When it comes to creating strong, precise threaded holes, accuracy is everything. Whether you’re working in metalworking, woodworking, or engineering, you cannot simply guess the size of a hole before threading it. This is where the drill tap chart becomes essential.
A drill tap chart (sometimes called a tap drill size chart) is a reference guide that shows the correct drill bit size to use before tapping threads for screws or bolts. Using the right drill size ensures that threads are strong, the fastener fits properly, and the material remains intact.
In this guide, we will explore what a drill tap chart is, why it’s important, and how you can use it effectively. We will also provide example charts for both metric and imperial systems.
What is a Drill Tap Chart?
A drill tap chart is a table that lists screw or bolt sizes alongside the drill bit size required to tap proper threads for them.
For example:
If you are threading for a ¼”-20 UNC screw, the chart recommends using a #7 drill bit (0.201″).
If you are threading for an M6 × 1.0 bolt, the chart suggests a 5.0 mm drill bit.
Without a chart, you risk drilling holes that are too small (breaking the tap) or too large (resulting in weak threads).
Why Do You Need a Drill Tap Chart?
Many beginners assume drilling a hole equal to the screw diameter is enough. Unfortunately, that leads to poor threading results. The drill tap chart eliminates guesswork and ensures accuracy.
Key Benefits:
Accuracy – Correct hole size for strong threads.
Strength – Prevents stripped or weak threads.
Efficiency – Saves time by reducing trial and error.
Tool Protection – Prevents broken taps and worn drill bits.
Universal Reference – Works across wood, metal, and plastic.
Understanding Drill Tap Sizes
Before using a drill tap chart, you should understand some key terms:
Tap – A tool that cuts internal threads inside a drilled hole.
Major Diameter – The outer diameter of a screw/bolt.
Minor Diameter – The diameter of the hole at the root of the threads.
Thread Pitch – The distance between threads (measured in mm for metric or threads per inch in imperial).
Percent of Thread Engagement – Refers to how much of the screw’s thread depth is engaged; 75% engagement is considered ideal.
A drill tap chart aligns these values to give you the correct drill size for threading.
Types of Drill Tap Charts
1. Metric Drill Tap Chart
Common worldwide.
Screw sizes are listed in millimeters (M3, M6, M8, etc.).
Thread pitch can be coarse or fine.
2. Imperial (UNC/UNF) Drill Tap Chart
Widely used in the U.S. and Canada.
Sizes are given in inches with threads per inch (TPI).
UNC = Unified Coarse Thread.
UNF = Unified Fine Thread.
3. NPT (Pipe Thread) Drill Tap Chart
Used for plumbing and fittings.
Taps are tapered to create leak-proof seals.
How to Use a Drill Tap Chart
Identify the Screw/Bolt Size – Example: M8 × 1.25 (metric) or ¼”-20 UNC (imperial).
Look Up the Chart – Find the recommended drill bit size.
Select the Drill Bit – Choose the exact size, or the closest available.
Drill the Hole – Ensure the hole is clean and straight.
Tap the Hole – Use a tap wrench and lubricant; rotate forward ½ turn, then back ¼ turn to clear chips.
Test the Fit – Insert the screw to make sure it threads smoothly.
Drill 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 |
Drill 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 |
How to Effectively Use a Drill Tap Chart
A drill tap chart is more than just a table of numbers—it’s a practical guide for ensuring precise, reliable threaded holes. Here’s how to use it effectively:
Identify Screw or Bolt Size: Determine the size of the fastener you plan to use. For metric screws, check the M size and pitch; for imperial screws, check the threads per inch (TPI).
Locate the Correct Drill Bit: Use the drill tap chart to find the recommended drill bit size for that specific screw and pitch. Choosing the right drill size ensures proper thread engagement and prevents weak or stripped threads.
Consider Material: Softer metals or plastics may allow slightly tighter tolerances, while hard metals often need precise sizing with lubrication.
Follow Proper Drilling Technique: Drill straight, clear chips often, and ensure the hole depth accommodates the screw or bolt length.
Benefits of Using a Drill Tap Chart
Using a drill tap chart brings several advantages for both professionals and hobbyists:
Accuracy: Ensures threads fit screws or bolts perfectly.
Tool Protection: Prevents broken taps and worn drill bits.
Efficiency: Saves time by reducing trial-and-error drilling.
Stronger Assemblies: Correctly sized threads maximize holding power and safety.
Versatility: Useful for woodworking, metalworking, automotive, plumbing, and general DIY projects.
By understanding and referring to a drill tap chart, you can avoid common mistakes such as drilling holes too small or too large, mixing metric and imperial sizes, or skipping lubrication. Keeping a printed or digital copy of the chart in your workshop allows quick access, ensuring each threaded hole is precise and professional.
Whether you are creating fine threads in electronics or heavy-duty bolts in machinery, a drill tap chart is an essential tool for accuracy, strength, and reliability in every project.
Common Mistakes to Avoid
Guessing drill sizes instead of checking the chart.
Mixing metric and imperial systems.
Skipping lubrication when tapping (leads to broken taps).
Using dull drill bits that create oversized or rough holes.
Forcing the tap instead of backing out to clear chips.
Tips for Better Results
Keep a printed drill tap chart in your workshop.
Use digital drill/tap calculator apps for quick access.
Always center punch before drilling to prevent wandering.
For hard metals, use high-speed steel taps and cutting oil.
Drill slightly deeper than the screw length for full engagement.
Benefits of Using a Drill Tap Chart
Strong Threads – Ensures fasteners hold securely.
Efficiency – Saves time and reduces trial and error.
Tool Safety – Prevents broken taps and bits.
Universal Use – Helpful in DIY, machining, construction, and plumbing.
Professional Results – Produces clean, accurate threads every time.
Final Thoughts
A drill tap chart may look like a simple table of numbers, but it’s one of the most important references for anyone working with threaded holes. By following the chart, you’ll avoid costly mistakes, protect your tools, and ensure your fasteners fit perfectly.
Whether you are threading a hole for a tiny screw in electronics or a large bolt in heavy machinery, the drill tap chart is your guide to precision and strength. Keep one handy—printed in your workshop or saved on your phone and you will never guess a drill size again.
Next time you reach for a tap, remember: check the drill tap chart first.
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