If you have ever worked with screws, bolts, or threaded holes, you know how important accuracy is. One of the most essential tools for achieving that precision is the drill and tap chart. This chart helps you select the correct drill size before tapping threads into a hole. Without it, you risk stripping threads, damaging fasteners, or creating weak connections.
Whether you are a machinist, mechanic, woodworker, or DIY enthusiast, understanding how to use a drill and tap chart will save you time, materials, and frustration. In this guide, we’ll dive into what drill and tap charts are, why they matter, and how you can use them effectively for professional results.
What is a Drill and Tap Chart?
A drill and tap chart is a reference table that shows the correct size drill bit to use before cutting threads with a tap. When you need to install a screw or bolt into a material, you first drill a hole. But that hole must be the correct size—not too large and not too small—for the tap to cut threads that match the fastener.
For example:
If you want to thread a hole for a ¼”-20 UNC screw, the chart will tell you the exact drill size (typically a #7 drill, or 0.201″).
If you are tapping for an M6 x 1.0 bolt, the chart will suggest a 5.0 mm drill bit.
This ensures that the threads are strong and the fastener fits securely.
Why Do You Need a Drill and Tap Chart?
Many beginners assume that drilling a hole the same size as the screw’s diameter will work. Unfortunately, this leads to poor results because there’s no room for the tap to cut proper threads.
Here’s why a drill and tap chart is essential:
Accuracy: Ensures correct hole size for threading.
Strength: Prevents weak or stripped threads.
Compatibility: Matches screws, bolts, and taps perfectly.
Time-Saving: Eliminates trial and error.
Material Protection: Prevents cracks, over-drilling, or broken taps.
Understanding Drill and Tap Sizes
Before using a chart, it helps to understand a few terms:
Tap: A tool used to cut internal threads in a hole.
Drill Size: The diameter of the hole drilled before tapping.
Thread Pitch: The distance between threads (measured in mm for metric, or as “threads per inch” for imperial).
Major Diameter: The outer diameter of the screw or bolt.
Minor Diameter: The inner diameter (after threads are cut).
The drill and tap chart connects these measurements, helping you choose the right drill size for the tap you plan to use.
Types of Drill and Tap Charts
There are different kinds of drill and tap charts depending on the system of measurement and the type of threads:
1. Metric Drill and Tap Chart
Used worldwide.
Sizes are given in millimeters (e.g., M6 × 1.0).
Pitch values are either coarse (default) or fine.
Example: For M8 × 1.25, use a 6.8 mm drill.
2. Imperial Drill and Tap Chart
Common in the U.S. and Canada.
Uses inches and “threads per inch” (TPI).
Example: For ¼”-20 UNC, use a #7 drill.
3. Pipe Thread (NPT) Tap Chart
Used for plumbing and fluid systems.
Requires tapered taps for leak-proof threads.
Example: For ⅛” NPT, use an 11/32″ drill.
4. Specialty Tap Charts
Used for plastics, sheet metal, or thin materials.
Sometimes include recommendations for material hardness.
How to Use a Drill and Tap Chart
Using a drill and tap chart is simple if you follow these steps:
Identify the Screw or Bolt Size
– For example, M6 × 1.0 (metric) or ¼”-20 (imperial).Check the Chart
– Find the recommended drill size for that tap.Select the Correct Drill Bit
– Use the exact size listed, or the closest available.Drill the Hole
– Ensure the hole is straight and properly sized.Tap the Threads
– Use a tap wrench and cutting fluid for best results.Test the Fit
– Insert the screw or bolt to ensure smooth engagement.
Example Drill and Tap Chart (Imperial UNC/UNF)
| Screw Size | Threads Per Inch (TPI) | Tap Drill Size | Decimal Equivalent (in) |
|---|---|---|---|
| #4-40 UNC | 40 | #43 | 0.0890 |
| #6-32 UNC | 32 | #36 | 0.1065 |
| #8-32 UNC | 32 | #29 | 0.1360 |
| #10-24 UNC | 24 | #25 | 0.1495 |
| ¼”-20 UNC | 20 | #7 | 0.2010 |
| ¼”-28 UNF | 28 | #3 | 0.2130 |
| 5/16″-18 UNC | 18 | F | 0.2570 |
| 3/8″-16 UNC | 16 | 5/16″ | 0.3125 |
Example Drill and Tap Chart (Metric)
| Metric Screw Size | Thread Pitch (mm) | Tap Drill Size (mm) |
|---|---|---|
| M3 × 0.5 | 0.50 | 2.5 mm |
| M4 × 0.7 | 0.70 | 3.3 mm |
| M5 × 0.8 | 0.80 | 4.2 mm |
| M6 × 1.0 | 1.00 | 5.0 mm |
| M8 × 1.25 | 1.25 | 6.8 mm |
| M10 × 1.5 | 1.50 | 8.5 mm |
| M12 × 1.75 | 1.75 | 10.2 mm |
| M16 × 2.0 | 2.00 | 14.0 mm |
Advanced Tips for Using Drill and Tap Charts
While a drill and tap chart provides the correct drill size for threading, combining it with practical techniques ensures stronger threads, cleaner holes, and longer tool life.
1. Choose the Right Drill Bit Material:
Different materials require specific drill bit types:
High-Speed Steel (HSS): Ideal for soft metals, wood, and plastics.
Cobalt Steel: Best for stainless steel and harder metals.
Carbide-Tipped Bits: Recommended for masonry or extremely hard materials.
Selecting the right bit preserves both the tool and the workpiece.
2. Drill Pilot Holes for Large Screws:
Even when using the correct tap drill size, a smaller pilot hole can guide larger screws, reducing stress on the tap and preventing splitting in softer materials like wood.
3. Mark and Secure Your Workpiece:
Use a pencil or center punch to mark the drilling location. Clamp the material firmly to prevent shifting, which ensures accuracy and safety.
4. Use Lubrication and Cooling:
For metals, apply cutting oil or tap lubricant to reduce friction and heat. This prevents premature wear on taps and drill bits while producing cleaner threads.
5. Keep Drill Bits Sharp:
Dull bits create oversized, rough, or uneven holes that can compromise thread strength. Regularly inspect and sharpen or replace drill bits for precise results.
6. Check Thread Engagement:
After tapping, test-fit the screw or bolt. A proper fit confirms the correct hole size, pitch, and thread depth, ensuring secure and reliable fasteners.
7. Leverage Digital Tools:
Digital calculators and mobile apps can convert between metric, fractional, and numbered drill sizes. This reduces errors when working with mixed fasteners or international components.
8. Consider Thread Depth and Material:
The depth of the hole relative to the screw length is critical. For deep threads, consider multiple passes and chip clearing. Softer materials may require slightly larger tap drill sizes to avoid thread deformation.
9. Maintain Safety Standards:
Always wear protective glasses when drilling or tapping. Avoid forcing taps, and work at recommended speeds to minimize the risk of breakage.
By combining your drill and tap chart with these best practices, you guarantee precision, reduce tool wear, and achieve professional-quality threads every time.
Common Mistakes to Avoid
When working with drill and tap charts, avoid these errors:
Skipping the Chart – Guessing often leads to oversized or undersized holes.
Using Dull Drill Bits – Results in inaccurate hole sizes.
Tapping Without Cutting Fluid – Increases the risk of broken taps.
Forcing the Tap – Always back off a half turn to clear chips.
Wrong Measurement System – Don’t mix metric with imperial.
Tips for Success
Keep a laminated chart in your workshop for quick reference.
Use digital drill/tap apps on your phone for convenience.
Always measure your fasteners with calipers before drilling.
Start with a center punch to prevent drill wandering.
For tough materials (like stainless steel), use specialized taps and proper lubrication.
Benefits of Using Drill and Tap Charts
Precision – Correct threads every time.
Efficiency – Less rework and wasted material.
Safety – Reduces broken tools and material damage.
Universal Reference – Works across woodworking, metalworking, plumbing, and machining.
Professional Results – Threads that fit perfectly, making your work look and perform better.
Final Thoughts
A drill and tap chart is more than just a table of numbers—it’s a tool that ensures accuracy, strength, and professionalism in your work. Whether you are threading holes for a machine part, installing screws in wood, or plumbing with NPT fittings, the chart is your best guide.
By learning how to read and apply a drill and tap chart, you will eliminate guesswork, protect your tools, and achieve strong, precise threads every time. Keep one in your shop, bookmark one online, or even download an app version—you will be surprised how often you use it.
Next time you reach for a drill and tap set, don’t guess. Use the chart and work like a pro.
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