonry bit, while a 3/16″ anchor depends on a 5/32″ bit. Standard jobber drill bits will fail in these materials.
Non-standard drill sizes require custom tooling, which adds 30-200% to machining costs and extends lead times by 5-14 days. Most machine shops do not stock non-standard drills, so they must be ordered specially from suppliers.
Yes. For soft materials (aluminum, plastic), you can use a slightly smaller drill size to increase thread depth. For hard materials (steel, titanium), use a slightly larger drill size to prevent tap breakage. Always consult your machinist for material-specific recommendations.
