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Metal Removal Rate Calculator - MRR, Machining Time & Horsepower for Milling, Turning & Drilling

Calculate how fast you are cutting, how long the job takes, and how much spindle power you need

Free metal removal rate (MRR) calculator for milling, turning, and drilling operations. MRR tells you how many cubic inches of material you remove per minute - the single most important metric for machining productivity. This calculator computes MRR from your cutting parameters, then converts that into estimated machining time for a given volume of material and required spindle horsepower based on the unit horsepower constant for your workpiece material. For milling: MRR = width of cut × depth of cut × feed rate (IPM). For turning: MRR = 12 × SFM × feed (IPR) × depth of cut. For drilling: MRR is based on drill diameter and feed per revolution. Enter your cutting parameters and the volume of material to be removed, and the calculator tells you how long the operation takes and whether your machine has enough horsepower to sustain the cut. The horsepower check is critical - many shops run aggressive parameters from a tooling catalog only to find their machine bogs down because the spindle motor cannot sustain the material removal rate. A Bridgeport with a 3-HP spindle cannot sustain the MRR that a 15-HP VMC can, even if the RPM and feed rate are identical. This calculator prevents those expensive learning moments.

Pro Tip: Unit horsepower (HP per cubic inch per minute) is the key to matching cutting parameters to your machine. Mild steel requires about 1.0 HP per in³/min, stainless needs 1.2-1.5, aluminum only 0.25-0.35. If your machine has 5 HP at the spindle and you are cutting mild steel, your maximum sustainable MRR is about 5 in³/min. Any combination of width, depth, and feed that exceeds that rate will stall the spindle or trip the overload. Work backward from available HP to set your maximum cut parameters.

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Metal Removal Rate Calculator
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How It Works

  1. Select Operation Type

    Choose milling, turning, or drilling. Each operation uses a different MRR formula. For milling, MRR depends on width and depth of cut plus table feed. For turning, it depends on cutting speed, feed per rev, and depth of cut. For drilling, it depends on drill diameter and feed per rev.

  2. Enter Cutting Parameters

    For milling: enter width of cut, axial depth of cut, and table feed rate (IPM). For turning: enter cutting speed (SFM), feed per rev (IPR), and depth of cut. For drilling: enter drill diameter and feed per rev. The calculator computes MRR in cubic inches per minute.

  3. Select Workpiece Material

    Choose your material to apply the correct unit horsepower factor: mild steel (1.0 HP/in³/min), alloy steel (1.4), stainless (1.5), cast iron (0.6), aluminum (0.3), brass (0.5), or titanium (1.2). These factors determine how much spindle power is needed.

  4. Enter Volume to Remove

    Input the total volume of material to be machined away in cubic inches. For a simple pocket, this is length × width × depth. For complex shapes, estimate the material envelope. The calculator divides total volume by MRR to give machining time.

  5. Review Results and HP Check

    See MRR in in³/min, estimated machining time, and required spindle HP. If required HP exceeds your machine's spindle rating, the calculator recommends reducing depth of cut or width of cut and shows the adjusted time. This prevents machine stalls and extends spindle motor life.

Built For

  • CNC programmers optimizing roughing strategies to maximize metal removal rate
  • Job shop estimators calculating cycle time for quoting machined parts
  • Machine operators verifying that cutting parameters will not overload the spindle motor
  • Production engineers comparing MRR across different tooling and strategy options
  • Shop owners evaluating whether a machine upgrade is justified based on MRR limitations

Frequently Asked Questions

Milling MRR (in³/min) = width of cut (inches) × axial depth of cut (inches) × table feed rate (IPM). For example: a 1/2-inch endmill taking a 0.250-inch width of cut, 0.500-inch depth, at 20 IPM: 0.250 × 0.500 × 20 = 2.5 in³/min. That requires about 2.5 HP at the spindle in mild steel (unit HP factor of 1.0). A 3-HP Bridgeport can handle it; a 1-HP benchtop mill cannot.
Maximum MRR = spindle HP / unit HP factor for the material. A machine with 10 HP at the spindle cutting mild steel (factor 1.0) can sustain 10 in³/min. The same machine in stainless (factor 1.5) can only sustain 6.7 in³/min. In aluminum (factor 0.3), it can push 33 in³/min - if the spindle speed and feed rate can keep up. Spindle HP is usually 70-85% of the motor nameplate HP due to drive losses.
Machining time (minutes) = volume to remove (in³) / MRR (in³/min). If you need to remove 15 cubic inches of mild steel and your MRR is 3 in³/min, the cutting time is 5 minutes. Add 20-40% for tool changes, rapids, and approach moves to get total cycle time. For multi-operation parts, calculate each operation separately and sum the times.
MRR directly determines how long each part takes to machine. Doubling your MRR cuts machining time in half. In a production shop running 2,000 parts per year, increasing MRR by 30% on a 20-minute cycle saves 100 hours of machine time annually. That is $5,000-15,000 in freed-up capacity depending on your shop rate. MRR optimization is the highest-return investment in machining after eliminating crashes and scrap.
Common unit HP factors (HP per cubic inch per minute): aluminum alloys 0.25-0.35, brass and bronze 0.40-0.60, cast iron 0.50-0.75, mild steel (1018, A36) 0.90-1.10, alloy steel (4140, 4340) 1.20-1.50, stainless steel (304, 316) 1.30-1.60, titanium alloys 1.10-1.40, Inconel and nickel alloys 1.50-2.00. Use the higher end of the range for harder conditions within each material group.
Disclaimer: This calculator provides MRR estimates and horsepower requirements based on standard machining formulas and unit power constants. Actual results depend on machine condition, tool geometry, coolant effectiveness, and cutting strategy. Spindle HP at the cutter is typically 70-85% of motor nameplate rating. Always verify machine capability before running aggressive parameters.

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