CNC speeds and feeds calculator (SFM, RPM, IPM): formulas + ShopCalc

Treat this page as a feeds and speeds calculator cheat sheet: SFM to RPM, chip load to IPM, MRR, and starter surface-speed ranges for common materials. The math matches what ShopCalc runs offline on your phone (mill, lathe, drilling, threading).

Speeds and feeds is the math that gets a tool through material without burning it up, breaking it, or producing a finish that looks chewed. The formulas are simple. The judgment isn't - but you have to start with the math.

The four numbers

• SFM - surface feet per minute (material × tool grade)
• RPM - spindle speed
• IPM - inches per minute (feed rate)
• MRR - material removal rate (cubic inches per minute)

The formulas

RPM   = (SFM × 12) / (π × D)
IPM   = RPM × Z × Fz
MRR   = IPM × DOC × WOC

D    = tool diameter (inches)
Z    = number of flutes
Fz   = chip load per tooth (in/tooth)
DOC  = depth of cut (inches)
WOC  = width of cut (inches)

SFM ranges to start with

• Aluminum 6061 - 600–1500 SFM
• Mild steel 1018 - 80–200 SFM
• Alloy steel 4140 - 60–150 SFM
• Stainless 304 - 60–130 SFM
• Titanium Gr5 - 50–120 SFM
• Brass 360 - 200–600 SFM

Carbide handles the high end of those ranges. HSS handles the low end. Coatings (TiN, TiAlN, AlTiN) push the high end further if you have rigidity and coolant.

Chip load - the easy way

• Tool ≤ 1/8" - start at 0.001 in/tooth
• 1/8" – 1/4" - start at 0.002 in/tooth
• 1/4" – 1/2" - start at 0.003 in/tooth
• 1/2" – 3/4" - start at 0.004 in/tooth
• Above 3/4" - start at 0.005+ in/tooth

Worked example: 1/2" 3-flute end mill in aluminum

Material: 6061 → SFM ≈ 1000
Tool:     1/2" Ø, 3 flutes
Chip:     0.003 in/tooth

RPM = (1000 × 12) / (π × 0.5)
    = 12000 / 1.5708
    ≈ 7,640 RPM

IPM = 7640 × 3 × 0.003
    ≈ 68.8 IPM

For DOC = 0.25, WOC = 0.5:
MRR = 68.8 × 0.25 × 0.5
    = 8.6 in³/min

Stop pulling numbers from forum threads

These formulas are easy. The slow part is doing them by hand for every job - looking up SFM ranges, converting units, second-guessing chip load, recalculating when you change tool diameter. ShopCalc does the math instantly with material presets and chip-thinning built in. Free on iOS and Android, fully offline, no signup.

Related

• 1/4" 4-flute carbide in 4140 prehard — full speeds/feeds example
• G-code: face a 2.5" bar on a CNC lathe (Fanuc/Haas)
• Tap drill chart and how to read thread sizes (UNC, UNF, M-series)
• All four trade calculator apps (hub)
• ShopCalc — full feature list and screenshots
• Ace & Alan Tool — carbide and end mills

Note: Use as a starting point. Always validate with your machine, fixture, and tooling.