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Surface Speed Calculator

Surface speed is the speed of the cutting edge relative to the material, and it matters more than the RPM itself. A 2-inch diameter at 600 RPM is about 314 surface feet per minute, or about 96 meters per minute. Use it to check that your RPM is right for the material: aluminum tolerates high surface speeds, and stainless steel and hard alloys need much lower ones.

Too fast burns the tool, and too slow wastes time and can cause built-up edge.

Surface Speed Calculator: inputs

Calculated result
Surface speed (SFM)314.2
Surface speed (meters per minute)95.8

Workpiece or tool diameter and surface speed (SFM)

Surface Speed Calculator: Workpiece or tool diameter and Surface speed (SFM)5 calculations vary the “Workpiece or tool diameter” input while holding the other entered inputs constant.1.6251.31.8282.72314.22.2345.62.4377

Enter the diameter of the workpiece or the tool in inches and the spindle speed in RPM. Surface speed in surface feet per minute equals pi times the diameter times the RPM divided by 12. On a lathe the diameter is the workpiece, and on a mill or drill it is the tool.

The result is compared with the recommended surface speed for the material, which comes from tool and material data.

Recommended values depend on material, tool, coolant and rigidity, so check the manufacturer's data. On a lathe the surface speed drops as diameter shrinks, so RPM must rise in facing cuts. The result is a nominal speed, and machines have RPM limits.

Safety matters at high speeds.

Surface speed (SFM) from workpiece or tool diameter

Workpiece or tool diameter2
Spindle speed600
Surface speed (SFM)314.2
Surface speed (meters per minute)95.8

Common error: Reading RPM as speed. Two tools at the same RPM cut at different speeds if their diameters differ.

Surface Speed: formula

SFM = π × diameter × RPM ÷ 12

ASME B1.1: Unified Inch Screw Threads · Sandvik Coromant: Milling formulas

Surface Speed: changing Workpiece or tool diameter

Surface speed (SFM): assumed Workpiece or tool diameter scenarios.
Workpiece or tool diameterSurface speed (SFM)Surface speed (meters per minute)
1.6251.376.6
1.8282.786.2
2 (example)314.295.8
2.2345.6105.3
2.4377114.9

Surface Speed: compare input ranges

Surface Speed Calculator: Surface speed (SFM) with each input moved on its own, every other input held at the worked-example value.
InputTested fromToSurface speed (SFM): low inputSurface speed (SFM): high inputSwing
Workpiece or tool diameter1.82.2282.7345.662.8 (20%)
Spindle speed540660282.7345.662.8 (20%)

Surface Speed: individual input comparisons

Surface Speed Calculator: change one input at a time; other values remain at the stated example.
InputValueSurface speed (SFM)Surface speed (meters per minute)
Workpiece or tool diameter1.6251.376.6
1.8282.786.2
2314.295.8
2.2345.6105.3
2.4377114.9
Spindle speed480251.376.6
540282.786.2
600314.295.8
660345.6105.3
720377114.9

Surface speed (SFM): Workpiece or tool diameter against Spindle speed

Surface Speed: Surface speed (SFM) at combined Workpiece or tool diameter and Spindle speed settings.
Workpiece or tool diameter / Spindle speed480600720
1.6201.1251.3301.6
1.8226.2282.7339.3
2251.3314.2377
2.2276.5345.6414.7
2.4301.6377452.4

Surface Speed: values and definitions

Worked-example inputs and the results they produce for the Surface Speed Calculator.
Quantity and definitionValue used
Workpiece or tool diameter — Diameter of the rotating workpiece or tool, in inches.2
Spindle speed — Spindle speed, in revolutions per minute.600
Surface speed (SFM)314.2
Surface speed (meters per minute)95.8

Workpiece or tool diameter: half and double

Surface Speed scale comparison; assumed inputs.
Workpiece or tool diameterSurface speed (SFM)
1157.1
2314.2
4628.3

Surface speed is how fast the edge of a tool or workpiece moves, and it is the number that materials care about. A two inch part at 600 RPM has a surface speed of about 314 feet per minute, or about 95.8 meters per minute. The same RPM on a smaller diameter is slower at the edge, which is why a small drill needs a higher spindle speed.

Each material and tool has a recommended range.

Compare the result with the tool maker's chart for the material. Going far outside the range wears the tool or leaves a poor finish.

Get RPM and feed on the feeds and speeds calculator, drill sizes on the tap drill size and thread engagement pages, and unit conversions on the length and speed converters. The frequency converter relates to spindle speed, and the circle area page relates to diameters.

How is surface speed defined?

The linear speed at which the cutting edge travels across the material, equal to pi times diameter times RPM divided by 12.

Why does RPM change with diameter?

Because surface speed depends on both, so a larger diameter needs a lower RPM to keep the same speed at the cutting edge.

How do I find the right surface speed?

Start from the tool manufacturer's recommendation for the material being cut, then adjust it up or down based on chip color and finish.