Choose a 7/8-Inch Drill Bit by Material and Shank Fit
A 7/8-inch drill bit is 0.875 inch or 22.225 mm. Compare wood, metal and masonry options, then check shank fit and usable drilling depth.
A 7/8-inch drill bit—sometimes written “7/8th”—has a nominal cutting diameter of 0.875 inch, or exactly 22.225 mm. That is the hole-size designation, not necessarily the diameter of the shank held by your drill.
Choose the cutting design for the material first, then check shank fit, drilling depth and your drill’s rated capacity. A 7/8-inch wood spade bit and a 7/8-inch metal twist bit are not interchangeable just because their diameters match.
Is 22 mm the same as 7/8 inch?
No. These differences are calculated using 25.4 mm per inch:
| Nominal size | Diameter in millimetres | Difference from 7/8 inch |
|---|---|---|
| 22 mm | 22.000 mm | 0.225 mm smaller |
| 7/8 inch | 22.225 mm | Exact equivalent |
| 23 mm | 23.000 mm | 0.775 mm larger |
A 22 mm bit may be acceptable for a noncritical opening, but it is not an exact substitute. For an anchor, bushing, grommet or fitted component, follow the specified hole size and tolerance. Likewise, do not assume a nominal 7/8-inch bit guarantees a finished hole measuring exactly 0.875 inch; check a test hole when fit matters.
Which 7/8-inch bit should you buy?
Wood: spade for through-holes, Forstner for recesses
For an ordinary through-hole in clean framing lumber, a spade bit is a practical starting choice. Diablo’s DSP2110, for example, is a 7/8-inch wood spade bit with a self-feeding tip, 1/4-inch hex shank and 6-inch overall length. Its listed applications include hardwood, softwood and plywood—not metal or masonry. Diablo DSP2110 specifications
For a flat-bottom recess or overlapping woodworking holes, choose a Forstner bit instead. Diablo’s 7/8-inch FB-006 is intended for those applications and is listed for both handheld drills and drill presses. Diablo FB-006 specifications
Metal: distinguish thick stock from sheet
For a deeper hole in metal, look for a twist bit rated for the particular alloy, along with a drill capable of that diameter. Milwaukee’s 7/8-inch S&D black-oxide twist bit lists steel and aluminum among its applications and uses a reduced 1/2-inch shank. That is a specification example, not a claim that it is the best choice for every steel. Milwaukee 48-89-2750 specifications
For sheet-metal openings, consider a step bit with a marked 7/8-inch step. Check the manufacturer’s material-thickness limit and leave room behind the sheet for the bit to project. Milwaukee’s #9, for example, provides 7/8-inch and 1-1/8-inch hole sizes for steel and plastic; feeding beyond the required step enlarges the opening. Milwaukee #9 step bit
For alloy-specific selection, see our guide to drill bits for metal.
Concrete: match the bit to the hammer’s holder
Use a purpose-built masonry bit and a compatible tool. Milwaukee’s 7/8-inch MX4 model 48-20-7228 has a solid-carbide head, is intended for concrete and masonry, and fits SDS-Plus rotary hammers. Do not buy it expecting the same shank interface as a reduced-shank twist bit. Milwaukee MX4 specifications
Will it fit your drill?
Read the shank specification, not just “7/8 inch” on the package:
- Milwaukee 48-89-2750 twist bit: 1/2-inch shank, 6-inch overall length, 4-inch listed drilling depth.
- Diablo FB-006 Forstner bit: 3/8-inch shank, 3-1/2-inch overall length.
- Diablo DSP2110 spade bit: 1/4-inch hex shank, 6-inch overall length.
These manufacturer-listed examples show why cutting diameter does not determine chuck fit. Overall length also does not equal usable hole depth; check the depth specification and space needed for chip clearance.
Fitting the chuck is not permission to exceed the drill’s capacity. For example, Milwaukee’s 2809-20 Super Hawg has a 1/2-inch keyed chuck, but its manual lists a maximum 1/2-inch steel twist-bit capacity in low gear. A reduced-shank 7/8-inch bit fitting that chuck does not change the rating. Super Hawg operator’s manual
Before drilling
Clamp loose work securely, wear eye protection and use the auxiliary handle where required. Back wood through-holes to limit breakthrough damage, clear twist-bit flutes frequently, and ease pressure near the exit. Milwaukee’s manual also recommends cutting oil when drilling iron or steel, checking for hidden pipes and wires, and using appropriate dust protection or extraction in dusty conditions. Drilling and safety instructions
For a large metal twist bit, follow the recommended speed rather than treating it like a small pilot drill. For example, Milwaukee specifies low gear for large-diameter bits on its 4292-1 and 4297-1 drill motors used with magnetic drill stands. That is not a universal RPM recommendation: follow your own tool and bit instructions for the actual setting. Magnetic drill system instructions Step bits can differ; Milwaukee describes its #9 step bit as optimized for high-speed operation in cordless drills.