Step Drill Bits: Sizes, Uses and How to Use One

A step drill bit is a cone-shaped bit whose diameter rises in steps, so one bit drills many hole sizes in thin material, typically up to about 1/8 in (3 mm) thick. Common bits cover 1/8 to 1/2 in, 1/4 to 3/4 in or 1/4 to 1-3/8 in. Run it slowly with cutting oil on steel, and go up one step at a time until the hole is the size you want.

Step drill bit sizes

Typical range	Common step size	Common use
1/8 to 1/2 in (3 to 13 mm)	1/32 in	Small holes in sheet and plastic
1/4 to 3/4 in (6 to 19 mm)	1/16 in	General sheet metal
1/4 to 1-3/8 in (6 to 35 mm)	1/8 in	Panels, conduit, automotive sheet

Metric bits are also sold, such as 4 to 12 mm or 4 to 32 mm. The exact sizes differ between makers, so read the markings on the bit you buy.

What a step bit drills

Step bits work in thin material:

• sheet steel, galvanized steel and stainless sheet
• aluminum and copper sheet
• plastic and acrylic
• thin plywood and laminate

Each step is short, so the cutting edge only meets thin stock. A step bit does not drill thick plate or deep holes. For anything thicker than about 1/8 in, a twist bit is the right tool, covered in drill bits for metal. For large holes in thicker material, a hole saw is the usual choice.

Why use a step bit instead of a twist bit?

A twist bit tends to grab as it breaks through thin sheet, leaving an oval or torn hole and often a burr. A step bit cuts a round hole in sheet with the edge slightly deburred, and the same bit covers several sizes. That is why it is the usual bit for panel work, electrical boxes and sheet metal enclosures.

For stainless in particular, the bit material matters; see best drill bits for stainless steel.

How to use a step bit

1. Mark the center and make a small dent with a center punch, so the tip does not skate.
2. Clamp the sheet so it cannot spin, and put wood behind it if you can.
3. Set a slow speed for steel and a medium speed for aluminum and plastic.
4. Add cutting oil on steel and stainless.
5. Drill with steady pressure until the step you want is flush with the surface.
6. Back the bit out occasionally, wipe off the swarf and add more oil.

Most step bits have a hex or three-flat shank, so they fit both a drill chuck and an impact driver; see which bits fit your drill.

Standard knockout hole sizes

Electrical trade knockouts use set sizes, and a step bit makes them cleanly.

Conduit trade size	Knockout hole
1/2 in	7/8 in (22 mm)
3/4 in	1-1/8 in (29 mm)
1 in	1-3/8 in (35 mm)

Mistakes to avoid

• Drilling thick plate. The steps cannot cut deep and the bit overheats.
• Running too fast on steel. A blue tip means it overheated.
• Skipping oil on steel and stainless.
• Forcing the bit up to the next size too early. Finish each step before moving on.