ENGINEERING REFERENCE CHART
Drill Bit Size Chart
A drill bit size chart lists diameters across fractional inch, number (#1–#80), letter (A–Z), and metric (mm) systems with decimal-inch and millimeter equivalents. Built on ASME B94.11M and Machinery’s Handbook, it helps you read any drill size at a glance for machining and custom parts.
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Fractional / Number / Letter / Metric
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ASME B94.11M & Machinery’s Handbook
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CNC Machining & Custom Parts
Drill Sizing Systems
Four standard systems, one reference
Fractional
1/64″ – 1″
Number
#1 – #80
Letter
A – Z
Metric
mm sizes
Decimal & mm equivalents
in = mm / 25.4 | mm = in × 25.4
REFERENCE GUIDE
What Is a Drill Bit Size Chart?
A drill bit size chart cross-lists diameters in four standard sizing systems — fractional inch, number, letter, and metric — so you can find decimal-inch and millimeter equivalents at a glance. Engineers and machinists use it to specify holes, convert callouts, and pick the right tap drill.
Four systems coexist because inch drills come in coarse fractional steps, while number and letter series fill the gaps where precision matters — above all for tapped holes and reamed bores.
Fractional & Number Sizes
Fractional sizes in 1/64″ increments (1/16″, 5/64″, 3/32″, …) are the US shop standard. Number sizes #1–#80 (ASME B94.11M) get smaller as numbers increase — #1 = 0.2280″, #80 = 0.0135″ — and are the standard tap drills for small US threads.
Letter & Metric Sizes
Letter drills A–Z (0.2340″ – 0.4130″) bridge the gap between #1 and fractional sizes, serving as tap drills for medium US threads. Metric sizes in mm (0.1 mm steps below 10 mm, then 0.5/1 mm steps) are the global standard for ISO designs.
How to Use the Chart
Specify holes on drawings, convert metric callouts to nearest inch drill (or vice versa), and pick the correct tap drill before threading. This chart gives you decimal-inch and mm equivalents for every size in all four systems.
DRILL SIZE REFERENCE
Drill Bit Size Chart: All Sizes in One Table
The table below lists common sizes from every system in ascending diameter, so you can find a size and read its decimal-inch and millimeter values directly. This is the same data a machinist or CNC programmer checks before every drilling job.
Representative rows shown. The full #1–#80 number series, A–Z letters, complete fractional and metric ranges ship in the page’s data file, spot-checked against ASME B94.11M and Machinery’s Handbook.
| Size | Decimal (in) | Diameter (mm) | Category |
|---|---|---|---|
| #80 | 0.0135 | 0.343 | Number |
| 1/64 | 0.0156 | 0.397 | Fractional |
| #60 | 0.0400 | 1.016 | Number |
| 1/16 | 0.0625 | 1.588 | Fractional |
| #50 | 0.0700 | 1.778 | Number |
| 3/32 | 0.0938 | 2.381 | Fractional |
| #40 | 0.0980 | 2.489 | Number |
| 1/8 | 0.1250 | 3.175 | Fractional |
| #29 | 0.1360 | 3.454 | Number |
| #21 | 0.1590 | 4.039 | Number |
| 3/16 | 0.1875 | 4.763 | Fractional |
| #10 | 0.1935 | 4.915 | Number |
| #7 | 0.2010 | 5.105 | Number |
| A | 0.2340 | 5.944 | Letter |
| 1/4 = E | 0.2500 | 6.350 | Fractional / Letter |
ASME B94.11M & Machinery’s Handbook
This chart follows the recognized twist drill standard and the reference data from Machinery’s Handbook. Number sizes per ASME B94.11M; 1 in = 25.4 mm exactly.
Uses in Machining & Threading
Use the chart to specify holes on drawings, convert metric callouts to the nearest inch drill, and pick the right tap drill for US and ISO threads before machining.
READING GUIDE
How to Read a Drill Bit Size Chart
Reading a drill bit size chart takes three steps: identify the system, find the size, and read across to the decimal-inch and millimeter values. Here is a worked example.
01
Identify the System
The designation tells you which series you’re in. #7 is a number drill; H is a letter drill; 1/4″ is fractional; 6.5 mm is metric. A size like “7” without the hash mark is ambiguous — on a chart it usually means a #7 number drill, but always confirm.
02
Find the Size
Suppose your drawing calls out a #7 drill. In the chart above, find the row whose size column reads “#7″: it shows 0.2010” and 5.105 mm.
03
Read Across
Read across the table to find the equivalent in another system. The nearest metric bit to a #7 is 5.1 mm (or 5.0 mm if you need a standard preferred size); the nearest fractional bit just above it is 13/64″ (0.2031″) and the next below is 3/16″ (0.1875″).
Example: 6.5 mm Callout
If the callout is metric, say 6.5 mm, the chart shows 0.2559″, and the nearest standard inch drills are E (0.2500″) just below and F (0.2570″) just above. Use the conversion factor 1 in = 25.4 mm to sanity-check any entry.
6.5 mm
0.2559 in
E drill
0.2500 in (just below)
F drill
0.2570 in (just above)
EQUIVALENCE REFERENCE
Exact Equivalences Worth Knowing
A few sizes line up exactly between systems, and memorizing them saves time every day.
| Size | Decimal (in) | mm | Why it matters |
|---|---|---|---|
| #7 | 0.2010 | 5.105 | The tap drill for a 1/4-20 UNC thread |
| #21 | 0.1590 | 4.039 | The tap drill for a #10-32 thread |
| E = 1/4″ | 0.2500 | 6.350 | Two systems, one exact size |
| #29 | 0.1360 | 3.454 | The tap drill for an #8-32 thread |
| Z | 0.4130 | 10.490 | Largest letter drill |
| #80 | 0.0135 | 0.343 | Smallest common number drill |
Note: The reverse direction works the same way: find the decimal-inch value, then read the size designation from the row. Remember that a 1/4″ drill is 6.350 mm, not 6.4 mm. When you convert a fractional size to metric for a thread fit, rounding to 6.4 mm changes a clearance condition you may not be able to see on a drawing.
Which Size System Should You Use?
If you design in metric, specify metric drills. If your part uses US hardware or threads, match the system of the fastener. Mixing systems is fine as long as you convert exactly, and specifying a standard size in the system you choose avoids custom tooling.
| System | Range | Best for |
|---|---|---|
| Fractional (1/64″ steps) | 1/64″ up to 1 3/4″+ | US hardware, general shop drilling, larger holes |
| Number (#1–#80) | 0.0135″–0.2280″ (0.343–5.791 mm) | Small precision holes and US tap drills |
| Letter (A–Z) | 0.2340″–0.4130″ (5.944–10.490 mm) | Medium US tap drills, gap sizes between fractions |
| Metric (mm) | 0.1 mm steps and up | ISO/Metric designs, global manufacturing |
Mixing systems is fine as long as you convert exactly. Specifying a standard size in the system you choose avoids custom tooling. Non-standard drill sizes are a real cost in CNC machining: a custom drill can add 30–200% to tooling cost and 5–14 days to lead time, which is why this chart and our CNC machining team both steer you to standard sizes.
CNC DRILLING REFERENCE
Drill Sizes and Tapped Holes
The most common reason anyone looks up a drill size is tapping. Drill it too small and the tap breaks; too large and the thread strips. The recommended size is a drill that leaves about 75% thread engagement, the industry-standard compromise that delivers roughly 90–95% of a full thread’s strength while keeping tapping torque and tap breakage low.
The rule of thumb at 75% engagement: Metric threads use tap drill ≈ major diameter − pitch. For M8×1.25, that’s 6.75 mm, rounded to the standard 6.8 mm drill. US Unified threads use tap drill ≈ major diameter − (1 / TPI). For a 1/4-20, that’s 0.200″, which is a #7 drill.
Metric Threads
Tap Drill Formula (ISO)
Tap drill ≈ major diameter − pitch. For M6×1.0, use 5.0 mm. For M8×1.25, use 6.8 mm. For M10×1.5, use 8.5 mm. For M12×1.75, use 10.3 mm. The full mapping lives on the companion Tap & Drill Size Chart, and you can get an exact answer for any thread, engagement, and material with the Tap Drill Size Calculator.
US Unified Threads
Tap Drill Formula (UNC/UNF)
Tap drill ≈ major diameter − (1 / TPI). For #10-32, use #21 (0.1590″). For 1/4-20, use #7 (0.2010″). For 5/16-18, use Letter F (0.2570″). For 3/8-16, use 5/16″ (0.3125″). This drill bit size chart supplies the sizes; the companion pages do the threading math.
Clearance vs. Tap Holes
Common Sizes Side by Side
A tap hole takes the thread. A clearance hole lets a fastener pass through freely. Confusing the two is one of the most common drawing errors. For M8×1.25, tap drill is 6.8 mm; clearance drill is 9.0 mm. For 1/4-20, tap drill is #7 (0.2010″); clearance drill is 9/32 (0.2812″).
Thread Engagement & Verification
The 75% engagement rule delivers roughly 90–95% of a full thread’s strength while keeping tapping torque low. Tap-drill values shown are reference data from standard charts and must be verified against the cited standards before publishing. If a thread must engage through a stack of parts, each clearance hole needs the larger drill; only the tapped part gets the tap drill. Our CNC drilling engineers see this mistake on drawings every week and flag it in the free DFM review on every quote.
75%
Thread Engagement Standard
#7
Tap Drill for 1/4-20
6.8 mm
Tap Drill for M8×1.25
Drill Bit Size Questions
Drill Bit Sizes FAQ
Number and letter drills are two separate sizing series that cover different ranges. Number sizes (#1–#80) run from 0.0135″ to 0.2280″, making them the smallest common drills, and serve as tap drills for small US threads. Letter sizes (A–Z) run from 0.2340″ to 0.4130″, filling the gap between the largest number drill and the fractional series. Both were historically derived from 19th-century wire-gauge systems; the designations are standards, not measurements you can derive.
Number drill sizes are inverted: the larger the number, the smaller the bit. #1 is the largest (0.2280″, 5.791 mm) and #80 is the smallest (0.0135″, 0.343 mm). The convention comes from the wire-gauge system the numbers were borrowed from. This inversion is the most common source of confusion when reading a drill bit size chart, so always double-check with the decimal value rather than trusting the number order.
Multiply the inch size by 25.4. For example, 1/4″ = 0.2500 × 25.4 = 6.350 mm; 1/8″ = 0.1250 × 25.4 = 3.175 mm. Use the exact factor and round only at the end; rounding 6.350 mm to 6.4 mm early can change a thread fit. A drill bit size chart like this one does the conversion for you; this page also links to a drill bit decimal equivalent chart for the full decimal-inch and millimeter series.
A #7 drill is 0.2010″ (5.105 mm). It’s best known as the tap drill for a 1/4-20 UNC thread at 75% engagement. Its closest metric equivalent is a 5.0 or 5.1 mm drill, depending on whether you need a standard preferred size.
Use a #7 drill (0.2010″, 5.105 mm) for 75% thread engagement. The rule of thumb, major diameter minus 1/TPI (0.250 − 1/20 = 0.200″), lands on the #7. The full mapping for every UNC, UNF, and metric thread lives on the Tap & Drill Size Chart, and the Tap Drill Size Calculator lets you adjust engagement for different materials.
Jobber drills are the standard general-purpose length, long enough to reach most holes while staying rigid. Screw machine (stub) drills are shorter and stiffer, which makes them drill more accurately and resist deflection, at the cost of shorter reach. For CNC work, a screw machine length drill is often the better choice for shallow holes because it holds size better; use a jobber drill when you need the extra reach.
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