Drill Size Chart
Find the nearest drill size
Enter a decimal like 0.257 or a fraction like 1/4 to find the closest match in every system.
Closest overall match
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Enter a target size to search.
- Nearest fractional
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- Nearest number/letter
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- Target in mm
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US drill bits come in three sizing systems — fractional, number and letter — and this calculator finds the closest match in each one. A target of 0.25" matches 1/4" exactly, and its closest letter equivalent is E (0.2500"), the same value.
Fractional, Number and Letter
Each system covers a different range with a different level of granularity, and together they span everything from a fine wire-gauge hole to a full inch.
Fractional
The most familiar system, stepping in 1/64" increments. Covers the range a standard fractional-inch drill index carries.
Number
0.0135" to 0.228", counting down as size increases. Fills the fine gaps below and between fractional sizes.
Letter
0.234" to 0.413", picking up where number drills leave off. Common in tap drill charts for larger threads.
Finding the Closest Match
Finding the nearest drill size is a minimization, not a formula with a single equation.
The best match is whichever size S, from every fractional, number and letter size, has the smallest absolute difference from the target T.
- TThe target size you're trying to match, in decimal inches.
- SAny candidate size across all three systems.
- |S − T|The absolute difference between a candidate and the target — always positive.
Worked example — target 0.257"
- 1/4" = 0.25", diff = 0.007"Closest fractional candidate
- F = 0.257", diff = 0"Letter F matches exactly
0.257" = letter F, an exact match
Where Each System Starts and Ends
| System | Size | Decimal | MM |
|---|---|---|---|
| Fractional | 1/64" (smallest) | 0.0156" | 0.397 mm |
| Fractional | 1" (largest shown) | 1" | 25.4 mm |
| Number | #80 (smallest) | 0.0135" | 0.343 mm |
| Number | #1 (largest) | 0.228" | 5.791 mm |
| Letter | A (smallest) | 0.234" | 5.944 mm |
| Letter | Z (largest) | 0.413" | 10.49 mm |
Every size within each system, and the fractional-to-number/letter cross-reference, is covered in full on the fractional drill size chart, number drill size chart and letter drill size chart.
Frequently Asked Questions
What are the three drill sizing systems in the US?
Fractional (1/64" to 1" and beyond, in 64ths), number (#80 smallest to #1 largest), and letter (A smallest to Z largest) — three separate scales that cover different size ranges at different levels of granularity.
What is the difference between a number drill and a letter drill?
A number drill uses a numeric designation from #80 (0.0135", smallest) to #1 (0.228", largest) — counting down as the size goes up. A letter drill picks up roughly where number drills leave off, running A (0.234", smallest) to Z (0.413", largest) — alphabetically up as size increases.
Which is bigger, a number 1 drill or a number 80 drill?
A #1 drill (0.228") is bigger than a #80 drill (0.0135"). Number drills count down as size increases, which is the reverse of what the numbers might suggest at first.
How do you find the closest drill size to a decimal measurement?
Compare the target decimal against every size across all three systems and pick whichever single size has the smallest absolute difference. The size-finder calculator above does exactly that automatically.
Why are there three overlapping drill sizing systems instead of one?
The systems developed at different times for different needs. Fractional sizing is oldest and simplest, while number and letter gauges were introduced specifically to fill in the finer steps that plain fractions leave between 1/64" jumps — which matters most for tap drill sizing and close-tolerance holes.
Drill Size References
Every other chart in this cluster, one click away.