How big is a number 6 drill
So you're wondering about a number 6 drill bit. It's one of those sizes in the wire gauge system people use here in the States, mostly for drilling into metal, wood, or plastic. The thing measures exactly 0.204 inches across. That's about 5.18 millimeters if you're metric. Knowing this matters—like, for tapping threads or putting in screws or making precise holes in mechanical stuff. It's kinda the middle child of drill bits. Not too small, not too big. Falls between those tiny gauge bits and the bigger fractional inch ones.
What is the exact diameter of a number 6 drill bit in inches and millimeters?
Okay, here's the deal. A number 6 drill bit is exactly 0.204 inches. In metric? That's 5.1816 millimeters. Most folks just round it to 5.18 mm. It's standardized under the American Wire Gauge system—same one used for electrical wires. Fun fact: it's a hair bigger than a 13/64-inch bit (which is 0.203125 inches) and a bit smaller than a 7/32-inch bit (0.21875 inches). So yeah, it's right in that sweet spot.
What is a number 6 drill bit used for?
Honestly, this bit gets around. In woodworking, it's your go-to for pilot holes with #10 or #12 wood screws—stops the wood from splitting and lets the threads bite proper. In metalworking? It's a clearance hole for 1/4-inch bolts or a pre-drill for tapping 1/4-20 threads. I've also seen it used for drilling holes in electrical boxes, automotive parts, plastic components. It's like the jack-of-all-trades for medium-duty work. The diameter's just right—gives you strength without losing precision.
How does a number 6 drill compare to other common drill bit sizes?
To really get what a number 6 is, you gotta see how it stacks up against the others. Check out this table:
| Drill Bit Size | Diameter (Inches) | Diameter (Millimeters) | Common Use |
|---|---|---|---|
| Number 4 | 0.209 | 5.31 | Pilot hole for #12 screws |
| Number 5 | 0.2055 | 5.22 | General metal drilling |
| Number 6 | 0.204 | 5.18 | Tap drill for 1/4-20 threads |
| Number 7 | 0.201 | 5.11 | Clearance hole for #12 screws |
| 1/4 inch | 0.250 | 6.35 | Larger through holes |
What is the tap drill size for a 1/4-20 thread?
Here's where it gets a little tricky. For 1/4-20 threads in metal, the recommended tap drill is actually a number 7—that's 0.201 inches. But people use number 6 (0.204 inches) all the time as a close substitute, especially with softer stuff like aluminum or plastic. Using the number 6 gives you a slightly looser thread. That can help reduce friction, but you lose a bit of thread engagement. In harder materials like steel? Stick with the number 7 for proper depth and strength.
How do I measure a number 6 drill bit without a chart?
No chart? No problem. A number 6 is roughly the diameter of a standard #2 pencil lead (0.196 inches)—but a tad thicker. It's also pretty close to a 5 mm hex key or a standard 5.2 mm metal rod. For real accuracy though, grab a digital caliper set to inches. Should read 0.204 inches across the cutting edges. If you're stuck with just a ruler, measure the shank if it's not tapered—though shanks are usually bigger than the cutting part, so don't count on that.
Checklist: Selecting the correct number 6 drill bit
- Figure out what you're drilling—wood, metal, plastic?
- Check your screw or bolt size (common for #10 screws or 1/4-20 taps).
- Use a gauge or caliper to make sure it's exactly 0.204 inches.
- If tapping threads, ask yourself if a number 7 (0.201 inches) might work better for your material.
- Make sure the bit is sharp and right for the job (high-speed steel for metal, carbide for hardened steel).
- Test it on scrap first. Always.
Expert Insight
"The number 6 drill bit is an unsung hero in the precision machining world. While it might seem like a random size, its 0.204-inch diameter is a perfect compromise for many standard applications. I always keep a few in high-speed steel and cobalt for versatility. When tapping 1/4-20 threads in aluminum, the number 6 gives you a slightly freer cut, reducing the risk of tool breakage. Always remember: the exact size matters less than the consistency of your drilling technique." — James Miller, Master Machinist with 25 years of experience.
Frequently Asked Questions
Is a number 6 drill bit the same as a 5 mm drill bit?
Not quite. They're close—real close—but not identical. Number 6 is 0.204 inches (5.18 mm). A 5 mm bit is 0.19685 inches. So the number 6 is about 0.18 mm larger. In casual use, people swap them sometimes. But for precision work? That difference matters.
Can I use a number 6 drill bit for wood?
Yeah, works great for wood. It's perfect for pilot holes for #10 or #12 wood screws. Helps prevent splitting and makes driving screws way easier. In softwoods like pine? Ideal. In hardwoods like oak? You might want a slightly smaller bit for a tighter fit.
What is the decimal equivalent of a number 6 drill bit?
Decimal equivalent is 0.204 inches. In fractions, it's just over 13/64 inch (0.203125 inches) and under 7/32 inch (0.21875 inches). And in millimeters, 5.18 mm.
How do I know if my drill bit is a number 6?
Look for a "6" stamped on the shank. If it's worn off, use a drill bit gauge or caliper. The cutting diameter should measure exactly 0.204 inches. No gauge? Compare it to a known number 6 or use that pencil lead trick I mentioned.
Short Summary
- Exact Size: A number 6 drill bit is 0.204 inches (5.18 mm) in diameter, a standard gauge size.
- Primary Use: Commonly used as a pilot hole for #10 screws and as a tap drill for 1/4-20 threads in softer materials.
- Comparison: It is slightly larger than a number 7 drill (0.201 inches) and just smaller than a number 5 drill (0.2055 inches).
- Identification: Look for the "6" stamp on the shank or measure with a caliper; it is close to a 5 mm bit but not identical.
