Machine Screw vs Wood Screw: What’s the Difference?
A machine screw has a straight shank and a standardised thread made to mate with a nut or a tapped hole. A wood screw has a tapered, pointed body and a coarse thread that cuts its own path through wood fibres, usually with a smooth shank under the head. The size numbers overlap, which is the trap: a #8 of either type is 0.164 in across, but the holes they need are different. Each family has its own chart, so use the machine screw size chart or the wood screw size chart for the screw in your hand, never one for the other.
Key takeaways
- A machine screw needs an internal thread to meet: a nut or a tapped hole. A wood screw makes its own thread in the wood.
- Look for the tell: a smooth shank under the head and a sharp point mean wood; a thread running the full length with a blunt end means machine.
- The same size number takes a different hole: a #8 machine screw is tapped with a #29 drill, while a #8 wood screw gets a pilot hole sized to the wood.
- Never borrow a hole size from the other family’s table.
How the threads differ
A machine screw has a parallel (straight) thread of constant diameter, finished with a blunt or chamfered end. The thread belongs to a standard series: Unified inch threads (UNC and UNF, ASME B1.1) or ISO metric threads. A standard series matters because the screw is meant to mate with a thread cut or moulded to the same standard, so a #8-32 machine screw fits any #8-32 nut or tapped hole.
A wood screw has no mating thread to match. Its thread is deep and widely spaced, and the body typically tapers toward a sharp point so the screw can start in wood and pull itself in. Wood screws are covered by ASME B18.6.1, which sets the thread length for cut-thread screws at roughly two thirds of the nominal length. Below that, the shank is smooth.
One wrinkle: not every wood screw tapers. Traditional wood screws have a shank that matches the outside diameter of the thread, and they pair with a tapered drill bit. Many modern production screws have a straight shank slightly smaller than the thread’s outer diameter, so a straight bit does the job. Both are wood screws; the second type is just more forgiving about the drill.
How each one holds
A machine screw holds through thread engagement with its partner. In a nut or a tapped hole, the screw thread and the internal thread mesh, and tightening clamps the parts together.
A wood screw holds by the thread cutting into the wood fibres of the lower piece. The smooth shank is not decoration. When joining two boards, the shank passes through a clearance hole in the top board, so the threads engage only the lower board and tightening draws the two together. If the threads also grip the top board, they can hold the boards apart.
Where each is used
Machine screws go where there is a thread to meet: tapped holes in metal, nuts, appliance panels, electronics, equipment housings. Wood screws go into wood and wood-based boards such as plywood and particleboard. Put a machine screw into plain wood and there is nothing for it to engage; put a wood screw into a tapped hole and its thread form does not match. They are not interchangeable.
A third family, sheet-metal or self-tapping screws, sits between the two and is outside this guide. The same rule applies: use the table for the family you actually have.
Why the same size number takes a different hole
The numbering is the source of the confusion. A #8 machine screw has a nominal major diameter of 0.164 in (4.166 mm), and the nominal diameter listed for a #8 wood screw is also 0.164 in. The number says how big the screw is. It says nothing about the hole, which depends on the job the hole does.
| Screw | Hole in the part that holds the thread | Hole in the part the screw passes through |
|---|---|---|
| #8 machine screw (8-32 UNC) | Tap drill #29 (0.136 in / 3.454 mm), then cut the thread with a tap | Close clearance hole #18 |
| #8 wood screw | Pilot hole, typical starting point: 7/64 in in softwood, 1/8 in in hardwood | Shank clearance hole, typical starting point: 11/64 in |
The machine screw figures are standard values for that thread. The wood figures are a typical starting point from manufacturer pilot-hole guidance (listed as approximate); wood hardness, moisture and the screw’s make all move them, so test in an offcut. Don’t cross the streams. A #29 hole is 0.136 in, far larger than a typical 1/8 in (0.125 in) hardwood pilot, and it would leave a wood screw’s threads very little wood to cut.
How to tell them apart in your hand
- Shank: a smooth, unthreaded section under the head points to a wood screw.
- Tip: a sharp point suggests wood; a blunt or flat end suggests machine.
- Thread spacing: wide and coarse suggests wood; fine and even suggests machine.
- Test fit: a machine screw turns freely into a matching nut, and a wood screw won’t.
- Calipers: read the thread crests, and expect the smooth shank of a wood screw to measure slightly over nominal.
Which one do you need?
Let the material decide. If the screw is going into a thread (a nut, a tapped hole in metal), you need a machine screw of the matching size and thread series. If it is going into wood or a wood-based board, you need a wood screw. For any load-bearing or safety-related fixing, follow the designer’s or manufacturer’s specification rather than a general guide. The screw size chart hub links to all three families side by side.
Standards behind the numbers: ASME B1.1 for unified threads, ASME B18.6.3 for machine screws and ASME B18.6.1 for wood screws. The wood hole sizes above come from the wood screw size chart. Sources and method explains how this site checks its own figures.
Last updated: