Drill Speed Chart — The Right RPM for Every Material and Bit Size

Two rules cover almost everything:

The bigger the bit, the slower the speed.

Stainless runs at about half the speed of mild steel.

Get the second one wrong and the bit blunts on the very first hole. More on why below.

Speed matters as much as the bit. Run too fast and the cutting edge overheats, loses its temper and goes blunt — often on the first hole. Run too slow and you just work harder than you need to. The chart below gives sensible starting speeds for HSS drill bits by material and diameter.

Drill speed (RPM) chart for HSS bits

Bit Ø Mild steel Stainless Cast iron Aluminium Brass
3 mm 2950 1250 2350 3000+ 3000+
4 mm 2250 950 1750 3000+ 3000+
5 mm 1800 750 1400 3000+ 2850
6 mm 1500 650 1150 3000+ 2400
8 mm 1100 500 900 2800 1800
10 mm 900 400 700 2250 1450
12 mm 750 300 600 1850 1200
14 mm 650 250 500 1600 1000
16 mm 550 250 450 1400 900
20 mm 450 200 350 1100 700
25 mm 350 150 300 900 550

Speeds in RPM, starting points for HSS bits. “3000+” means run at your machine’s top speed. Bits under 3 mm and soft materials can spin faster than most drills go — in practice you’re capped by the tool: a typical cordless or hand drill tops out around 2,500–3,000 RPM, so use the highest setting you have. Cobalt bits can run a little faster than HSS in steel and stainless; the figures above are the safe HSS baseline.

The stainless trap. Stainless steel work-hardens — drill it too fast, or let the bit rub without cutting, and the surface goes glass-hard and the bit stops cutting and burns out. This is the single most common reason people think a drill bit is “faulty.” Slow right down (see the stainless column), keep firm pressure so it’s always cutting, use cutting fluid, and reach for cobalt rather than plain HSS.

How to work out the speed for any size

If your exact size isn’t in the chart, the formula is:

RPM = (cutting speed × 1000) ÷ (π × bit diameter in mm)

Typical HSS cutting speeds: mild steel ~28, stainless ~12, cast iron ~22, aluminium ~70, brass ~45 (all in metres/min). Example: a 10 mm bit in mild steel = (28 × 1000) ÷ (3.14 × 10) ≈ 900 RPM.

Wood, plastic and masonry

  • Wood is forgiving — run fast for small bits (1,500–3,000 RPM) and slow down for large flat and forstner bits (around 500–1,500 RPM) to keep the edges clean and avoid burning.
  • Plastic melts if you run too fast or let heat build — use a moderate speed, ease off the pressure as you break through, and clear the swarf. Acrylic especially likes a slower speed and a sharp bit.
  • Masonry and concrete aren’t speed-critical in the same way — it’s the hammer action and the right SDS or masonry bit that matter, run at a moderate speed with steady pressure.

Three signs you’re going too fast

  1. Blue or brown discolouration on the bit tip or the swarf — that’s heat, and heat is what kills the edge.
  2. Powder instead of curls. A sharp bit at the right speed peels off proper spiral swarf. Fine dust means it’s rubbing, not cutting.
  3. Squealing and no progress — classic on stainless. The surface has work-hardened. Stop, slow down, more pressure, cutting fluid.

The right bit makes the speed easier to get right

Over a million drill bits in stock, HSS in 0.1 mm increments plus cobalt for steel and stainless. Same-day dispatch, free next-working-day UK delivery on orders over £50.

HSS drill bits · M35 cobalt · M42 cobalt · All drill bits

Frequently asked questions

What speed should I drill stainless steel?

Slow — roughly half the speed you’d use for mild steel. About 650 RPM for a 6 mm bit, 400 RPM for 10 mm. Keep firm, constant pressure so the bit is always cutting, use cutting fluid, and use cobalt rather than plain HSS. Stainless work-hardens, so a bit that rubs instead of cuts will glaze the surface and blunt.

Why does my drill bit go blunt so quickly?

Nine times out of ten it’s heat from running too fast, drilling dry, or letting the bit rub on hard material like stainless. Match the speed to the material and diameter (see the chart), use cutting fluid on metal, and keep enough pressure to cut rather than skate.

What RPM for a 10 mm bit in mild steel?

About 900 RPM with an HSS bit. Drop to roughly 400 RPM for stainless of the same size.

Does a bigger drill bit need a higher or lower speed?

Lower. The bigger the bit, the faster its edge travels at a given RPM, so you reduce the RPM to keep the cutting speed — and the heat — under control.

Can I drill faster with cobalt bits?

A little. Cobalt (M35 and M42) resists heat better than plain HSS, so it tolerates slightly higher speeds and lasts longer in steel and stainless. It doesn’t change the golden rule though: slow down for stainless and for large diameters.

UK Drills has supplied drill bits to UK trade and industry since 1985, from our trade counter in Hindley, Wigan. Not sure which bit or speed for your job? Email info@ukdrills.com — we’re happy to advise.