The calculation
Read a designation, then compare its diameter
A fraction such as 1/4 in describes a diameter directly. A number such as #7 and a letter such as F are designations whose nominal diameters must be looked up. This chart includes #80–#1, A–Z, every 1/64 in from 1/64 to 1 in, and 0.5–25 mm in 0.5 mm increments. It does not include every metric drill sold.
Fractional inches are kept at their calculated values internally. For example, 1/64 in is 0.015625 in; a chart printed to four decimal places may display 0.0156. That display rounding is not a different nominal size.
Close does not mean interchangeable
A 1/4 in drill is exactly 6.35 mm. A 6 mm drill is smaller by 0.35 mm. Whether that difference is acceptable depends on the hole’s job, tolerance and material. A close match for a loose clearance hole may be wrong for a fitted part or a tapping operation.
The comparison ranks only the sizes included in the selected system. If the target is outside that system’s range, the nearest endpoint is still shown; that is not a suitable-tool recommendation.
Pilot, clearance and tap holes serve different jobs
A pilot hole prepares material for a screw’s threads. A clearance hole lets a screw pass through the first piece. A tap-drill hole leaves material for threads that will be cut by a tap. This diameter table does not choose any of those hole specifications. Use the matching fastener or tool manufacturer’s data.
Questions from the workbench
Why do a letter and a fraction sometimes share a diameter?
The systems overlap. For example, E and 1/4 in have the same nominal 0.250 in diameter in this reference.
Can I print just one drill system?
Yes. Select a table system, optionally search, then print the chart. Only currently visible rows are printed.
Does the nearest-size result use the table’s search filter?
No. The calculator uses its own “Compare sizes from” selector. The table search filters the reference rows independently.
Sources and reference scope
Reference data checked November 7, 2026. Number/letter nominal diameters are sourced. Fractional and metric equivalents are calculated, not a statement of a manufacturer’s inventory.
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