---
title: "Feeds and Speeds Basics: RPM and Feed Rate Formulas"
description: "Two numbers decide whether a cutter lasts or dies: spindle RPM and feed rate. Both come from one formula pair you can run by hand."
image: "https://nestingcalc.com/og-default.png"
url: "https://nestingcalc.com/guides/feeds-and-speeds-basics/"
locale: "en"
---

# Feeds and Speeds Basics: RPM and Feed Rate Formulas

Two numbers decide whether a cutter lasts or dies: spindle RPM and feed rate. Both come from one formula pair you can run by hand.

## The two numbers

Speed is how fast the spindle turns — RPM. Feed is how fast the tool moves through the material — mm/min or in/min. Get both right and the cutter removes chips cleanly and runs cool; get either wrong and you rub, heat, burn or snap the tool. Every CNC job starts with these two values.

## The RPM formula

RPM = Vc × 1000 ÷ (π × D), where Vc is the cutting speed in m/min and D the tool diameter in mm (in imperial: RPM = SFM × 3.82 ÷ D in inches). So a 6 mm carbide end mill in aluminium at Vc 300 runs about 15,900 RPM — machine caps and rigidity decide the rest.

## The feed formula

Feed rate = RPM × flutes × chip load. Chip load is the thickness each tooth bites — typically 0.05–0.2 mm in wood and plastics, 0.01–0.08 mm in metals. At 15,900 RPM, 2 flutes and 0.05 mm chip load, feed = 1,590 mm/min. Feed and RPM move together: change one and the other follows.

## Reading the cut

Too low a chip load and the tool rubs instead of cutting — heat, glazing, work hardening in metals and melted edges in plastics. Too high and the spindle loads, the cutter deflects or breaks. Listen for a steady chip sound: squealing means rubbing, chattering means too much engagement.

## Start from tested presets

The [Feeds & speeds](https://nestingcalc.com/feeds-speeds-calculator/) bundles starting values for common materials — softwood, plywood, MDF, hardwood, acrylic, aluminium, ABS — for routers and mills, and computes RPM and feed rate from your tool geometry in both unit systems.

[Feeds & speeds](https://nestingcalc.com/feeds-speeds-calculator/) [Open the linear cutting calculator](https://nestingcalc.com/linear-cutting-calculator/)

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