How to Calculate Material Removal Rate (MRR) in CNC Milling & Turning

In production CNC machining, cycle time is money. While speeds and feeds tell you how fast the cutter moves, Material Removal Rate (MRR) (often denoted as $Q$) measures how much metal you are physically carving away per minute.

MRR is the gold-standard metric for comparing roughing efficiency, selecting CAM toolpath strategies, and validating whether your CNC machine’s spindle motor has sufficient horsepower to handle the cut without stalling.

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1. Milling MRR Formula (Rectangular Engagement)

For standard peripheral and face milling cuts with constant axial and radial engagement:

$\text{MRR } (Q) = \text{Axial Depth of Cut } (a_p) \times \text{Radial Width of Cut } (a_e) \times \text{Feed Rate } (F)$

Metric Units

• $a_p$ in millimeters ($\text{mm}$)
• $a_e$ in millimeters ($\text{mm}$)
• $F$ in millimeters per minute ($\text{mm/min}$)
• Resulting volume is in $\text{mm}^3/\text{min}$. In professional practice, machinists divide by $1,000$ to express MRR in cubic centimeters per minute ($\text{cm}^3/\text{min}$):

$\text{MRR } (\text{cm}^3/\text{min}) = \frac{a_p \times a_e \times F}{1,000}$

Imperial Units

• $a_p$ in inches ($\text{in}$)
• $a_e$ in inches ($\text{in}$)
• $F$ in inches per minute ($\text{IPM}$)
• Resulting volume is directly in cubic inches per minute ($\text{in}^3/\text{min}$):

$\text{MRR } (\text{in}^3/\text{min}) = a_p \times a_e \times F$

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2. Drilling MRR Formula

Because a twist drill cuts an entire circular hole volume as it advances axially:

$\text{MRR} = \text{Hole Cross-Sectional Area} \times \text{Feed Rate}$ $\text{MRR} = \frac{\pi \times D^2}{4} \times F$

• Metric ($\text{cm}^3/\text{min}$): $\text{MRR} = \frac{\pi \times D^2 \times F}{4,000}$
• Imperial ($\text{in}^3/\text{min}$): $\text{MRR} = \frac{\pi \times D^2 \times F}{4}$

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3. Estimating Required Spindle Power from MRR

Every metal requires a specific amount of cutting energy to shear away a unit volume of material. This is known as specific cutting energy ($k_c$) or unit power rating ($P_u$):

$\text{Spindle Power } (P_{\text{net}}) \approx \text{MRR} \times P_u$

Workpiece Material	Metric Unit Power ($P_u$ in $\text{kW}/(\text{cm}^3/\text{min})$)	Imperial Unit Power ($P_u$ in $\text{HP}/(\text{in}^3/\text{min})$)
Aluminum Alloys (6061/7075)	$\approx 0.02 - 0.03$	$\approx 0.25 - 0.40$
Cast Iron (Gray / Ductile)	$\approx 0.05 - 0.07$	$\approx 0.60 - 0.90$
Carbon Steels (1018/1045)	$\approx 0.07 - 0.09$	$\approx 0.90 - 1.20$
Alloy Steels (4140/4340)	$\approx 0.09 - 0.12$	$\approx 1.20 - 1.60$
Stainless Steels (304/316)	$\approx 0.10 - 0.14$	$\approx 1.30 - 1.80$
Titanium (Ti-6Al-4V)	$\approx 0.10 - 0.15$	$\approx 1.40 - 1.90$
Nickel Superalloys (Inconel 718)	$\approx 0.15 - 0.22$	$\approx 2.00 - 3.00$

Horsepower Calculation Example

Suppose an imperial roughing cut in 4140 steel achieves an MRR of $6.5\text{ in}^3/\text{min}$: $\text{Required Cutting Power} \approx 6.5 \times 1.35 = 8.78\text{ HP}$

Accounting for machine spindle drive efficiency ($\approx 80\%$), the required spindle motor rating is: $\text{Motor Horsepower} = \frac{8.78}{0.80} \approx 11.0\text{ HP}$

If your machine only has a $7.5\text{ HP}$ spindle, this cut will stall the spindle or trip the drive overload breaker!

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High-Depth Low-Width vs. Low-Depth High-Width

A major breakthrough in modern CAM machining is understanding that MRR can be maintained while radically reducing tool deflection:

• Traditional cut: $a_p = 3\text{ mm}$, $a_e = 12\text{ mm}$ (Full slot), $F = 600\text{ mm/min}$ $\rightarrow \text{MRR} = 21.6\text{ cm}^3/\text{min}$.
• Dynamic trochoidal cut: $a_p = 24\text{ mm}$ (Full flute), $a_e = 1.2\text{ mm}$ ($10\%$ stepover), $F = 1,800\text{ mm/min}$ (with chip thinning compensation) $\rightarrow \text{MRR} = 51.8\text{ cm}^3/\text{min}$.

The dynamic cut achieves 2.4x higher MRR while distributing tool wear across the entire flute length rather than wearing a single notch at the tip.

Calculate your cutting volume and productivity instantly with our free Material Removal Rate Calculator or Feed Rate Calculator.