How to Calculate Feeds and Speeds (+ Free Calculator)

Learn how to calculate CNC feeds and speeds step by step, with formulas and a simple reference table for common materials and tools.

Category: Shop Floor Practices

Introduction

Getting feeds and speeds right is one of the most important skills in CNC machining. Too fast, and you break tools or ruin the finish. Too slow, and you waste time and risk work-hardening the material. This guide walks through the formulas step by step, so you understand what’s happening — not just plugging numbers into a black box.

Key Terms You Need First

Spindle Speed (RPM) — how fast the cutting tool rotates
Feed Rate — how fast the tool moves through the material (inches or mm per minute)
Surface Speed (SFM or SMM) — the speed at which the cutting edge moves through the material, based on tool diameter and RPM
Chip Load — how much material each cutting edge removes per revolution

Step 1: Calculate Spindle Speed (RPM)

Formula:

RPM = (SFM × 3.82) ÷ Tool Diameter (inches)

SFM (Surface Feet per Minute) depends on your material — this is typically provided by the tool manufacturer or found in a reference chart.

Example: Cutting aluminum (SFM ≈ 300) with a 0.25″ end mill:

RPM = (300 × 3.82) ÷ 0.25 = 4,584 RPM

Step 2: Calculate Feed Rate

Formula:

Feed Rate (IPM) = RPM × Number of Flutes × Chip Load

Example: Using the RPM above (4,584), a 2-flute end mill, and a chip load of 0.002″:

Feed Rate = 4,584 × 2 × 0.002 = 18.3 IPM

Quick Reference Table (Starting Points)

Material	SFM Range	Typical Chip Load (per flute, small end mill)
Aluminum	300–800	0.002″–0.006″
Mild Steel	80–150	0.001″–0.004″
Stainless Steel	50–120	0.001″–0.003″
Wood (router)	N/A (use manufacturer RPM)	0.010″–0.020″
Plastic (acrylic, HDPE)	300–600	0.003″–0.008″

These are starting points — always check your specific tool manufacturer’s recommendations, and adjust based on machine rigidity, tool condition, and finish requirements.

Practical Tips

Start conservative. It’s easier to increase speed after a successful test cut than to recover from a broken tool.
Listen and watch. Chatter or squealing usually means your speed or feed needs adjusting.
Chip color matters. For metals, chips should come off as small curls or shavings — blue/discolored chips usually indicate too much heat (speed too high or coolant insufficient).

Frequently Asked Questions

Do I need to calculate this manually every time? No — most CAM software has built-in feeds and speeds calculators, and many machinists use free online calculators (like G-Wizard) once they understand the underlying formulas.

What happens if my feed rate is too high? Tool breakage, poor surface finish, or machine stalling on underpowered equipment.

What happens if it’s too low? Excess heat buildup, tool rubbing instead of cutting, and premature tool wear.