Drill Speed Calculator
The right rpm for the bit in the material — from surface feet per minute, bit diameter and the press you actually own.
Drill Speed Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- HSS-chart speeds; cobalt and carbide differ
- Rounded rim speeds — no point angle or pilot term
In short: A quarter-inch bit in mild steel wants about 1,222 rpm — 80 surface feet a minute at the cutting edge. The same bit in aluminum wants three times that, and in stainless a third. The rule that never changes: double the bit, halve the speed, because the rim speed is what the material feels. Set the nearest belt below the number, feed hard enough to cut a real curl, and let the chips tell you when it is right.
Formula
rpm = SFM × 3.82 ÷ D(in) — 3.82 = 12 ÷ π
Surface feet per minute describes how fast the rim travels; rpm converts that to rotations for a given diameter. The 3.82 is just twelve over pi — the unit bridge from feet at the rim to revolutions at the chuck. Feed rides along: about a thousandth of an inch per revolution for every sixteenth of bit diameter keeps a twist bit cutting instead of rubbing. These are HSS-chart speeds; cobalt and carbide run faster, and the tool chart that ships with the bit governs.
Worked Example
- Pick the SFM the material wants from the chart.
- Divide SFM × 3.82 by the bit diameter.
- Set the nearest belt below and feed for a curl.
Defaults: a quarter-inch bit in mild steel at 80 SFM — 1,222 rpm, about 0.004 in of feed per revolution and 4.9 in/min of feed. Swap to stainless and the same bit drops to 611 rpm; the press never noticed.
Strengths & Limits Of This Model
Where this engine is strong
- Three inputs, one belt decision
- Feed rule rides along with the speed
Where it stops
- Chart speeds are conservative middles, not optimums
Practical Use Cases
Drill press belts
pick the pulley before the hole
Hand drills
respect the top-speed ceiling
Pilot holes
small bits spin, big bits walk
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Drill Speed Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why do bigger bits turn slower?
The rim speed is what heats the cutting edge. A half-inch bit travels twice as far per revolution as a quarter-inch, so it needs half the rpm to keep the same surface speed — 611 rpm against 1,222 in mild steel.
What does the 3.82 in the formula mean?
It is twelve divided by pi. SFM is feet per minute at the rim; circumference in feet is pi times diameter in inches over twelve. The feet and inches cancel and 3.82 is what survives.
How hard should I feed?
The old shop rule is about a thousandth of an inch per revolution for every sixteenth of diameter — a quarter-inch bit takes roughly 0.004 in per turn. Feed until the chip is a real curl; a dust means you are rubbing, a squeal means you are starving the edge.
My press can't reach the number. Now what?
Run the fastest belt you have and ease the feed — especially in aluminum and brass, where the charts assume more speed than a bench press owns. In steel err slow; speed burns edges, slowness only costs time.
Are these speeds right for wood too?
They are a start. Softwood tolerates far more speed than the chart, hardwood a little less; the limit that matters in wood is burning and tear-out, not edge wear. Forstner and spade bits drop to a few hundred rpm regardless.
What about stainless?
Work-hardening is the trap. Slow to about 40 SFM, keep the feed heavy so the edge cuts under the hardened layer, and do not let the bit dwell — a stainless hole rubbed at high speed hardens itself against the very bit cutting it.
Cutting fluid — always?
In steel and stainless, yes; it doubles edge life and keeps chips breaking. Aluminum likes a squirt of light oil, brass runs dry, and cast iron dusts along happily without any.
How does this differ from the milling speeds?
The milling page starts from a cutting speed and a cutter and derives rpm and table feed together. This page is the drill-press version: the material's SFM and the bit you already picked, with the feed rule that goes with it.