Impeller tip speed calculator
Choose millimetres or inches for the impeller diameter and enter revolutions per minute. The calculator returns metres per second and feet per second. It describes the rim's motion; it does not calculate the airflow delivered by the fan.
Illustrative inputs. This calculation does not select equipment or establish a safe operating limit.
Use a dimensioned drawing or the correct model data for the diameter. For a belt-driven fan, motor rpm and wheel rpm can differ. Check the transmission ratio or a safely obtained speed measurement before using the result.
Fan tip speed formula
One revolution moves a point on the rim through the circumference πD. Multiplying by revolutions per minute and dividing by 60 gives distance per second. With D in metres and N in rpm, tip speed U = π × D × N ÷ 60.
With diameter in millimetres, use U = π × D × N ÷ 60,000. For inches, first multiply diameter by 0.0254 to obtain metres. Multiply m/s by approximately 3.28084 for ft/s. These conversions describe the same geometric speed.
To find rpm from a specified tip speed, rearrange the equation: N = 60U ÷ (πD), with D in metres. This rearrangement is arithmetic, not permission to operate a wheel at the resulting speed.
A worked fan example
Take an illustrative impeller 250 mm in diameter rotating at 1,400 rpm. Its circumference is π × 0.250, approximately 0.785 m. It turns 23.33 times each second, giving a tip speed of about 18.33 m/s, or 60.13 ft/s.
At the same rpm, doubling diameter doubles tip speed. At the same diameter, doubling rpm doubles tip speed. Neither change alone tells you the new airflow, pressure or absorbed power, because the full impeller and casing geometry matters.
For a speed change on the same fan, our fan affinity laws guide explains the related flow, pressure and power estimates and their assumptions.
Why fan selection needs more than rim speed
The fan curve, wheel construction, material, balance, bearings and drive determine the permitted operating range. Chemical exposure and temperature also matter for extraction equipment. Use the manufacturer's stated limits for the exact assembly rather than a general maximum from another kind of machine.
Mixers, pumps, aircraft propellers and extraction fans can share this geometric formula while having very different design limits. A propeller Mach-number rule or a mixer process target is not an extraction-fan rating. Our industrial fan selection guide begins with the required duty and operating environment.
Tip speed versus air velocity
Air velocity in a duct is volumetric airflow divided by the duct's internal area. Tip speed is a rotating solid's tangential speed. The two quantities use metres per second but describe different things, so one cannot be substituted for the other.
To review a proposed fan change, record the fan model, impeller diameter, actual wheel speed and intended duty. Include motor and transmission details and the source of each value. Compare the duty with the fan performance curve, and have adjustments assessed before changing the equipment.
Your questions
Should I use the fan inlet size as the impeller diameter?
Use the impeller's outside diameter from the correct drawing or model data. An inlet or duct connection size may differ.
Can tip speed tell me whether a fan is safe?
It is one calculation. Safe operation also depends on the manufacturer's limits, the complete assembly, condition and operating environment.