Screw Size Chart Explained: M3, M4, M6 and Gauge Numbers

The first time I stood at a hardware store bin with a drawing that said "M4" in one place and "#8" in another, I bought both, drove home, and discovered neither matched the holes I had already drilled. The screw size chart is one of those things that looks like trivia until you are mid-build with a split board and a stripped pilot hole. So let me lay out exactly what these numbers mean, how the two systems line up, and which drill bit to grab.

What the M number actually tells you

Here is the one fact that makes the metric system painless: the M number is the thread outer diameter in millimetres. That is the whole rule. An M6 screw measures 6.0 mm across the outside of the threads. An M3 is 3.0 mm. An M10 is 10.0 mm. There is no formula to memorise and no lookup table to trust — the label is the measurement.

So when someone hands you an M4 screw and asks how wide it is, the answer is 4.0 mm, full stop. M3, M4 and M6 are the three sizes you meet most in flat-pack furniture, electronics enclosures and light brackets, and now you can read them at a glance: 3, 4 and 6 millimetres at the threads.

The one thing the M number does not tell you is the thread pitch — how far apart the threads sit. That is a separate spec. M6 in its standard coarse form runs a 1.0 mm pitch, meaning the thread advances one millimetre per turn. So a complete M6 callout is "M6 × 1.0": 6 mm diameter, 1.0 mm pitch. Fine-thread variants exist (M6 × 0.75, for instance) for vibration-prone or thin-walled jobs, but if a drawing just says M6, assume coarse 1.0 mm pitch.

How imperial gauge numbers work

The imperial side is where people get tripped up, because the numbers feel arbitrary. A #8 wood screw is not 8 of anything obvious. The gauge number maps to a major diameter through a fixed linear formula:

major diameter (inch) = 0.060 + 0.013 × gauge number

Run it and the pattern falls out. A #0 is 0.060 inch. A #6 is 0.060 + 0.078 = 0.138 inch. A #8 is 0.060 + 0.104 = 0.164 inch. A #12 is 0.060 + 0.156 = 0.216 inch. Every step up in gauge adds exactly 0.013 inch, or about 0.33 mm, of diameter. Multiply any inch figure by 25.4 to land in millimetres.

That last conversion is what lets you bridge the two systems. A #8 at 0.164 inch is 4.17 mm, which sits right between M4 and M4.5 — which is why charts call a #8 the rough metric cousin of an M4 wood screw. A #6 lands near 3.5 mm (call it M3.5), a #10 sits at 4.83 mm between M4.5 and M5. The match is approximate: pitch and head style differ between systems, so treat the cross-reference as a starting point, not a promise that the two are interchangeable in a tapped hole.

If you want all of this resolved without arithmetic, the Screw Size Chart does both directions for you. Pick a gauge number or an M size and it shows the major diameter in inch and mm plus the nearest equivalent in the other system.

Tap drill and pilot hole sizes

Knowing the diameter is half the job. The other half is drilling a hole that actually holds. Two different holes matter, and confusing them is the fastest way to crack a workpiece.

A pilot hole is the small hole the threads bite into. You want it narrower than the screw so the thread has fibre to grab, but not so narrow that it forces the wood apart. The working rule for wood:

Softwood (pine, spruce, cedar): drill about 70 percent of the major diameter. For a 4.17 mm #8 that is roughly a 2.9 mm pilot — grab the 3 mm bit.
Hardwood (oak, maple, beech): drill about 85 percent, near 3.5 mm for that same #8. Dense species split when the thread shoves too much fibre aside, so the bigger hole gives it room.

A clearance hole is different. It goes through the top board, wide enough for the screw's shank to slide straight through without biting — roughly 1.1 times the major diameter, so about 4.6 mm for a #8. This is the trick most people skip. Without a clearance hole the threads grab both boards and jack them apart instead of clamping them together. Pilot in the bottom piece, clearance in the top piece: that is how a screw pulls a joint tight.

For machine screws in metal, the equivalent of a pilot hole is the tap drill — the hole you drill before cutting threads with a tap. The standard rule there is "diameter minus pitch": an M6 × 1.0 needs a 5.0 mm tap drill (6 − 1), an M4 × 0.7 needs roughly a 3.3 mm drill. That single subtraction covers most coarse metric taps.

A worked example, end to end

Say the build sheet calls for #8 wood screws into an oak face frame, and your drill index is metric. Walk it through:

Diameter. #8 → 0.164 inch → 4.17 mm.
Metric cousin. 4.17 mm ≈ M4, so an M4 box will stand in for diameter if you are short on #8s.
Pilot, hardwood. 85 percent of 4.17 mm ≈ 3.5 mm. Drill 3.5 mm into the bottom rail.
Clearance. 1.1 × 4.17 ≈ 4.6 mm. Drill that through the top stile so the shank floats.
Drive. The thread grips the rail, the head pulls the stile down, the joint clamps flat instead of splitting.

That is the difference between two screws that split your last face frame and a joint that pulls dead tight.

Keeping your fastener system straight

A few habits save real headaches. Never read a gauge number as a millimetre size — a #10 is 4.83 mm, not 10 mm; treating it as 10 mm doubles the real diameter and wrecks every hole you drill. Always step pilot and clearance figures to the nearest real drill bit you own, and test on scrap before the workpiece. And when a metric callout omits the pitch, assume the coarse standard (M6 × 1.0, M4 × 0.7, M3 × 0.5) unless the drawing says fine.

Gauge-style numbering is not unique to screws, by the way — wire uses its own backwards-running gauge scale where a bigger number means a thinner conductor. If you bounce between fasteners and electrical work, the Wire Gauge Calculator handles that conversion the same way the screw chart handles this one.

Get the diameter right, drill the two holes the wood actually needs, and respect the pitch on metric threads. Do that and the chart stops being trivia and starts being the thing that keeps your panels whole.

Made by Toolora · Updated 2026-06-13