Drill Bit Size Chart: Metric, Fractional & Number Sizes Converted

Metric to Fractional and Gauge Drill Converter

The Four Drill Size Series

Those four cards contain the two reversals that catch people out. Number drills run backwards, #80 is the smallest at 0.0135" and #1 the largest at 0.228", while letter drills run forwards from A at 0.234" to Z at 0.413". A #1 and an A are adjacent sizes, though they look like they come from opposite ends of anything. Metric drills, by contrast, simply state their diameter, which is why converting a metric drawing dimension to an inch drill is mostly a matter of the millimetre to inch conversion plus a lookup. The same metric-against-inch problem shows up in fastener sizes, where the socket conversion calculator handles it for tool openings rather than hole diameters.

Number Drill Sizes: #80 to #1

Reading down the table shows the steps are not even: they crowd tightly at the top of the range where #61 to #56 move by thousandths, then open out below #30 where a single number can be worth several thousandths. That uneven spacing is historical rather than designed, which is why the number series resists any formula and has to be looked up. The sizes that matter most in practice are the tap drills, #7 for 1/4-20 and #29 for 8-32 among them, and those are listed against their threads on the tap drill size chart.

Letter Drill Sizes: A to Z

The letter series covers a narrow band of 0.234" to 0.413", barely three sixteenths of an inch in total, but it covers that band with 26 steps, which makes it the finest-grained option near the quarter to three-eighths range. That is precisely the range most inch tap drills live in, which is why letters appear so often on a tap chart even though it is the series people know least well. E is worth remembering on its own: at 0.250" it is exactly a quarter inch.

Fractional Drill Sizes: 1/64" to 1"

Every step here is exactly 1/64", or 0.0156", which makes this the only series with predictable spacing, and also the reason it is too coarse for small work. At 1/16" a single 64th step is a 25 per cent change in diameter; at 1" it is 1.6 per cent. Converting these fractions to decimals by hand is the same arithmetic as reading a socket size, covered on the fraction to decimal chart.

Metric Drill Sizes: 0.5mm to 25mm

Half-millimetre steps work out to about 0.0197", slightly larger than a 64th, so a metric index is marginally coarser than a fractional one across the shared range. Sets intended for tapping usually add tenth-millimetre sizes through the small diameters, because metric tap drills land on values like 6.8mm and 10.2mm that a half-millimetre set skips entirely. A gap worth checking before you start a threading job.

Choosing the Right Drill Size

Two habits prevent most drilling problems regardless of size. Centre-punch before you start, because a drill point wanders across a flat surface and no amount of pressure straightens it afterwards. And match speed to diameter, large drills want slow speeds and steady feed, small drills want high speeds and light pressure, and reversing that pair is how bits get burned or snapped. When the hole is finished and a fastener goes into it, the socket that tightens it is sized from the head rather than the thread, which the bolt size chart maps out and the socket conversion calculator converts between systems.

More Conversion Tools

The rest of the fastener, thread and drill references on this site, every one of them is one click away from here.