Letter drill bit sizes
Letter sizes A–Z fill gaps above many number sizes — the stamps you see on denser jobber sets and tap charts. Each row lists the letter, decimal inch, and millimeter equivalent so you can match a drawing without opening the full four-system chart. Need fractional, metric, or number? Use the links below or the flagship chart.
Full size chart · Size finder · Drill hub · Fractional · Metric · Number
How to use this table
Scan the Size column for the letter stamp on the bit or the letter on a tap-drill chart. Decimal inch and mm are nominal workshop values. Click Find on a row to open the size finder with that label preselected — set matching stays on the tool page.
Letters increase in diameter from A toward Z (opposite the number-gauge intuition). If 1/4" is slightly tight and you need a few thousandths more, letter F is a common neighbor — always confirm with decimal columns rather than memory. For screw clearance versus pilot, diameter alone still does not choose thread engagement; pair with the screw→drill tool when that is the job.
Typical flow: read the letter from the chart or stamp, find the row, note mm if you hold a metric box, then Find → size finder. Critical fits need a gauge. Do not treat every row as a buy button.
Drill bit set (nearest shop staple)
Letter sizes are rarely listed on consumer set lids. One optional fractional set is for general shop use — use Find on a row when you want set matching tied to that letter.
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FAQ
What are letter drill bit sizes?
Letter sizes run A through Z and sit above many number sizes in diameter. They fill gaps that fractional steps skip, especially in metalworking and tap-drill charts.
Which letter sizes are on this page?
This table lists 8 common letters (A, C, F, J, N, R, U, Z). It is not every A–Z size — use the decimal column to compare with a fraction you already own, or open the full four-system chart.
How do letter sizes compare to fractional bits?
Use the decimal-inch and mm columns. Example: letter F is about 0.2570" — slightly larger than 1/4" (0.2500"). Pick letter when a fraction is slightly undersize for the hole you need.
Can I buy a kit from this page?
No product grid here. Click Find on a row to open the size finder. Many consumer sets print fractional lids only; if the set does not list your letter, the finder notes that instead of claiming a match.
Sources
Letter diameters follow common published letter-drill indexes. Values are nominal. Confirm critical fits on the bit or with a gauge.
See the site disclaimer for limits of use.