Spade bit, auger or Forstner — which do I need for a large hole in wood?

Three bits for big holes in timber, what each is genuinely good and bad at, and the one to reach for when the hole will be seen.

Difficulty: beginner
Time: 12 min
Read: 3 min

Short answer

Spade bits are cheap and fast for rough through-holes in soft timber. Augers are for deep holes and joists, because the screw point pulls them through and the flute clears waste. Forstner bits give the cleanest hole, cut flat-bottomed and overlapping holes, and are the only sensible choice when the hole is visible.

Twist bits above about 10mm get expensive, slow and inaccurate in timber. Three specialist families take over, and they are not interchangeable.

Step by step

Identify what the hole is for.Hidden service hole, deep hole through structure, or a visible hole in a finished piece. That answer picks the bit on its own.
Use a spade bit for cheap rough work.A flat blade with a long point. Fast in softwood, very cheap, poor at accuracy, tears the exit face badly and struggles in hardwood.
Use an auger for depth.The threaded lead screw draws the bit through and the deep single flute lifts waste out, so it will drill a joist end to end. Run it slowly — it feeds itself and does not need pressure.
Use a Forstner for quality.A rim cutter severs the fibres around the circumference before the interior is removed, so the wall of the hole is clean. It will also cut a flat-bottomed hole, drill at an angle to a face, and overlap a previous hole.
Match the drill to the bit.Large augers and Forstners generate a lot of torque. Use a drill with a side handle at low speed, and be ready for it to grab.
Run large bits slowly.A 35mm Forstner at high speed will burn the timber and the bit. Slow speed and steady feed keeps the edge and the finish.
Back the work whatever you use.All three tear the exit face without support. A clamped scrap board behind the workpiece fixes it.
Clear waste on deep holes.Withdraw periodically. Forstners in particular have little waste capacity and will pack, overheat and scorch.

Tips

Forstner bits are the ones worth buying properly. Cheap ones chatter and burn; a decent one lasts for years.
An auger bit needs no downward pressure — the screw does it. Pushing only makes it snatch.
For a hole for a kitchen waste pipe or a cable run through a joist, an auger is the correct and much faster answer.

Common mistakes

Using a spade bit where the hole will be seen — The flat cutters scrape rather than shear, leaving a fibrous wall and a torn exit. It is a rough-work bit and always looks like one.
Running a large bit at full speed — Rim speed at the outside of a big bit is very high. It burnishes and burns the timber, blunts the edge and can set fire to the dust in the flutes.

If it doesn't work

Hole walls scorched and blackened

Cause: Too fast, or waste packed in the flutes — Fix: Slow the drill down and withdraw to clear waste every few seconds.

Bit grabs and twists the drill out of your hands

Cause: Auger or large Forstner catching in the cut — Fix: Use the side handle, reduce speed, and let an auger feed itself rather than pushing.

Exit face badly torn

Cause: No backing board — Fix: Clamp scrap behind the work, or drill halfway from each face.

Forstner wandering off the mark

Cause: Small centre spur not located before the rim engaged — Fix: Start slowly with the spur in a punched or bradawled dimple, and hold the drill square.

Questions people ask

Which one for a door lock or a letterbox?

A Forstner for the visible face and for flat-bottomed recesses. For the long latch hole into the edge, an auger or a lock morticing bit.

Can a spade bit be sharpened?

Yes, easily, with a flat file on the two cutting edges, keeping the original angles. It is one of the few bits genuinely worth touching up.

Why does my auger bit stop in a joist?

Usually a nail or screw, or waste packed in the flute. Withdraw, clear, and check with a detector before going back in.