About this tool
This converter turns any number drill (#1–#80), letter drill (A–Z), fractional drill (1/64" to 1"), or metric drill into decimal inches and millimetres, and shows the nearest drills in every other series with the exact difference. It also works the other way: enter a hole size from a drawing or tap chart and it finds the closest drills you can actually buy.
How to use
- Choose Drill size → equivalents to convert a drill you have, or Any size → nearest drills to start from a dimension.
- To convert, pick the series — number, letter, fraction, or metric — and the drill size, such as #7, F, or 13/64".
- To find drills, choose millimetres or inches and type the size; fractions such as 13/64 and decimals such as 0.201 both work.
- Read the equivalents: a same-size match is marked, otherwise you see the nearest smaller and larger drill and how far each is from your size.
- Use the chart below to scan every number, letter, and fractional drill in size order, or select one to open its equivalents.
The four drill size series
Inch drills come in three series. Number (wire gauge) drills run backwards, from #80 (0.0135") up to #1 (0.228"). Letter drills continue above them, from A (0.234") to Z (0.413"). Fractional drills step in 64ths of an inch. Number and letter sizes fill the gaps between fractions, which is why so many tap drills are numbers or letters. The ASME B94.11M twist drill standard lists these sizes; they are not generated by a formula. Metric drills are named by diameter and are commonly sold in 0.1 mm steps up to 13 mm and 0.5 mm steps above.
Reading the equivalents
Each equivalent shows whether it is the same size, larger, or smaller, by how much in thousandths of an inch and millimetres, and which one is closest. Some pairs are identical: an E drill and a 1/4" drill are both 0.250". Most are not. A #7 drill is 0.2010" (5.105 mm): 5.1 mm is 0.005 mm smaller and 13/64" is 0.054 mm larger. A 13/64" drill (5.159 mm) sits between #7 and #6, and its nearest metric drill is 5.2 mm.
Finding a drill for a metric or decimal size
Switch to Any size → nearest drills when a drawing or tap chart gives a size you do not have. For a 6.8 mm hole (the usual tap drill for M8 × 1.25), the nearest inch drills are letter H (0.044 mm smaller) and 17/64" (0.053 mm smaller). For 10.2 mm (M12 × 1.75) they are letter Y (0.062 mm larger) and 13/32" (0.119 mm larger). The tool shows the nearest drill on each side so you can choose whether to go slightly over or under.
Larger or smaller: which way to round
For a clearance hole, round up so the bolt passes freely. For a tapped hole, use the tap drill the chart specifies; a smaller substitute makes the tap harder to turn and a larger one weakens the thread. For a hole that will be reamed, drill slightly under the finished size. Twist drills often cut a little oversize, more so with a worn or unevenly sharpened bit or in soft material, so measure a test hole when the size matters.
Privacy
The sizes you choose or type are processed in your browser. This tool does not send them to a server or store them after you close or reload the page.
Continue this workflow
Use the adjacent tool when the next step calls for a different input, output, or method.
Assumptions and limitations
- Covers number drills #1–#80, letter drills A–Z, fractional drills from 1/64" to 1", and metric drills from 0.3 to 25 mm. The rarely used number sizes #81–#97 are not included.
- Metric drills are assumed to come in 0.1 mm steps to 13 mm and 0.5 mm steps above. Not every step is stocked everywhere.
- All sizes are nominal. Drill manufacturing tolerances and the oversize a drill actually cuts are not included.
- This converts drill sizes only. Use a tap drill chart for thread engagement and a clearance hole chart for bolt fits.
Sources and standards
These authoritative references were used to verify the method and guidance on this page.
Frequently asked questions
What size is a #7 drill in mm?
A #7 drill is 0.2010" or 5.105 mm. The nearest metric drill is 5.1 mm, just 0.005 mm smaller, and the nearest fractional drill is 13/64" (0.2031").
Is a letter E drill the same as 1/4"?
Yes. Both are exactly 0.250" (6.350 mm).
Which number drill is the largest?
Number drills get smaller as the number rises: #1 is the largest at 0.228" and #80 the smallest at 0.0135". Letter drills continue upward from A (0.234").
What is the metric equivalent of a 13/64" drill?
13/64" is 0.2031" or 5.159 mm. The closest metric drill is 5.2 mm, about 0.04 mm larger; 5.1 mm is about 0.06 mm smaller.
What inch drill can I use for a 6.8 mm hole?
Letter H (0.266", 6.756 mm) is the closest, about 0.04 mm smaller. 17/64" (6.747 mm) is the closest fractional drill.
Why do number and letter drills exist?
They fill the gaps between 64ths of an inch, so holes such as tap drills can be drilled close to the ideal size. A 1/4-20 tap, for example, traditionally uses a #7 drill.