How do I calculate Surface Feet per Minute for my blade?

Calculate Surface Feet per Minute

RPM ÷ (Diameter(inches) x ? ÷ 12) = Surface Feet per Minute

12" Granite Blades 4600 to 5400 = surface feet / minute 1465 to 1720 RPM
14" Granite Blades 4600 to 5400 = surface feet / minute 1250 to 1470 RPM
16" Granite Blades 4600 to 5400 = surface feet / minute 1090 to 1285 RPM
18" Granite Blades 4600 to 5400 = surface feet / minute 980 to 1150 RPM
20" Granite Blades 4600 to 5400 = surface feet / minute 885 to 1040 RPM