Horsepower Calculator

Before you push depth of cut and feed to the limit, check whether your spindle actually has the power to sustain it.

Optional — pick your machine once and results here will flag anything that exceeds its rated RPM or horsepower.

Axial depth of cut — how deep the tool is cutting, not to be confused with stepover.
Radial width of the cut — how wide a path the tool is removing, not the tool's own diameter.
Table feed rate in inches per minute — read this off your program or the Feed Rate Calculator.
Sets the unit power constant used to estimate cutting horsepower for this material.
Accounts for drivetrain losses between the motor and the cutting edge — 80% is a reasonable default for a belt- or gear-driven spindle.

Unit power constants used

MaterialUnit power (hp per in³/min)
Aluminum0.25
Brass0.7
Cast iron (gray)0.7
Mild steel (1018)1.0
Alloy steel (4140)1.25
Stainless steel (304)1.3
Titanium (Ti-6Al-4V)1.2

Representative midpoint values — actual unit power varies with hardness, condition, and how dull the tool is. Treat this as a sanity check against your spindle's rated power, not a precision spec.

Where the numbers come from

Material removal rate (MRR, in³/min) = depth of cut × width of cut × feed rate tells you how much material you're taking off per minute. Multiply that by the material's unit power — how many horsepower it takes to remove one cubic inch per minute of that specific material — and you get the horsepower actually being consumed at the cutting edge. Divide by machine efficiency (accounting for spindle motor and drivetrain losses) to get the horsepower your spindle motor needs to be rated for.

Why efficiency matters

A spindle motor rated at 5 HP doesn't deliver a full 5 HP to the cutting edge — belt drives, gearing, and bearing friction all eat into it, typically leaving somewhere around 75–85% available at the tool. Using 80% as a default assumes a reasonably efficient drivetrain; a older or direct-drive/geared head machine might run closer to 70%, meaning it needs more rated horsepower to deliver the same cutting power.

What to do if the number is close to your spindle's rating

If the calculated horsepower is at or above what your machine is rated for, that's a sign to back off depth of cut, width of cut, or feed before you stall the spindle or overheat the motor — this calculator estimates steady-state cutting power, and doesn't account for interrupted cuts or shock loads, which can demand more instantaneous power than the average value shown here.

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