Enter a surface speed and a diameter and get the spindle speed, or enter the rpm you run and get the surface speed in both SFM and m/min. Speed and diameter units are chosen separately, so a catalog in SFM works with a metric tool and the other way round. Runs entirely in your browser.
For turning use the workpiece diameter at the cut, for milling and drilling the cutter diameter. Starting speeds by material: machining hub, for example 316L, C45, Ti-6Al-4V, Inconel 718 and 6061.
Surface speed is the distance a point on the circumference travels per minute, and rpm is how often the circumference comes round. Circumference is π × D, so:
The unit definitions used are 1 ft = 0.3048 m and 1 in = 25.4 mm. When speed and diameter come in different systems, the page converts the speed to m/min and the diameter to mm first, so every mix gives the same answer.
Hold the surface speed constant and rpm is inversely proportional to diameter: halve the diameter and the rpm doubles. That is why a small drill or end mill needs a high spindle speed to reach the same cutting speed, and why constant surface speed on a lathe raises the rpm as the diameter shrinks toward the centre.
RPM = 12 × SFM / (π × D), with D in inches. At 600 SFM and a 1 in diameter that is about 2292 rpm. If the diameter is in millimetres, use rpm = 304.8 × SFM / (π × D), which is the same formula with 12 in per foot converted to mm.
RPM = 1000 × vc / (π × D), with vc in m/min and D in mm. At 200 m/min and a 50 mm diameter that is about 1273 rpm. To go from m/min to SFM divide by 0.3048 (1 SFM is 0.3048 m/min).
Take it from the catalog of the tool you run, because the right speed depends on grade, coating and material condition. We list starting data by workpiece material on our machining pages, which you can reach from the machining hub, and our speeds and feeds calculator carries ISO-group starting values for turning.
This page converts units. For a starting surface speed, use the maker data for your grade, our material pages, or ask the advisor for a grounded, brand-neutral answer.
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