A 1-inch metal drill bit bores a 25.4 mm hole and comes in three main materials — HSS, cobalt, or carbide — with cobalt holding up best in hard steel and stainless.
A 1-inch metal drill bit is defined by one thing: its diameter. One inch equals exactly 25.4 millimeters, so any bit sold under that label will cut a hole of that exact size in steel, aluminum, or the fittings that go through them. The variation that actually matters is what the bit is made of, and that choice depends entirely on what you’re drilling.
How Big Is A 1-Inch Bit In Millimeters?
A 1-inch bit measures 1.000 inch, which converts to 25.4 mm — and that number is fixed, not approximate. Conversion between the two systems is exact, so a 25.4 mm bit and a 1-inch bit are the same tool under two labels.
The size belongs to a standard fractional family. You’ll find 1-inch bits listed alongside 1 1/32″, 1 1/16″, 1 1/8″, and 1 1/4″ in the same catalog pages. At this diameter, the bit falls into the large-bore category, which changes how it behaves in a drill press or handheld drill: bigger bits remove more material per turn, generate more heat, and demand slower speeds than the small bits most people are used to.
That size range is why these bits show up in so much hardware work. A 1-inch bore fits conduit, pipe, bolts, bushings, wiring runs, and pipe fittings — anything where hardware has to pass through metal cleanly.
Which Material Should You Choose For Metal?
Cobalt HSS (M35 or M42) is the right pick for stainless steel and hard alloys, while standard HSS handles softer metals like aluminum without trouble. The material classes below cover nearly everything sold for metal work.
- Carbon steel: fine for wood and plastics, but not built for metal. Skip it here.
- HSS (high-speed steel): the general-purpose standard, and perfectly adequate for aluminum and other softer metals.
- Black oxide HSS: the same HSS core with a surface treatment that helps retain lubricant and resist heat buildup.
- Cobalt HSS (M35 or M42): harder and more heat-resistant, which is why it’s the recommendation for steel, stainless steel, and hard alloys.
- Carbide / tungsten carbide: the hardest class, most at home in high-volume or abrasive work.
One trade-off comes with cobalt: the same hardness that makes it durable in steel also makes it more brittle and easier to snap than other materials. Steady feed pressure beats forcing a cobalt bit through a hole.
Drill Bit Material Comparison
| Material | Best For | Key Trade-Off |
|---|---|---|
| Carbon steel | Wood, plastics | Not suited to metal |
| HSS | Aluminum, soft metals | Dulls faster in hard steel |
| Black oxide HSS | General metal drilling | Same HSS core underneath |
| Cobalt HSS (M35/M42) | Steel, stainless steel, hard alloys | More brittle, easier to break |
| Carbide | Hard alloys, abrasive work | Costs more, needs rigid setup |
| HSS long-shank twist | Deep holes in steel | Extra length reduces rigidity |
For deep holes in steel, an HSS long-shank twist bit is the standard answer, because the extra length reaches through thick stock that a standard bit can’t. Supplier catalogs like McMaster-Carr organize 1-inch drill bits by both material and diameter for exactly this reason — size alone doesn’t tell you whether a bit will survive the job. McMaster-Carr’s metal drill bit catalog shows how material and size are listed together, and if you’d rather compare specific options side by side, this roundup of the best 1-inch metal drill bits breaks down tested picks.
How Fast Should You Drill?
Cutting speed drops as bit diameter rises, and the numbers are steeper than most people expect. Recommended spindle speeds are derived from surface feet per minute, a formula that accounts for both the material and the size of the bit.
The pattern is what matters: softer material tolerates more speed, harder material demands less, and a larger diameter always means slower.
Apply that logic to a 1-inch bit and the takeaway is straightforward: run it slowly and keep steady pressure. Heat is what kills large bits, and speed is what generates heat. A cutting fluid and a firm, unhurried feed will do more for bit life than any single upgrade.
FAQs
Is a 1-inch drill bit the same as 25 mm?
Not quite. A 1-inch bit is 1.000 inch, which converts to exactly 25.4 mm. A 25 mm bit is slightly smaller, at roughly 0.984 inch. For most hardware the difference is negligible, but if a fitting requires a precise fit, use the 25.4 mm equivalent.
Can I use a wood bit on metal?
No. Carbon steel bits are built for soft materials like wood and plastics and aren’t designed for metal. Using one on steel or aluminum will dull or snap it quickly. Reach for HSS at minimum, and cobalt for anything harder than aluminum.
Why do cobalt bits break more easily?
Cobalt HSS is harder and more heat-resistant than standard HSS, but that hardness makes it more brittle. Side-loading the bit or forcing it through at high speed stresses it in ways the material can’t absorb. Straight pressure and slow speed prevent most breakage.
References & Sources
- McMaster-Carr. “Metal Drill Bits.” Lists 1-inch and sheet-metal-cutting drill bits organized by size and material.
- CMT. “Complete Guide to Drill Bits and Construction Materials.” Covers bit material classes and use cases by material.
- Diager. “Standard 752C.” Reference for drill bit specifications and material standards.
