Drill Bit Size Chart
Metric hole size to match
Enter a hole size in millimetres to find the closest US drill bit.
Closest US bit
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Enter a hole size to search.
- Nearest fractional
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- Nearest number/letter
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- Target in inches
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To match a metric hole size to a US drill bit, convert to inches by dividing by 25.4, then find the closest fractional, number, or letter size. A 6.5 mm hole (0.2559") is closest to letter F (0.257").
Fractional, Number or Letter?
Most drilling never needs anything beyond a basic fractional set — the other two systems exist for the jobs a fractional set can't handle precisely enough.
Start with a fractional index covering 1/16" to 3/8" or 1/2" in sixteenth-inch steps — it handles the vast majority of general drilling, from pilot holes to hardware clearance. Reach for a number or letter set specifically when tapping threads (where the tap drill size is rarely a clean fraction) or when a hole needs to be closer to a target size than 1/16" allows.
Metric Hole to US Bit
Two steps convert a metric target to the closest US bit size.
Convert the millimetre target to decimal inches, then find whichever bit size S sits closest to it.
- MMThe target hole size in millimetres, such as 6.5.
- INThe target converted to decimal inches: MM ÷ 25.4.
- SAny candidate bit size across all three systems.
Worked calculation — 6.5 mm
- 6.5 ÷ 25.4 = 0.2559"Convert to decimal inches
- Letter F = 0.257", diff = 0.0011"Closest overall match
6.5 mm ≈ letter F (0.257")
Common Fractional Sizes to Keep on Hand
The sizes that see the most use in a basic fractional drill index.
| Fraction | Decimal | MM |
|---|---|---|
| 1/8" | 0.125" | 3.175 mm |
| 3/16" | 0.1875" | 4.762 mm |
| 1/4" | 0.25" | 6.35 mm |
| 5/16" | 0.3125" | 7.938 mm |
| 3/8" | 0.375" | 9.525 mm |
| 7/16" | 0.4375" | 11.112 mm |
| 1/2" | 0.5" | 12.7 mm |
For the complete set of fractional, number and letter sizes, see the fractional drill size chart, number drill size chart and letter drill size chart.
Frequently Asked Questions
Should I buy a fractional, number, or letter drill index first?
A fractional set, typically 1/16" to 1/2" or 3/8" in 1/16" steps, covers the overwhelming majority of everyday drilling and is the right first purchase. Number and letter sets are worth adding once you need finer steps for tapping threads or holding a tighter tolerance.
What size drill bit do I need for a 1/4 inch hole?
A 1/4" fractional bit drills a 1/4" hole directly — no conversion needed, since 1/4" is a standard fractional size carried in every basic drill index.
Are letter and number drill bits worth buying separately?
For general work, not essential — most hobbyist and DIY tasks never need finer than sixteenth-inch steps. They become worthwhile once you're tapping threads or holding a tolerance a fractional set can't hit precisely enough.
How do I convert a metric hole size to the nearest US drill bit?
Divide the millimetre value by 25.4 to get a decimal inch, then compare that decimal against every fractional, number and letter size to find whichever is closest. The calculator above does this automatically.
What's the most useful drill bit index for a home shop?
A standard fractional set running 1/16" to 3/8" or 1/2" in 1/16" steps (or 1/32" steps for finer control) handles the vast majority of home-shop drilling. Add a number and letter index later specifically for tap drill work.
Drill Size References
Every other chart in this cluster, one click away.