What is the difference between an impact driver and an impact wrench?

Two tools with the same hammering mechanism, two completely different jobs, and the confusion that costs people a set of wheel studs.

Difficulty: beginner
Time: 10 min
Read: 3 min
Safety: caution

Short answer

An impact driver takes hex bits and drives screws into timber. An impact wrench takes a square drive and sockets, produces several times the torque, and undoes nuts and bolts. They are not substitutes: a driver will not shift a rusted wheel nut, and a wrench will destroy a wood screw before it seats.

Both work by spinning up a hammer inside the tool and letting it strike an anvil rotationally, thousands of times a minute, so that a small light tool delivers bursts of torque that would twist your wrist if applied steadily. What differs is the scale of that torque and what the output end is shaped to hold.

Safety

An impact wrench will snap a wheel stud, strip an alloy thread or shear a bolt without any warning through the handle — the reaction goes into the mechanism, not your wrist, so you get no feedback before the failure. Never use one for final tightening on anything safety-critical. Wheel nuts, suspension and engine fasteners are tightened to a figure with a torque wrench.

Step by step

Identify them by the output end.An impact driver has a hexagonal collet that accepts quarter-inch hex bits. An impact wrench has a square drive — commonly three-eighths or half-inch — that accepts impact sockets. That is the reliable way to tell them apart in a photograph.
Understand the torque gap.An impact driver produces enough to bury a long timber screw. An impact wrench produces several times that, enough to undo fasteners that have been tightened by machine and then rusted for a decade. The gap is not incremental.
Use the driver for screws.Decking, fencing, joist hangers, structural screws, sheds, framing. Anything where a drill would stall or twist in your hand and the screw head would cam out.
Use the wrench for nuts and bolts.Wheels, tow bars, trailer hitches, rusted exhaust fixings, scaffold and machinery. Removing seized fasteners is what it is genuinely brilliant at.
Never final-tighten with a wrench.Use it to run the fastener down, then finish with a torque wrench to the specified figure. This applies absolutely to wheel nuts and to anything on a vehicle's suspension or brakes.
Use impact sockets, not chrome ones.Chrome hand sockets are hard and brittle. Under impact they shatter and throw fragments. Impact sockets are black, softer and designed to deform rather than explode.
Decide which you actually need.For household woodwork and building, the driver. For cars, trailers, mowers and machinery, the wrench. Very few home users need both, and the driver is the one that overlaps with general DIY.

Tips

A hex-to-square adaptor lets an impact driver turn small sockets, which covers coach screws and modest bolts. It does not turn it into a wrench — the torque is still the driver's.
Impact wrenches come in cordless and air-powered forms. Air is cheaper for the same torque but needs a substantial compressor, which is why cordless has taken over.
If your problem is a single seized nut, a long breaker bar and penetrating oil usually beats buying a tool, and gives you feel for whether the fastener is about to shear.

Common mistakes

Tightening wheel nuts with an impact wrench — Overtightened nuts warp brake discs, stretch studs and make roadside wheel changes impossible. It is also how studs snap.
Using ordinary chrome sockets on an impact wrench — They crack and shatter. The failure is sudden and the fragments move fast.

Questions people ask

Can an impact driver undo a wheel nut?

Sometimes, on a nut that was hand-tightened and is not rusted. Usually not, and trying it is how the hex adaptor gets twisted. It is the wrong tool.

Can I drill with an impact driver?

With hex-shanked bits, for rough holes in timber, yes. The hammering makes it inaccurate and it will destroy fine bits, so anything that needs to be round and where you put it stays with the drill.

What is an impact wrench's torque figure actually measuring?

Usually peak breakaway torque under impact, which is a loosening figure, not a tightening one. It is not comparable to a torque wrench setting and should never be used as one.