CNC Feeds & Speeds Calculator

Inch system. Drill or endmill. Compute RPM & IPM.

Unit
Inch (in)
Tool type
Tool Diameter
Material SFPM
Glossary — what do these terms mean?
SFPM (Surface Feet per Minute)
The speed at which the cutting edge of the tool travels across the workpiece surface, measured in feet per minute. It depends on the material being cut — softer materials allow higher SFPM, harder materials require lower SFPM. SFPM is the foundation: pick it from the material, then derive RPM from it.
RPM (Revolutions per Minute)
How fast the spindle (and the tool) rotates. Calculated from SFPM and tool diameter: smaller tools need higher RPM to maintain the same surface speed, larger tools need lower RPM. Formula: RPM = (3.82 × SFPM) / D.
IPR (Inches per Revolution)
How far the tool advances into the work for each full rotation of the spindle. Used primarily for drilling. A common rule of thumb for drills: IPR = (D / 0.0625) × 0.001, which scales feed rate with drill diameter.
IPM (Inches per Minute)
The actual feed rate — how fast the tool moves through the material in inches per minute. This is the value you program into the machine. For drills: IPM = IPR × RPM. For endmills: IPM = RPM × flutes × chip load.
Chip Load (in/tooth)
The thickness of the chip removed by each cutting edge (flute) per revolution. Too small and the tool rubs (heat, dull edges); too large and the tool deflects or breaks. Chip load is set by the tool manufacturer based on tool diameter, material, and HSS vs carbide.
Flutes
The cutting edges on an endmill. Common counts are 2, 3, or 4. More flutes mean a higher feed rate at the same chip load (since each rotation removes more material), but also reduce chip-clearance space — important when cutting soft, gummy materials like aluminum where 2-flute is often preferred.
Tool Diameter (D)
The cutting diameter of the drill or endmill, in inches. For drills, you can specify the size as a Starrett designation (number drill like #42, letter drill like L, fractional like 1/4, or metric tap drill like M3 x 0.5) and the calculator looks up the decimal equivalent automatically.
Material SFPM table
Approximate cutting speeds for HSS tools in inches: Tool Steel 50, Cast Iron 70, Mild Steel 100, Brass / soft bronze 200, Aluminum 300. These are conservative starting points — carbide tooling allows 2–4× higher SFPM. Check the tool and material datasheets for production work.

Drill-size lookups based on the Starrett Inch/Metric Tap Drill Sizes & Decimal Equivalents chart.