Tools / Pump Laws

Pump Laws

Enter a known best-efficiency point and a new speed. Flow scales with speed, head and NPSHr with speed², power with speed³ × ρ₂/ρ₁. Hydraulic power is Q H SG / 3960; specific gravity is only on that check, not on the affinity prediction.

Utility only — not a PE stamp, certified weather product, or code-approved calculation. Outputs are estimates from your inputs/files; verify before design, bidding, or compliance use. Terms

Known condition (at BEP)

Flow (gpm)Speed (rpm)Total head (ft)Brake power (hp)NPSH required (ft)

New speed (rpm)

Specific gravity (hydraulic power)Density ratio ρ₂/ρ₁ (affinity)

Pump efficiency (%)

SG (1.0 = water) is only on P_hyd = Q H SG / 3960. Affinity power uses ρ₂/ρ₁ (default 1 — same fluid). Efficiency is the hydraulic / brake check only.

Hydraulic check (condition 1)

Hydraulic power2.525 hp

Brake horsepower3.367 hp

Specific speed1,107

TypeRadial flow (centrifugal)

Condition 2 (predicted)

Flow50.00 gpm

Speed1750 rpm

Head25.00 ft

Power0.625 hp

NPSHr2.50 ft

Ratios

Flow0.500×

Head0.250×

Power0.125×

Q₂/Q₁ = N₂/N₁, H₂/H₁ = (N₂/N₁)², P₂/P₁ = (N₂/N₁)³ × (ρ₂/ρ₁). NPSHr scales with speed². P_hyd = Q H SG / 3960. N_s = N √Q / H^0.75.

Utility only — not a PE stamp, certified weather product, or code-approved calculation. Outputs are estimates from your inputs/files; verify before design, bidding, or compliance use. Terms