Drill Bit Size Chart — Decimal Equivalents

Drill Bit Size Chart

Complete printable drill bit size chart with decimal inch and millimeter equivalents for all four standard series: number (#97–#1), letter (A–Z), fractional (1/64"–2"), and metric (1.0–50.0 mm). All values verified against Machinery’s Handbook, 29th Edition.

A drill bit size chart is a reference table listing every standard twist drill diameter with its decimal inch and millimeter equivalents. Drill bits are organized into four series defined by ANSI/ASME B94.11M-1993: number drills (#97–#1), letter drills (A–Z), fractional drills (1/64" increments), and metric drills (millimeter sizes per ISO 235). This chart includes all four series sorted by diameter, with search, filtering, and the nearest fractional equivalent for every size.

Drill bit sizes: number, letter, fractional, and metric drills with decimal equivalents per ANSI/ASME B94.11M-1993

Source: Machinery’s Handbook 29th Ed. — Drill sizes: pp. 868–890, Table 1 (ANSI/ASME B94.11M-1993). Metric drill sizes per ISO 235.

How to Use This Drill Bit Size Chart

Use the Series dropdown to filter by number, letter, fractional, or metric drills, or leave it on “All” to see every standard drill size sorted by diameter. Type a drill designation or decimal value into the Search field to find a specific size instantly. Click any table value to copy it to your clipboard. Click a row to highlight it for reference.

If you need to find the correct drill for a tapped hole, see our tap drill size chart, which maps thread sizes to drill diameters at adjustable engagement percentages.

For pipe threads, see our NPT pipe thread size chart.

Example: Finding the Right Drill for a 0.201" Hole

You need to drill a 0.201" hole in a steel part. Type “0.201” in the search field — the chart shows a #7 drill at 0.2010". If you only have fractional drills, the nearest fraction column shows 13/64" (0.2031"), which is 0.002" oversize. For this application, the #7 number drill is the better match.

For a metric example: you need a 6.5 mm hole. Search “6.5” to find the 6.5 mm metric drill (0.2559"). The nearest fractional drill is 1/4" (0.2500"), which is about 0.006" (0.15 mm) undersize — the metric drill is the correct choice when precision matters.

Drill Size Terminology

Number Drills (#97–#1)
Wire-gauge-based sizes defined by ANSI/ASME B94.11M. #97 is the smallest (0.0059") and #1 is the largest (0.2280"). Number drills provide the finest increments in the small-diameter range, making them essential for tap drilling, precision holes, and miniature work. The numbering runs opposite to diameter: a higher number means a smaller drill.
Letter Drills (A–Z)
Twenty-six sizes from A (0.2340") to Z (0.4130") that bridge the gap between the largest number drill (#1 at 0.2280") and fractional drills. Letter drills are commonly used as tap drills for thread sizes in the 1/4"–7/16" range.
Fractional Drills
Drills sized in 1/64" increments, ranging from 1/64" (0.0156") upward. These are the most widely stocked drill sizes and the default choice for general-purpose work. Above 1", only selected fractional sizes are standard.
Metric Drills
Drills sized in millimeters per ISO 235. Standard metric drills are available in 0.05 mm increments from 1.0–2.5 mm, 0.1 mm increments from 2.5–10.0 mm, and 0.5–1.0 mm increments above 10 mm. Metric drills are essential when working with ISO-standard drawings or tapping metric threads.

Drill Bit Types & Materials: How to Choose

The diameters above are the same whatever the drill is made of — but the material and construction set how fast you can run, how long the tool lasts, and how deep you can go in one shot. Drills split into two families: one-piece tools ground from solid stock (high-speed steel, cobalt high-speed steel, and solid carbide), and tools that carry a replaceable cutting element on a reusable body (spade, indexable-insert, and modular drills). Machinery’s Handbook notes that high-speed steel and carbide are the two most common cutting-tool materials, which is why most published feed-and-speed data covers them.

Comparison of drill bit types by material and construction, key strength, best use, and relative cutting speed.
Drill Type Material & Construction Key Strength Best For Cutting Speed (relative)
High-Speed Steel (HSS) One-piece HSS, hardened to 63–65 Rc; keeps its edge up to about 1,000–1,100°F (540–595°C) and can be reground many times. Tough and forgiving — the standard twist-drill material for its mix of red hardness and wear resistance. General-purpose holes, drill presses, and less-rigid setups. Baseline.
Cobalt HSS (5–12% Co) High-speed steel with 5–12% cobalt for higher hot hardness; slightly more brittle, so the edge is a little more chip-prone. Higher red hardness than plain HSS; cuts heat-resistant alloys and materials harder than Rockwell 38 C. Tough, hard, or abrasive workpieces and heavy roughing. Faster than HSS, not equal to carbide (132 SFM in non-alloy steel on Haas’s chart).
Solid Carbide One-piece cemented (sintered) tungsten carbide; far harder than HSS but more brittle, so it needs a rigid setup. Holds hardness past 1,400°F (760°C), so it runs at very high cutting speeds with long tool life. Production drilling on rigid CNC machines. High — 263–362 SFM in non-alloy steel on Haas’s chart, about 2–3× a cobalt HSS drill.
Through-Coolant (TSC) Carbide Carbide drill with internal coolant holes that direct cutting fluid under pressure to the cutting edges. Cools the tip and flushes chips out of deep holes, so you can drill in one pass instead of pecking. Deep holes — beyond about 3× the drill diameter (3XD). Carbide-class, sustained in deep holes.
Modular / Replaceable-Tip Carbide Haas’s “Modular Drills” line: renew the worn cutting end rather than the whole drill — the same idea as spade and indexable drills. Carbide cutting performance with less carbide thrown away when the cutting end wears. Repeat production holes; Haas advises through-coolant beyond 3XD. Solid-carbide class — Haas lists ~246–459 SFM starting for steel groups (up to 591 max).
Indexable-Insert Drill Steel body with replaceable carbide inserts; index to a fresh edge, then discard the insert when all edges dull. Highest cutting speeds of any drill type; no regrinding. Larger-diameter holes in rigid setups; with through-coolant, well beyond 2× diameter (Haas adjusts feed/speed out to 4–5× D). Highest — about 600 SFM starting in steel on Haas’s chart (up to 930).
Carbide Spade Drill Flat replaceable blade locked in a holder by a retaining screw; blade can be HSS, cast nonferrous, or carbide-tipped, with a 130° point. Economical large and deep holes; a shorter chisel edge lowers thrust. Deep holes and larger diameters — the cheap replaceable blade is the only wear part. About 10–15% below an equivalent twist drill; needs a heavy feed.

Source: Machinery’s Handbook 29th Ed. — drill and cutting-tool materials pp. 1008–1010, cobalt HSS p. 896, cemented carbides pp. 785–790, spade drills pp. 1074–1078, indexable drills pp. 1059–1071, cutting fluids pp. 1182–1187. Representative cutting speeds from Haas Tooling Carbide, Cobalt, Modular, and Indexable drill speeds-and-feeds charts.

Carbide holds its hardness to about 1,400°F (760°C) — well past the ~1,000–1,100°F (540–595°C) limit of high-speed steel — so it runs much faster, and a thin PVD coating adds another 20–50% on top of uncoated carbide. For steel and stainless, TiAlN and AlTiN dominate (Haas coats its carbide drills in TiAlN): in the cut, their aluminum oxidizes into a thin, self-renewing aluminum-oxide (Al₂O₃) skin that resists heat and oxidation far better than older TiN. Aluminum is the exception — those aluminum-bearing coatings build up on it, so aluminum drills run uncoated/polished or use a low-adhesion coating like ZrN, TiB₂, or diamond-like carbon (DLC). The trade-off is brittleness: carbide needs a rigid setup, while tougher HSS tolerates flex, hand-feeding, and interrupted cuts. For holes past about three diameters deep, chip evacuation drives the choice — clear the chips with a peck cycle, or use a through-coolant drill that pushes fluid straight to the tip.

Once you’ve settled on a drill type, set its spindle speed and feed with our drilling feed rate calculator. For the milling side, our chip load calculator converts between chip load and feed rate.

Frequently Asked Questions

What are number, letter, and fractional drill bit series?

Drill bits are organized into four standard series. Number drills (#97 through #1) are wire-gauge-based sizes that provide the finest diameter increments, especially in the small-diameter range below 0.228". Letter drills (A through Z) cover diameters from 0.234" to 0.413", bridging the gap between number and fractional drills. Fractional drills are sized in 1/64" increments (e.g., 1/16", 3/32", 1/8") and are the most commonly stocked in shops. Metric drills are sized in millimeters per ISO 235. All four series are defined in ANSI/ASME B94.11M-1993.

How do I convert drill bit sizes to decimal inches?

Number and letter drill sizes do not follow a simple mathematical formula — each size has a specific decimal equivalent defined by ANSI/ASME B94.11M-1993 (see the chart above). For fractional drills, divide the fraction: a 7/32" drill is 7 ÷ 32 = 0.2188". For metric drills, divide the millimeter size by 25.4: a 5.5 mm drill is 5.5 ÷ 25.4 = 0.2165". This chart shows the decimal equivalent for every standard drill size.

What size drill bit do I need for a pilot hole?

Pilot hole size depends on the application. For machine screws with tapped threads, use a tap drill chart to find the correct drill for your thread size and engagement percentage — see our tap drill size chart for the complete lookup table. The drill bit size chart above helps you identify the nearest standard drill once you know the target diameter.

What is the difference between a drill size chart and a tap drill chart?

A drill bit size chart is a reference of all standard drill diameters organized by series (number, letter, fractional, metric) with their decimal and millimeter equivalents. A tap drill chart tells you which specific drill to use before tapping a particular thread size — it maps thread specifications (like 1/4-20 UNC) to the correct drill diameter for a target thread engagement percentage. This page is a drill size chart; for tap-specific lookups, see our tap drill size chart.

What are the standard metric drill bit sizes?

Standard metric drill sizes are defined by ISO 235. Common sizes range from 1.0 mm to 50.0 mm. From 1.0 to 2.5 mm, drills are available in 0.05 mm increments. From 2.5 to 10.0 mm, increments widen to 0.1 mm. Above 10 mm, increments are 0.5 mm up to 32 mm, then 1.0 mm up to 50 mm. Use the Series filter above set to “Metric” to view the complete list with inch equivalents.

What is the difference between HSS and carbide drill bits?

The core difference is hardness versus toughness. Cemented carbide is much harder than high-speed steel and keeps cutting at higher temperatures, so it runs faster and lasts longer — but it is brittle and needs a rigid machine. HSS is softer yet tough and forgiving, which is why it stays the standard for general-purpose twist drills and less-rigid setups. Cobalt HSS sits between them, adding 5–12% cobalt for more heat resistance on harder or abrasive materials. Choose HSS or cobalt HSS for one-offs, harder materials, and conventional machines; choose carbide for production speed and tool life on rigid CNC equipment.

What is a through-coolant (TSC) drill bit?

A through-coolant drill — also called a through-spindle coolant, or TSC, drill — has internal channels that carry cutting fluid under pressure straight to the cutting edges. The fluid cools the tip and flushes chips back out of the hole, which matters most in deep holes. Machinery’s Handbook advises clearing chips once a hole passes about three drill diameters deep, and Haas recommends through-coolant for holes deeper than three times the drill diameter (3XD). Without it, you generally have to peck-drill to clear the chips, which is slower.

What is a modular drill bit?

A modular drill renews the worn cutting end rather than the whole tool — the same replace-the-cutting-element idea behind spade and indexable drills. Haas sells them as its Modular Drills line; on Haas’s chart they run at solid-carbide cutting speeds (Haas lists starting values of about 246–459 SFM for steel groups, up to 591 SFM max), and Haas recommends through-coolant for holes deeper than three times the drill diameter (3XD). They suit repeat production holes where replacing the cutting end beats scrapping a whole solid-carbide drill.

When should I use a spade drill?

Spade drills are a low-cost way to make larger and deep holes. A flat, replaceable blade locks into a holder with a retaining screw, so you swap the inexpensive blade instead of a whole drill, and the blade can be high-speed steel, cast nonferrous metal, or carbide-tipped. The 130° point — versus 118° on a twist drill — and a shorter chisel edge lower the thrust and push chips toward the flutes. Run a spade drill about 10–15% slower than an equivalent twist drill and feed it heavily so the chips keep breaking.

How much faster can carbide drills run than HSS?

Quite a bit. On Haas’s own charts — a fair, like-for-like comparison in the same material — a solid-carbide drill cuts non-alloy steel at 263–362 SFM versus 132 SFM for a cobalt high-speed-steel drill, about two to three times faster. That edge comes from carbide holding its hardness to roughly 1,400°F (760°C), well past the ~1,000–1,100°F (540–595°C) limit of HSS. A thin PVD coating — usually TiAlN or AlTiN on carbide drills — adds another 20–50% on top of uncoated carbide, and indexable carbide-insert drills go higher still — Haas lists about 600 SFM starting for steel. The exact numbers depend on the material, coating, machine rigidity, and coolant.

Related Calculators

References

  • Oberg, E. et al. Machinery’s Handbook, 29th Edition, Industrial Press, 2012, pp. 868–890. ANSI Straight Shank Twist Drill sizes (number, letter, fractional, and metric drill diameters per ANSI/ASME B94.11M-1993).
  • ANSI/ASME B94.11M-1993 — Twist Drills (Inch and Metric). Defines standard drill diameters for number, letter, fractional, and metric series.
  • ISO 235 — Twist drills with parallel shanks; metric sizes. Defines the standard metric drill size series.
  • Oberg, E. et al. Machinery’s Handbook, 29th Edition, Industrial Press, 2012 — drill and cutting-tool materials (pp. 1008–1010), cobalt high-speed-steel drills (p. 896), spade and indexable drills (pp. 1059–1078), cemented carbides and coatings (pp. 785–790, 1097), and cutting fluids (pp. 1182–1187).
  • Haas Automation, Inc. (Haas Tooling) — Carbide, Cobalt Jobber, Modular, and Indexable Drill speeds-and-feeds charts. Source of the representative carbide-vs-cobalt cutting speeds and the through-coolant (3×D) recommendation.

Data last verified: May 2026

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