Clearance Hole Chart for Metric Screws: M2 to M24 Through-Hole Sizes

Two jobs go wrong for the same reason. The bracket that will not line up on its four bolts, and the joint that stays a hair open no matter how hard the screw is driven. Neither is a fastener problem. Both come from the through hole — the hole the bolt passes through, which is a different size from the hole it threads into, and which almost nobody looks up.

The short answers first, because these are the ones searched most: an M2 screw needs a 2.4 mm through hole, M2.5 needs 2.9 mm, M3 needs 3.4 mm, M8 needs 9.0 mm. Those are the ISO 273 medium-fit sizes, the column that suits ordinary fabrication. The full chart, and when to use the other two columns, is below.

Metric clearance hole chart — M1.6 to M24

The last column rounds up, never down. An imperial bit smaller than the clearance size is not a substitute — the bolt simply will not pass, and the hole has to be redrilled with the part already in place.

Close, medium or free — which column to use

• Close fit — when the hole also has to locate the part. Machine guards, cover plates that must sit concentric, anything where the bolt is doing double duty as a rough dowel. It costs you: four close-fit holes in a bracket will fight you on assembly unless the pattern is drilled accurately.
• Medium fit — the default, and what a drawing means when it just says "clearance". Enough slack to absorb normal drilling error, tight enough that the washer still covers the hole.
• Free fit — hot-rolled steel, site fabrication, anything with weld distortion or a bolt pattern drilled by two different people. Use it when the parts have to go together on the first attempt and nobody is going to ream anything.

The small sizes: M2, M2.5 and M3

Small screws are where clearance holes get skipped most, and where skipping them causes most damage. On an M2 or M2.5 screw there is no torque headroom at all — the head will shear before it pulls two threaded parts together. In electronics enclosures, instrument panels and small assemblies, the top part gets a clearance hole and only the bottom part gets a thread. Always.

Note the 2.05 mm tap drill for M2.5 — a size no half-millimetre set contains. In practice shops use 2.0 mm and accept a slightly tighter thread, which is fine in aluminium and brass and risky in stainless. If you tap M2.5 in stainless regularly, buy the 2.05 mm singles; a snapped M2.5 tap in a finished part is a scrapped part.

From our range

Industrial 10-Piece Micro HSS Twist Drill Bit Set (0.8-3.0mm) – OEM

The 0.8–3.0 mm micro set holds the small clearance and tap sizes that standard index cases skip.

Counterbore sizes for socket head cap screws

A counterbore lets the head sit below the surface. Two numbers matter — the diameter must clear the head with room for the tool, and the depth must be at least the head height or the screw never seats:

Drill the clearance hole first, then the counterbore — the small hole guides the larger cutter and keeps it concentric. Reversing the order gives you a counterbore that wanders and a clearance hole drilled off-centre inside it.

Clearance hole vs tap drill — the mix-up that breaks taps

These two numbers live next to each other on every chart and do opposite jobs. The tap drill is smaller than the thread and leaves material for the thread to be cut into. The clearance hole is larger than the thread and leaves the bolt free to pass. Drill an M8 hole at 6.8 mm when you wanted clearance and the bolt will not go through; drill it at 9.0 mm when you wanted to tap it and the tap cuts nothing but air.

Tap size in one part, clearance in the other. A bolt that threads into both parts cannot pull them together.

That last point is the one worth taping to a wall. It is the same physics as a wood screw with no clearance hole — thread gripping both halves of a joint locks the gap in place instead of closing it. Full metric tap drill sizes are in the M2–M24 tap drill chart.

Drilling a clearance hole that is actually round

• Centre punch, or start with a smaller bit. Above about 8 mm a twist drill will walk before the full lip engages, and a walked hole is oval — which eats your clearance allowance before the bolt arrives.
• Clamp thin sheet. An unclamped panel lifts on breakthrough and the bit snatches, leaving a triangular hole. That is the real reason bolts "do not line up" on sheet assemblies.
• Deburr both faces. The burr on the exit side stops the parts sitting flat, which reads exactly like a hole that is too small.
• Match-drill where you can. Clamp the two parts and drill through both, then open the top one to clearance size. The pattern cannot be out of alignment because it was never drilled twice.

For workshops drilling clearance holes all day in mild steel and stainless, a cobalt set holds its edge on the larger sizes far longer than plain HSS — the big-diameter holes are where heat builds and edges soften. Bulk buyers can mix cobalt in the large sizes and standard HSS in the small ones on the same order; ask us for that split when you request a quote.