For any hole that needs to finish at seven-eighths of an inch across, a 7/8 inch bit is the simple answer. Confusion starts when someone wants to tap that hole for a bolt. A 7/8-inch bit is not automatically the tapping drill for a 7/8-inch fastener — those are separate decisions, and mixing them up is the most expensive mistake in this size class.
What Size Hole Does a 7/8 Inch Drill Bit Make?
McMaster-Carr lists a dedicated 7/8 inch drill-bit category, confirming it as an ordinary stocked fractional size. Fractional bits carry their diameter in the name, so the number on the shank is the hole you get for a plain hole.
- Rounding to “about 22 mm” loses 0.225 mm, enough to matter in a clearance fit.
- Real-world holes run slightly oversize. Runout, resharpened edges, and material grain push a drilled hole past nominal. Go down a size when the fit is tight.
What Is a 7/8 Inch Drill Bit Used For?
It handles clearance holes for large bolts, pipe and conduit penetrations, anchor-bolt passes, and enlarging existing holes in wood, plastic, and soft metal — a workhorse size in framing, plumbing rough-in, and fabrication.
- Clearance for large fasteners. A hole slightly larger than the bolt lets it slide through without threading.
- Conduit and pipe pass-throughs. Common where a run passes through a stud or plate.
- Anchor and lag bolts. A pilot or clearance hole prevents splitting and keeps the anchor seated square.
- Enlarging an existing hole. Step up in small increments rather than jumping to 7/8 inch.
Match the bit to the material: twist bits for metal, brad-point for clean entry in wood, masonry for concrete or brick. Our roundup of the top 7/8 inch drill bits for tough jobs covers tested picks by material.
Where the 7/8 Inch Number Stops Helping: Tapped Holes
For threaded holes, drill size is set by the thread standard and pitch, not the nominal 7/8-inch figure. Two different threads at the same diameter need two different drills.
The formula: tap drill diameter = major thread diameter − (1 ÷ threads per inch). For a 7/8-inch thread at 9 TPI, that lands near 0.764 inch — noticeably smaller than 7/8 inch. At 14 TPI, the required drill shrinks further.
That is why a generic drill chart fails here. Tap-drill sizes belong to the fastener standard; Bolt Depot’s US tap/drill table matches a thread specification to its drill. Identify the thread callout — diameter and pitch — then look up the drill.
| Goal | Drill Size Rule | Example |
|---|---|---|
| Plain 7/8 inch hole | Use the 7/8 inch bit directly | 0.8750 in / 22.225 mm finished |
| Clearance for a bolt | Nominal bolt diameter plus a small margin | Bolt slides through without threading |
| Tapping a 7/8-9 thread | Major diameter minus 1/TPI | 0.875 − 0.111 ≈ 0.764 in |
| Tapping a 7/8-14 thread | Major diameter minus 1/TPI | 0.875 − 0.071 ≈ 0.804 in |
| Conduit or pipe pass-through | Sized to the outside diameter of the run | Framing and panel work |
| Enlarging an existing hole | Step up in small increments | Reduces grab and chatter |
Setup Mistakes That Ruin the Hole
Most failures trace back to wrong bit for the material, too much speed, and no support under the workpiece.
- Confusing bit size with fastener size. A 7/8-inch bit is not the tapping drill for every 7/8-inch thread — check pitch first.
- Running too fast. Large bits want lower RPM; spin a 7/8 inch twist bit fast in steel and you’ll burn the edge.
- Skipping the pilot. A small pilot hole keeps a large bit from wandering on entry.
- No backing. Clamp a scrap board behind the workpiece to stop blowout.
- Rounding the metric. Treat 7/8 inch as 22.225 mm when the fit is tight, not 22 mm.
FAQs
Is 22 mm the same as 7/8 inch?
No. That 0.225 mm gap can change a tight fit into interference. Use the exact metric equivalent when tolerance matters.
Can I use a 7/8 inch bit to tap a 7/8-inch bolt hole?
Usually not. Tap-drill size depends on thread pitch via major diameter minus one divided by TPI. For most 7/8-inch threads the correct drill lands well under 0.875 inch. Identify the thread specification first, then match the drill.
What material can a 7/8 inch bit drill through?
With the right bit type, it handles wood, plastic, mild steel, and masonry. Twist bits cover metal, brad-point bits clean up wood entry, and masonry bits handle concrete and brick. Slow the speed for harder stock.
References & Sources
- McMaster-Carr. “7/8 inch Drill Bits.” Confirms 7/8 inch as a standard stocked fractional drill-bit size.
- McMaster-Carr. “Drill Size Charts.” Reference for fractional, metric, and wire-gauge drill sizing.
- Bolt Depot. “US Tap and Drill Size Chart.” Maps thread specifications to the correct tapping drill size.