This calculator determines the peripheral velocity (also called circumferential speed or tip speed) at an impeller’s outside diameter. Choose the variable to calculate, then enter the other two. It can find velocity from diameter and rpm, rpm from velocity and diameter, or diameter from velocity and rpm.
The formulas
One point on the impeller rim travels one circumference, πD, during each revolution. Multiplying by rotational speed and converting minutes to seconds gives V = πDN/60 when D is in metres, N is in rpm, and V is in metres per second. The inverse forms are N = 60V/(πD) and D = 60V/(πN).
Solve in any direction
- Peripheral velocity (V) — enter the impeller outside diameter and rotational speed.
- Rotational speed (N) — enter the required peripheral velocity and impeller diameter.
- Impeller diameter (D) — enter the required peripheral velocity and rotational speed.
Worked example
A 12 in (0.3048 m) impeller rotating at 1,750 rpm has a peripheral velocity of π × 0.3048 × 1750 ÷ 60 = 27.93 m/s, or 91.63 ft/s. Reversing the calculation with that velocity and diameter returns 1,750 rpm.
Which diameter should you use?
Use the diameter of the circular path at the point whose velocity you need. For impeller outlet tip speed, this is normally the current outside diameter after any trim—not the casing diameter, shaft diameter, or the maximum untrimmed impeller size on a family curve.
Interpret the result carefully
Peripheral velocity is a kinematic result, not a permissible-speed rating. KSB notes that typical impeller-outlet circumferential speeds are about 20 to 60 m/s, with exceptional cases up to 140 m/s. The acceptable value for a real pump depends on impeller material, geometry, balance, liquid, cavitation performance, noise, stresses, and the manufacturer’s published limits.
Assumptions and limits
- N is actual impeller rotational speed, not merely motor nameplate synchronous speed.
- D and V refer to the same radius on the rotating impeller.
- The relationship describes solid-body rim motion; it does not calculate the liquid’s absolute or relative velocity.
- Confirm any speed or diameter change against the manufacturer curve and mechanical speed rating.