Step bit (Unibit) size chart: every hole by size number

A step bit drills a ladder of holes off one cone: each step you pass through opens the hole to that step's diameter. The problem is that the size numbering is a brand system, not a standard, and nobody prints the whole ladder in one place. This page does: the complete Irwin Unibit fractional system #1 through #21 with every hole each bit drills, the metric Unibits, the titanium and cobalt variants, and the cross-reference that maps Lenox Vari-Bit numbers to Unibit numbers. Every row is taken from manufacturer catalog pages, listed in the sources ^{[1] [2] [3] [4]}.

The full step-hole matrix

Every hole the seven ladder bits drill, one row per step position. Hole lists are derived from each bit's published range, increment, and hole count in the Irwin catalog ^[1], and the #1, #2, and #4 ladders match the hole lists Lenox prints for its equivalent VB1, VB4, and VB8 bits exactly ^[2]. Aircraft Spruce's retail reference corroborates the counts and shanks ^[5].

Empty cells mean the bit has no step at that position. Column headers give each bit's increment. The #1 is the fine bit of the family: its 1/32 inch steps are half the increment of every other ladder bit, which is why it packs 13 sizes into a 1/2 inch cone ^[1].

The fractional Unibit system, bit by bit

The catalog listing for the HSS fractional line, with carded item numbers so a replacement order lands on the right bit ^[1].

Two catalog details worth knowing before you order. First, the #1, #2, #10, and #12 ship with a quick-change shank, so they load directly into a hex chuck adapter ^[1]. Second, the SpeedPoint self-starting tip is included on all self-starting sizes except the #1, which is the one bit you may want to center punch first ^[1]. Aircraft Spruce's reference adds the per-step geometry: the #2 carries 3/8 inch of step thickness on its 1/4 inch shank, while the #3 and #4 steps are 1/8 inch thick, which is the practical material thickness limit per pass ^[5].

Single-hole Unibits #10 to #14

Five bits break the ladder rule and drill exactly one size each. They exist because a single diameter on a step cone body gives a full-width cutting edge and a deburring shoulder, which is what you want when every hole in the panel is the same size ^[1].

The #9 knockout bit belongs with this group in practice: it drills the same two conduit sizes electricians buy singles for, 7/8 inch for 1/2 inch conduit and 1-1/8 inch for 3/4 inch conduit, on one body ^[1]. Lenox's VB7 and VB11 cover the same two conduit sizes in its numbering ^[2].

Metric Unibits: #1M to #5M, #20M, #21M, PG29

The metric line mirrors the fractional logic in millimeters, with one twist: the #1M and #2M share the same 4 to 12 mm span but step it differently, 1 mm versus 2 mm, so the #1M drills 9 sizes where the #2M drills 5 ^[1].

The PG29M is the metric conduit specialist: PG29 knockout holes are the European enclosure standard, and 28.3 mm is the diameter that fits them, with the plain 29 mm version alongside it for nominal sizing ^[1]. The metric line also exists in titanium coated versions, #1MT through #5MT ^[1], and Lenox sells the same metric ground covered as three compact kits, MVB412, MVB1424, and MVB618 ^[2].

Titanium and cobalt variants: T and CB suffixes

Three materials run through the same size ladders. Plain HSS is the general purpose line. The T suffix is titanium nitride coating, which Irwin credits with resisting heat and friction and extending cutting edge life up to six times longer than uncoated high speed steel ^[1]. The CB suffix is M35 cobalt steel, engineered for tough material, explicitly including stainless steel ^[1].

Sets follow the same letters: the 3-piece HSS set 10502ZR carries #1, #2, #3; the 3-piece TiN set 15502 carries #1T, #2T, #3T; the 3-piece cobalt set 10502CB carries the cobalt #1, #2, #3; and the 4-piece 10225 adds the #21 enlarging bit to #1, #2, #4 ^[1].

Lenox Vari-Bit to Unibit cross-reference

Lenox's VARI-BIT is the other long-running step drill line, and Lenox publishes the equivalence itself: its catalog page carries a Unibit column mapping each VB number to the Irwin sizes it replaces, plus the Klein catalog numbers for the same ground ^[3]. This is the table to use when your electrician's list says VB but the counter stocks Unibit, or the reverse.

Lenox's own footnotes: the VB6 combines Unibit #10 and #12 on one body, and where the Unibit #13 drills only its 1-1/8 inch step, the VB11 drills both 7/8 and 1-1/8 ^[3]. Lenox also lists the VBX8, an 8 inch extension that fits any 3/8 inch shank Vari-Bit and larger ^[2].

Construction differs from the Irwin single-flute: Vari-Bits use a patented two-flute design that Lenox credits with fast, clean, burr-free holes, a split point that starts on contact, and a steam oxide coating for tool life, with a cutting lubricant recommended ^[2]. The same catalog page carries Klein's Unibit-era part numbers for each VB size, 1-59001 through 11-590011, for cross-referencing old stock ^[3].

Klein's current step bit line: QRST and KTSB

Klein Tools sells its own step bits today in two parallel lines: QRST, quick-release 1/4 inch hex shank with dual spiral flutes for impact drivers, and KTSB, durable 3/8 inch hex shank with dual straight flutes for standard drills ^[4]. The size ladders are the classic ones, so they line up with the Unibit chart above.

The Unibit ladder column is our mapping from Klein's published ranges to the Irwin numbering this page charts; Klein does not print those equivalences itself. The Klein line adds an aluminum titanium nitride coating it rates more durable than black oxide, laser-etched primary hole markings, and electrician kits that bundle the 3/16 to 7/8, 7/8 to 1-1/8, and 7/8 to 1-3/8 bits with cutting oil ^[4].

Reading a step: count, increment, and thickness

Three numbers describe any step bit, and the catalog carries all three. The hole count is how many discrete sizes the cone cuts, 13 on a #1 down to 1 on a single-hole bit. The increment is the jump between steps, 1/32 inch on the #1, 1/16 inch on most ladders, 1/8 inch on the #5, and it sets how close the bit gets to an arbitrary target size. Step thickness is the depth each step can cut per pass, 1/8 inch on the #3 and #4, which is why step bits are thin-material tools ^{[1] [5]}. The laser-etched size marks inside the flute identify the steps for the life of the bit, and the single-flute Unibit geometry deburrs the hole with the next step as you pass through ^{[1] [5]}.

Buying rules, compressed

• First step bit: the #4 / VB8 / KTSB14 ladder, 3/16 to 7/8 in by 1/16, 12 sizes, covers panel work in one tool ^{[1] [2] [4]}.
• Fine sizes: add the #1 ladder for its 1/32 inch steps when you drill screw clearance holes in sheet ^[1].
• Electrical: the #9 knockout or the Klein 7/8 to 1-1/8 pair for 1/2 and 3/4 conduit holes ^{[1] [4]}.
• Enlarging an existing hole: #20 from 9/16 in or #21 from 13/16 in; the method is on our enlarge an existing hole page ^[1].
• Stainless regularly: the CB cobalt versions, run slow with lubricant ^[1].
• Metric panels: the #1M for 1 mm steps, PG29M for metric conduit knockouts ^[1].

Related

For drilling capacities per material, the metric import cones, and Milwaukee's version of the same ladders, see our step drill bit size chart. For the thin-stock technique of opening a hole that already exists, enlarge an existing hole. When the hole you need is past 1-3/8 in, the step ladder ends and the hole saw size chart begins; for shank standards on the rotary hammer side, the SDS bit size chart. Speeds by material and diameter are on the drill speed chart, and the coating question behind the T and CB suffixes is treated in full on drill bit coatings compared.

Sources

1. Irwin Tools, Metal Drilling catalog pages 63-64, UNIBIT Step Drill Bits (CU91615): fractional #1-#21 ranges, hole counts, increments, shanks, carded items, metric #1M-#5M and #20M/#21M/PG29M/29mm, TiN T-suffix and Cobalt CB lines, sets, SpeedPoint and laser-etch notes: Irwin UNIBIT catalog PDF
2. Lenox, VARI-BIT Step Drill Bits spec page (emacstores scan 30881VB1): VB1-VB12 hole size lists in inches and mm, shanks, metric kits MVB412/1424/618, VBX8 extension, two-flute and split-point design notes: Lenox Vari-Bit PDF
3. Lenox, VARI-BIT Step Drill Bits catalog page 57 (emacstores scan 30882VB2): the Unibit cross-reference column mapping VB numbers to Unibit equivalents with Klein part numbers, plus the #6 and #13 footnotes: Lenox Vari-Bit cross-ref PDF
4. Klein Tools, Step Drill Bits sellsheet 78354 Rev 12/23: QRST and KTSB lines, hole counts, increments, ranges, shank types, AlTiN coating, laser-etched markings, kits: Klein Tools sellsheet PDF
5. Aircraft Spruce, Unibit Step Drill retail reference: Irwin #1-#5 listings with hole counts and shanks, step thickness 3/8 in on the #2 and 1/8 in on the #3 and #4, single-flute deburring description: aircraftspruce.com