A #7 drill bit measures 0.2010 inch in diameter, or about 5.105 mm. It is most often used as the standard tap drill for 1/4-20 UNC threads. The closest common substitutes are 5.1 mm and 13/64 inch, but 13/64 is slightly larger.
Drill bit sizes get confusing fast. Numbered bits, fractional bits, and metric bits do not match up in a simple way. I run into this all the time in shop work and home repairs. In this guide, I’ll show you the exact #7 size, the best substitutes, and when that small difference really matters.
Quick Answer: What Size Is a #7 Drill Bit?
Exact #7 Drill Bit Size
A #7 drill bit has an exact diameter of 0.2010 inch. In metric, that is about 5.105 mm. In real-world use, most people round that to 5.1 mm when comparing metric bits.
- Decimal: 0.2010 inch
- Millimeters: 5.105 mm
- Number system: #7
- Closest fractional size: 13/64 inch
- Closest metric size: 5.1 mm
Quick Reference Table
| Drill Bit Type | Size |
|---|---|
| Number size | #7 |
| Decimal size | 0.2010 in |
| Fractional equivalent | 13/64 in (closest common) |
| Metric equivalent | 5.1 mm (closest common) |
| Common use | Tap drill for 1/4-20 UNC |
Understanding Numbered Drill Bit Sizes
What Are Numbered Drill Bits?
Numbered drill bits are a standard size system used a lot in the USA. They usually run from #80 on the small end to #1 on the large end. The tricky part is this: the lower the number, the bigger the bit. So a #7 bit is larger than a #8 bit.
I like numbered bits because they fill the gaps between common fractional sizes. That is why they show up so often in metalworking, tapping, and repair work.
Numbered vs Fractional vs Letter vs Metric Drill Bits
In the U.S., you will see four common drill size systems:
- Numbered bits: Great for tapping and close size steps
- Fractional bits: Common in home shops and general drilling
- Letter bits: Often used in machinist sets
- Metric bits: Common on imported tools and hardware
They all work, but they do not line up perfectly. That is why conversions matter. For broader comparisons, the Engineering ToolBox drill size chart is a handy reference.
Why a #7 Drill Bit Is Not the Same as 7/32 Inch
This is one of the most common mistakes I see. A #7 drill bit is 0.2010 inch. A 7/32 inch drill bit is 0.21875 inch. That is a big enough jump to ruin a tapped hole.
If you use 7/32 when the job calls for #7, the hole will come out too large for a standard 1/4-20 tap. The threads can end up weak, shallow, or unusable.
Exact Size of a #7 Drill Bit in Inches, Decimal, and MM
#7 Drill Bit in Decimal Inches
The exact decimal size of a #7 drill bit is 0.2010 inch. Decimal sizing matters most when you need a close fit, especially in metal. It also makes it easier to compare numbered bits to fractional or metric substitutes.
#7 Drill Bit in Millimeters
A #7 drill bit converts to about 5.105 mm. If you need a metric substitute, 5.1 mm is the closest common match. That is only about 0.0002 inch smaller, which is close enough for many jobs.
If you want to check inch-to-metric conversion basics, NIST explains unit conversion standards in plain language.
Closest Fractional Equivalent
The closest common fractional size to a #7 drill bit is 13/64 inch. But it is not exact.
- #7: 0.2010 in
- 13/64: 0.203125 in
That means 13/64 is slightly larger. For a simple clearance hole, that may be fine. For a tapped hole, it can reduce thread engagement.
Size Comparison Table
| Drill Bit | Decimal (in) | Metric (mm) | Notes |
|---|---|---|---|
| #8 | 0.1990 | 5.055 | Slightly smaller than #7 |
| #7 | 0.2010 | 5.105 | Exact target size |
| 5.1 mm | 0.2008 | 5.100 | Very close metric substitute |
| 13/64 in | 0.2031 | 5.159 | Slightly larger |
| #6 | 0.2040 | 5.182 | Slightly larger than #7 |
What Is a #7 Drill Bit Used For?
Most Common Use: Tap Drill for 1/4-20 UNC
The most common job for a #7 drill bit is making the tap hole for a 1/4-20 UNC thread. That is why so many people search for this size. In my shop, if I’m cutting standard 1/4-20 threads in steel or aluminum, #7 is the bit I reach for first.
Other Common Uses
A #7 drill bit also works well for:
- Precision holes in metal parts
- Pilot holes for machine work
- Repair jobs on brackets, plates, and tool parts
- General shop work where a fractional bit is not quite right
Materials You Can Drill with a #7 Bit
You can use a #7 bit in many materials:
- Steel: Use slower speed and cutting oil
- Aluminum: Drills easily but can grab
- Brass: Use light pressure
- Plastic: Keep heat low
- Wood: Works fine, though fractional bits are more common there
For general work, HSS is fine. For stainless or harder steel, I prefer cobalt.
Recommended Tool
Numbered Drill Bit Set
Useful for keeping #7 and nearby sizes ready to use.
#7 Drill Bit Tap Chart
Common Tapping Applications for a #7 Drill Bit
| Thread Size | Thread Type | Recommended Tap Drill | Decimal Size |
|---|---|---|---|
| 1/4-20 | UNC | #7 | 0.2010 |
| 1/4-20 | UNC (close substitute) | 5.1 mm | 0.2008 |
| 1/4-20 | UNC (approx. substitute) | 13/64 in | 0.2031 |
Mini Tap Drill Reference Chart Around #7 Size
| Tap Size | Recommended Drill Bit | Decimal |
|---|---|---|
| 10-24 | #25 | 0.1495 |
| 10-32 | #21 | 0.1590 |
| 12-24 | #16 | 0.1770 |
| 1/4-20 | #7 | 0.2010 |
| 1/4-28 | #3 | 0.2130 |
| 5/16-18 | F | 0.2570 |
Important Note About Form Taps vs Cut Taps
The chart above is for standard cut taps. If you use a form tap, you usually need a larger drill size. That is because a form tap displaces metal instead of cutting it away. Material type also matters. Softer metals and higher thread percentages may call for a different size.
When the fit matters, I always check a trusted tap chart before drilling.
Recommended Tool
1/4-20 Tap and Drill Set
Good for cutting threads and repair work.
How to Choose the Right Substitute if You Don’t Have a #7 Drill Bit
Best Alternatives to a #7 Drill Bit
If your #7 bit is missing, the best backups are:
- 5.1 mm: Best all-around substitute
- 13/64 inch: Works in some cases, but slightly oversize
- #8: Slightly smaller, gives more thread engagement but harder tapping
If I need a close match and only have metric bits nearby, I use 5.1 mm first.
When You Can Use 13/64 Instead
You can often use 13/64 inch when:
- The hole is not highly critical
- The material is soft
- You want a slightly easier tap
- The hole is for general repair, not precision work
When You Should Avoid Substitutes
Avoid substitutes when:
- You are tapping threads in hard steel
- You need full thread strength
- The part has tight tolerance requirements
- You cannot risk a loose or weak thread
For those jobs, use the exact #7 bit.
Substitute Comparison Table
| Substitute | Difference from #7 | Good Choice? | Best Use Case |
|---|---|---|---|
| 5.1 mm | Very small | Yes | Closest practical replacement |
| 13/64 in | Slightly larger | Sometimes | Non-critical or loose tap work |
| #8 | Slightly smaller | Sometimes | More thread engagement, harder tapping |
| #6 | Slightly larger | Usually not ideal | Only if a looser hole is acceptable |
Step-by-Step Guide: How to Drill a Hole with a #7 Drill Bit
Tools and Materials You’ll Need
- #7 drill bit
- Hand drill or drill press
- Center punch
- Cutting oil
- Clamp or vise
- Safety glasses
- Deburring tool
- Tap and tap wrench if you plan to thread the hole
Step 1: Mark the Hole Location
Measure the location carefully. Mark it with a pencil, scribe, or layout dye. A clean mark saves time later.
Step 2: Center Punch the Spot
Use a center punch to make a small dent at the mark. This keeps the bit from wandering, especially on metal.
Step 3: Clamp the Workpiece Securely
Do not hold the work by hand. Clamp it to the bench or lock it in a vise. This makes the hole safer and more accurate.
Step 4: Set the Right Drill Speed
Use slower speeds for steel, medium speed for aluminum, and faster speed for wood or plastic. The harder the material, the slower you should go. Add cutting oil when drilling metal.
Step 5: Drill the Hole
Start straight. Apply steady pressure. Do not force the bit. For deeper holes, use peck drilling. That means drilling a little, backing out, and clearing chips before going deeper.
Step 6: Deburr and Inspect
After drilling, remove sharp edges with a deburring tool or countersink. Then check the hole. Make sure it is clean, round, and in the right spot.
Step 7: Tap the Hole If Needed
If you are threading the hole, line the tap up straight. Turn it forward a little, then back it off to break chips. Keep it lubricated. This is the step where using the correct #7 size really pays off.
Recommended Tool
Cutting Oil for Drilling and Tapping
Helps reduce heat and improve thread quality.
Common Mistakes When Using a #7 Drill Bit
Confusing #7 with 7/32 Inch
This is the big one. A 7/32 bit is much larger than #7. If you swap them, the hole can be too loose for proper threads.
Using the Wrong Bit for Tapping
Always match the drill size to the exact thread size and pitch. A 1/4-20 tap does not use the same drill as a 1/4-28 tap.
Drilling Too Fast in Metal
Too much speed creates heat. Heat dulls the bit, roughs up the hole, and can make the drill grab. Slow down and let the bit cut.
Skipping Cutting Oil
Oil helps cool the bit and move chips out of the hole. On steel and stainless, it makes a big difference.
Not Securing the Workpiece
A loose part can spin, shift, or ruin the hole location. It is also a safety problem.
Using a Fractional Substitute Without Checking Tolerance
Just because a size is close does not mean it is right. For tapped holes, a few thousandths of an inch can change the way the thread feels and holds.
Pro Tips for Better Results with a #7 Drill Bit
Use a Drill Gauge to Confirm the Size
If the markings on the bit are worn off, use a drill gauge. I keep one near my bench because number sizes can be hard to tell by eye.
Choose the Right Bit Material
- HSS: Good for general drilling
- Cobalt: Better for stainless and harder steel
- Titanium-coated: Fine for lighter-duty use
Use a Pilot Hole When Appropriate
For thick steel or tough materials, a small pilot hole can help accuracy. Just do not make the pilot too large, or the #7 bit may grab less cleanly.
Keep a Tap Chart Near Your Bench
This saves time and prevents mistakes. If you do repair work often, a printed tap chart is worth having.
Use a Drill Press for More Accurate Holes
A drill press helps keep the bit straight and the hole consistent. For tapping prep, that extra alignment matters.
For 1/4-20 Threads, Check Thread Percentage if Precision Matters
Standard charts are a great starting point, but thread percentage, material, and tap style can change the best drill size. On critical parts, I always verify before drilling.
#7 Drill Bit vs Other Common Drill Bit Sizes
#7 vs 13/64 Drill Bit
13/64 inch is slightly larger than #7. It can work as a backup, but it is not ideal for precision tapping.
#7 vs 5.1 mm Drill Bit
5.1 mm is almost the same size as #7. If you only have metric bits, this is the best substitute in most cases.
#7 vs #8 Drill Bit
#8 is slightly smaller. That can give you more thread engagement, but it also makes tapping harder, especially in steel.
#7 vs #6 Drill Bit
#6 is slightly larger than #7. It can make tapping easier, but the finished threads may be looser than you want.
Comparison Table
| Size | Decimal | Relative to #7 | Typical Use |
|---|---|---|---|
| #8 | 0.1990 | Smaller | Tighter tap hole |
| #7 | 0.2010 | Exact | Standard 1/4-20 tap drill |
| 5.1 mm | 0.2008 | Nearly same | Metric substitute |
| 13/64 in | 0.2031 | Slightly larger | Approximate replacement |
| #6 | 0.2040 | Larger | Looser alternative |
How to Read a Drill Bit Size Chart
Numbered Drill Charts
Start by finding the number size you need, then look across to the decimal value. In a good chart, #7 will show as 0.2010 inch. That decimal number is the easiest way to compare systems.
Fractional and Metric Conversion
Once you know the decimal size, you can compare it to fractional and metric options. This is much safer than guessing by eye. A tiny difference can matter when the hole is for a tap or press fit.
When to Use a Conversion Chart vs a Tap Chart
Use a conversion chart when you want to compare sizes. Use a tap chart when you need the right drill for a specific thread. They are not the same tool, and mixing them up causes mistakes.
FAQ About #7 Drill Bit Size
What is a #7 drill bit in inches?
A #7 drill bit measures 0.2010 inch in diameter.
What is a #7 drill bit in mm?
It is about 5.105 mm, usually rounded to 5.1 mm.
What fraction is closest to a #7 drill bit?
The closest common fractional size is 13/64 inch.
Is a #7 drill bit the same as 13/64?
No. 13/64 inch is slightly larger than a #7 drill bit.
What tap uses a #7 drill bit?
A #7 drill bit is most commonly used as the tap drill for a 1/4-20 UNC tap.
Can I use a 5.1 mm drill bit instead of #7?
Yes. A 5.1 mm bit is extremely close to a #7 and works well in many cases.
Can I use 13/64 instead of a #7 drill bit?
Sometimes, yes. But it is slightly oversized, so it is not the best choice for critical tapped holes.
Final Takeaway
Simple Summary
A #7 drill bit equals 0.2010 inch or about 5.105 mm. The closest common substitutes are 5.1 mm and 13/64 inch, with 13/64 being slightly larger. Its most common use is as the standard tap drill for 1/4-20 UNC.
Best Practice
Use the exact #7 bit when you can, especially for tapping. It gives you the most reliable fit and thread strength. If you do this kind of work often, keep a numbered drill set and tap chart close to the bench.
