We often hear from customers who say, "I need a 6.2 mm drill bit - what's that in standard size?"
It sounds like a simple question, but it opens up one of the biggest confusions in the drilling world: why are there so many different size standards?
Let's unpack it together.
Why Are There Different Drill Size Systems?
The answer lies in history and geography [1].
In Europe and most of Asia, the metric system dominates manufacturing. Everything is measured in millimeters. Simple, logical, universal.
In contrast, the imperial system-used mainly in the U.S.-relies on fractions of an inch (like 1/4", 3/8") and wire gauge numbers (#29, #40, etc.), which date back to 19th-century wire-making standards.
That's why the same drill bit can have three different ways of being described - depending on where you are.
When American engineers talk about a "standard drill", they usually mean fractional-inch or wire-gauge sizes.
When a European or Australian machinist says "standard drill", they mean metric sizes.
So, if you're working across regions - or sourcing tools internationally - it's easy to see how confusion arises.
[1] According to the Wikipedia 'Drill bit sizes' entry, metric, fractional inch, and wire-gauge systems reflect regional standards and historical manufacturing practices
Metric-to-Standard Conversion Table (3.3 mm to 16.5 mm)
| Metric (mm) | Closest Fractional (inch) | Closest Number / Letter | Comment |
|---|---|---|---|
| 3.3 mm | 1/8" (0.1250") | #30 (0.1285") | #30 is a tighter match |
| 3.5 mm | 9/64" (0.1406") | #29 (0.1360") | Use #29 for tighter fit |
| 5.2 mm | 13/64" (0.2031") | #7 (0.2010") | #7 is very close |
| 6.2 mm | 15/64" (0.2344") | #1 (0.2280") | Slight undersize if using #1 |
| 6.5 mm | 1/4" (0.2500") | - | Practically interchangeable |
| 6.8 mm | 17/64" (0.2656") | - | Use if clearance allows |
| 8.5 mm | 21/64" (0.3281") | - | Common in metric hardware |
| 10.5 mm | 13/32" (0.4063") | - | Often used for tapping M12 threads |
| 16.5 mm | 21/32" (0.6563") | - | Typical for large through holes |
Note: The conversion is approximate - always verify with tolerances before production.
Curious about 4 mm conversions and when you can safely substitute? Check out our earlier article, "4 mm Drill in Imperial - Do You Really Need a Metric Drill Bit?", where we explore this exact dilemma from the manufacturer's perspective.
Which Should You Use - Metric or "Standard"?
Here's our rule of thumb after years of working with global distributors:
If you're matching an existing drawing or part, stick with the original standard. Don't mix systems unless you're sure.
For general-purpose or on-site drilling, close imperial equivalents (like 1/4" for 6.5 mm) are often acceptable.
For precision assemblies, tapping, or CNC machining, always use the exact metric bit - a small deviation can cause fit or tolerance issues later.
One of our European distributors once told us, "Before switching to metric drills, we were wasting hours rechecking fit tolerances. Now, we just order directly by millimeter."
It's a small change, but it saved them countless reworks and mismatched stock.
Regional Differences: Why the Confusion Persists
In the U.S., fractional and number drills are still the go-to standard for domestic manufacturing and repair shops. Hardware stores list 1/8", 3/16", 1/4"-but rarely 3.3 mm or 6.2 mm.
Meanwhile, in Europe, the UK, and Australia, metric sizing is fully standardized (ISO 235)[1]. So a 6.2 mm bit is just… 6.2 mm.
This mismatch leads to the "translation problem" we see so often online - especially when engineers in one country read specs written in another.
[1] ISO 235 specifies the drill dimensions in mm and inches for parallel-shank drills.According to ISO 235:2016, drill dimensions are standardized in both millimeter and inch units, defining the jobber and stub series
Choose Precision Over Guesswork
At Ceres Tools, we always recommend using metric drills whenever your design specifies them.
Why risk inconsistent tolerances when precise metric tools are available and affordable?
We manufacture ISO-standard metric drills in HSS, cobalt, and carbide materials - tested for accuracy and hardness.
And if you also stock imperial or wire-gauge tools, we can help simplify your inventory with custom-labeled packaging and mixed-size shipments under one purchase order.
If you ever find yourself wondering whether your 6.2 mm drill equals a 15/64" or a #1 - the real question might be: why not just use a 6.2 mm bit that's made for the job?
Ceres Tools - Precision in Every Size
Different standards exist for historical reasons - but precision, efficiency, and global trade all point toward the same conclusion:
We deliver certified metric and imperial drill bits, all manufactured under strict ISO 9001 control.
Get exact sizing, OEM customization, and reliable global shipping - whether you need 3.3 mm, 6.5 mm, or 16.5 mm drills, we've got them all.
👉 Contact us today to find your perfect fit.