Specific Speed Pump Calculator
Specific speed (N_s) is a dimensionless index that characterizes the geometry and performance of a centrifugal pump impeller. It is calculated at the Best Efficiency Point (BEP)....
Formula
Source: Engineering Toolbox, Hydraulic Institute Standards | Last reviewed: June 8, 2026
Examples
0 GPM (US)
= 1237 GPM (US)
- N = 1750
- Q = 500
- H = 100
Ns ≈ 1,237 — Radial vane impeller
0 GPM (US)
= 5876 GPM (US)
- N = 3500
- Q = 1000
- H = 50
Ns ≈ 5,876 — Mixed flow impeller
0 GPM (US)
= 18534 GPM (US)
- N = 1750
- Q = 10000
- H = 20
Ns ≈ 18,534 — Axial flow (propeller) pump
Where is this used?
Efficiency estimation: maximum achievable efficiency correlates with specific speed — values between 2,000-3,500 generally yield the highest efficiencies.
NPSH assessment: suction specific speed (related to N_s) indicates cavitation resistance.
Frequently Asked Questions
What specific speed indicates a radial pump?
Ns < 1,500 indicates a radial-flow (centrifugal) impeller design. These pumps produce high head at relatively low flow. Values 500-1,000 are typical for boiler feed and high-pressure process pumps. The impeller disk is narrow with large diameter.
How does specific speed relate to pump efficiency?
Peak pump efficiency typically occurs at Ns between 2,000 and 3,500 (US units). Below 1,000, efficiency drops because of high disc friction. Above 5,000, efficiency drops due to high internal fluid velocities. The most efficient pump designs cluster around Ns ≈ 2,500.
What units are used for specific speed?
In US customary units: Ns = rpm × (gpm)^0.5 / (ft)^0.75. In SI (metric): Ns = rpm × (m³/s)^0.5 / (m)^0.75. The SI value is approximately Ns(US) × 0.0194. Always verify which unit system is being used when comparing values.
Reviewed for accuracy
· Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.