Step drill bit sizes: the charts, inch and metric

A step drill bit is a cone with stacked cutting diameters: each step you pass through opens the hole to that step's size, so one bit replaces a drawer of twist drills in thin material. This page prints the actual size charts, Irwin Unibit and Milwaukee for inch steps, the 4-12, 4-20, and 4-32mm cones for metric, with the published capacities per material. Every row cites two or more named sources; nothing here is converted between unit systems, so the inch tables stay inch and the metric tables stay metric.

What a step drill bit is for

The use case is thin material and many sizes: electrical panels, sheet metal enclosures, PVC, and deburring holes you already drilled. Irwin positions the Unibit for thin materials such as mild steel, copper, brass, aluminum, plexiglas, wood, and laminate, with no need for center punching ^{[6] [5]}. Milwaukee aims its step bits at small and large diameter holes in steel and plastic, optimized for cordless drills run in high speed gear ^{[2] [1]}. What a step bit is not for is thick stock and precise depth: past the step thickness it is opening, not drilling, and for holes in thick wood a brad point or Forstner bit is the right tool, covered on our drill bits by material page.

Irwin Unibit step chart (inch sizes)

The classic Unibit numbering, with the hole count, increment, and shank for each. Sizes and steps below are confirmed by Irwin's own set packaging copy and by the Aircraft Spruce Unibit reference page, which adds the shank sizes ^{[5] [6] [7]}.

Row sources: [5] Irwin 3-piece set packaging via Blaisdells, [6] the same set via True Value, [7] Aircraft Spruce Unibit page, [8] Acme Tools listing for the Irwin 10234 (#4), [10] Lowe's listing for the #4, [1] Milwaukee's category table for the equivalent size ladder. Aircraft Spruce lists the #2 with 3/8 in step thickness and the #3 and #4 with 1/8 in step thickness; Acme Tools confirms the 1/8 in step thickness and 3/8 in shank on the #4.

Milwaukee step drill bit sizes

Milwaukee runs the same inch ladder under its own SKUs, in black oxide finish with laser-engraved reference marks and a 3-flat shank. The SKU table below is Milwaukee's own category listing, cross-checked against the individual product pages and the 4-piece set contents ^{[1] [2] [3] [4]}.

Shank values are from the individual product pages for the 9201 (1/4 in) and 9203 (3/8 in) and the Zoro set listing; the 4-piece set 48-89-9223 combines the 9201, 9204, 9207 (7/8 in single size), and 9212 for 31 hole sizes from 1/8 to 1-3/8 in. Milwaukee also lists single-size and dual-size entries (9206, 9207, 9209, 9211) omitted here because they drill one or two sizes rather than a ladder.

The two brands' ladders are the same ladder: 13 steps from 1/8 to 1/2 in, 6 from 3/16 to 1/2 in, 9 from 1/4 to 3/4 in, and 12 from 3/16 to 7/8 in appear in both tables ^{[5] [1]}. What differs is the body: Milwaukee bits are black oxide HSS with dual straight flutes, and Milwaukee claims up to 2X faster hole times, up to 4X longer life, and up to 50 percent more holes per battery charge for the line ^{[2] [3]}. Those are marketing multipliers, not measurements we can check; the geometry claims, dual flute, laser marks, 3-flat shank, are on every product page.

Metric step drill bits: 4-12, 4-20, 4-32mm

Metric cones are the import side of the family, sold mostly under seller brands in M35 cobalt or HSS with a titanium or titanium-colored finish. The step ladders below come from two independent listings of the same cones, with dimensions as published ^{[11] [12]}.

Sources: [11] Tilga M35 cone listing (step lists, M35 = 5 percent cobalt, 135 degree point), [12] Syncee cobalt cone listing (same step lists, stainless guidance), [13] Tenyun Tool's manufacturer table. Variance flag: Tenyun's table lists the 4-32mm cone as 14 steps at 102mm overall length, where the retail listings say 15 steps; the 15-step, 2mm-increment ladder is what both retail listings print, so that is what we print. Tenyun also publishes lengths for the family: 4-12 at 62mm, 4-20 at 71mm, 4-30 at 95mm, 4-32 at 102mm.

Published shanks for the cones: 4mm on the 4-12, 6mm on the 4-20, and 10mm on the 4-32 in one listing ^[11], with hex shanks also common ^[12]. A 10mm shank already asks more than a 3/8 in chuck holds, which is the practical ceiling of the family on a handheld drill.

Maximum drilling capacity by material

No manufacturer publishes one clean capacity table, so this table collects what the spec pages and seller guidance do publish, quoted verbatim in its own units. Where sources give a number for a material, it appears; where they only list the material as an application, the row says so.

Row sources: [6] True Value Irwin set listing, [7] Aircraft Spruce (1/8 in per step), [1] Milwaukee category page, [9] Aircraft Tool Supply cobalt Unibit page, [10] Lowe's #4 listing, [12] Syncee M35 cone listing, [2] Milwaukee 9201 product page, [14] Flintools metric set listing (3 mm metal, 5 mm wood and PVC). The 1/8 in per-step figure and the mm figures are quoted as published, not converted.

Step drill bit for stainless steel

Stainless is where the cheap cone fails and the alloy matters. Irwin sells cobalt M35 versions of the Unibit, #1 through #4 in the same size ladders as the HSS chart above, for extended life in tough materials ^{[9] [10]}. The M35 metric cones carry the same answer: 5 percent cobalt alloy, marketed explicitly for stainless and hardened plate ^{[11] [12]}. Technique does the rest of the work: the cone sellers' guidance is a hand drill only up to about 1 mm stainless, a bench drill for 2 to 3 mm, and RPM kept low, a few hundred, with lubricant ^{[12] [9]}. That matches the general stainless rule on our drill speed chart: slow, sharp, and wet, or the work hardens and the hole stops progressing.

Step drill bit for metal, plastic, and wood

For mild steel and sheet metal, the standard HSS or titanium-finish dual-flute bit is the tool; the finish question, black oxide versus titanium versus cobalt, is the same one we treat in full on the drill bit coatings page ^{[2] [15]}. For plastic, Milwaukee's published application is steel and plastic, and the Unibit set lists plexiglas; the failure mode in plastic is melting from too much speed and too little chip clearing, not bit wear ^{[2] [6]}. For wood, a step bit will open a clean hole in thin stock, up to the published 5 mm cone capacity, and it is the wrong geometry beyond that: it has no center spur, so it wanders and blows out exit faces where a brad point bit does not ^{[14] [6]}. The full geometry side of that argument is on the drill bits by material page.

Speed, lubricant, and technique

Two published guidance points bracket the speed question. Milwaukee designs its step bits to run in the high speed gear of a cordless drill in steel and plastic, with a Rapid Strike tip that starts fast and generates less heat ^{[2] [1]}. The M35 cone sellers say the opposite for stainless: hundreds of RPM, not thousands, or the work hardens ^[12]. Both are right, because they are about different materials; the organizing rule is on the drill speed chart, where surface speed per material converts to RPM by diameter, and for a step bit the diameter that matters is the step you are currently cutting. Lubricant on steel and stainless, dry or soapy water on plastic, pilot hole only if the smallest step exceeds about 1/4 in, since below that the tip self-starts ^{[6] [2]}.

Which step bit to buy first

• One bit only: the 3/16 to 7/8 in, 12-step size (Irwin #4 or Milwaukee 48-89-9204). It spans the sizes electrical and HVAC sheet work actually uses ^{[8] [1]}.
• Mostly small holes: the 13-step 1/8 to 1/2 in bit, whose 1/32 in increments are fine enough for screw clearance work ^{[5] [2]}.
• Metric work: the 4-20mm cone covers M5 through M8 clearance and tapping-adjacent sizes in one tool; add the 4-32mm only if you regularly open large holes in thin stock ^{[11] [12]}.
• Stainless regularly: buy the cobalt version, Irwin's cobalt Unibit or an M35 cone, and run it slow with oil ^{[9] [12]}.
• Holes bigger than 1-3/8 in: that is hole saw territory, and our hole saw sizes page picks up exactly where the step ladder ends ^[4].

Related guides

Also on this site: drill bits by material for the geometry choices in wood, metal, and tile, drill bit coatings compared for the titanium versus cobalt question on these same bits, the drill speed chart for RPM by material and diameter, the pilot hole chart for the wood-screw side of hole sizing, the tap and drill chart if the hole is headed for threads, and hole saw sizes for the large holes a step bit cannot reach.

Sources

1. Milwaukee Tool, Step Drill Bits category page (SKU, size, and step-count table): milwaukeetool.com
2. Milwaukee Tool, #1 Step Drill Bit 48-89-9201 product page (13 steps, 1/32 in increments, 1/4 in shank, steel and plastic application, high speed optimization): milwaukeetool.com
3. Milwaukee Tool, #3 Step Drill Bit 48-89-9203 product page (9 steps, 1/16 in increments, 3/8 in shank): milwaukeetool.com
4. Zoro, Milwaukee 4-piece step drill bit set 48-89-9223 listing (set contents, 31 hole sizes, shanks and lengths): zoro.com
5. Blaisdells, IRWIN Unibit 3-piece HSS set listing (bit sizes #1, #2, #3 with steps and increments): blaisdells.com
6. True Value, Irwin Unibit 3-piece set listing (same sizes, plus materials list and no-center-punch claim): truevalue.com
7. Aircraft Spruce, Unibit step drill reference (hole counts, shank sizes, step thickness, 1/8 in per-step capacity): aircraftspruce.com
8. Acme Tools, Irwin #4 Unibit 10234 listing (12 sizes, 3/16 to 7/8 in, 1/16 in increments, 3/8 in shank, 1/8 in step thickness): acmetools.com
9. Aircraft Tool Supply, Unibit cobalt step drill page (M35 cobalt Unibits #1 to #4, tough-material positioning): aircraft-tool.com
10. Lowe's, IRWIN Unibit #4 dual-flute listing (ideal for stainless steel and sheet metal, 2X faster claim): lowes.com
11. Tilga, M35 cobalt step cone drill bit listing (4-12, 4-20, 4-32mm step lists, shank diameters, 135 degree point): tilga.com.kw
12. Syncee, cobalt steel step cone drill bit listing (same cones; stainless hand-drill 1 mm and bench-drill 2 to 3 mm guidance, low RPM): syncee.com
13. Tenyun Tool, metric step drill manufacturer table (step counts and lengths; lists 4-32mm as 14 steps, a variance flagged above): tenyutool.com
14. Flintools, 4-piece 4-12/4-20/4-32/4-39mm HSS titanium set listing (3 mm metal and 5 mm wood, PVC, insulation board capacities): flintools.com
15. Zhonghuan Tools, step drill bit coatings comparison (titanium mid-range for step bits, black oxide cheaper with less life): zhonghuantools.com