Specific gravity is the ratio of a fluid’s density to the density of water at a defined reference condition. For a centrifugal pump handling fluids of similar viscosity, a change in specific gravity does not change the head the pump produces at a given speed but it directly changes the pressure that head represents and the power the pump and motor must deliver. Getting this distinction wrong is a common cause of undersized motors and misread pressure gauges.
Specific gravity, density and head
Specific gravity (SG) has no units because it is a ratio. Water at standard conditions has an SG of 1. A brine solution might have an SG of 1.2, while a light hydrocarbon might sit around 0.7 to 0.8.
A centrifugal pump generates head, which is energy per unit weight of fluid, expressed in metres. Head depends on impeller diameter, speed and the hydraulic design of the pump — not on the density of the fluid being pumped. This is why a centrifugal pump’s head-versus-flow curve is usually plotted once and applied across a reasonable density range, provided viscosity stays broadly similar.
Why specific gravity changes pressure and power
Head describes energy per unit weight, so the same head represents a different pressure depending on how heavy the fluid is. A denser fluid carries more weight per litre, so the same head corresponds to a higher pressure. Where a system is specified by pressure rather than head, this conversion has to run in the correct direction using the actual fluid density.
Hydraulic power can be estimated using:
P_h = ρ × g × Q × H
Where:
- P_h = hydraulic power in W
- ρ = fluid density in kg/m³
- g = gravitational acceleration, approximately 9.81 m/s²
- Q = flow rate in m³/s
- H = pump head in m
A denser fluid needs more power for the same flow and head. This is a real, estimable figure, not a rule of thumb, use it as a starting point and confirm against the pump manufacturer’s power curve for the actual fluid. Motor selection, shaft loading and drive torque should be checked against the maximum expected density across the operating range, not just the normal value.
Worked example
A pump moves 20 L/s of a fluid with SG 1.15 against 30 m of head.
Density ρ = 1.15 × 1000 kg/m³ = 1,150 kg/m³
Flow Q = 20 L/s = 0.02 m³/s
P_h = 1,150 × 9.81 × 0.02 × 30 = 6,771 W, approximately 6.8 kW hydraulic power
This is hydraulic power only. Shaft power and motor input power will be higher once pump efficiency and drive losses are included. Treat this as an estimate for sizing checks, not a substitute for the manufacturer’s confirmed power curve.
What actually changes with specific gravity
| Parameter | Effect of higher specific gravity |
|---|---|
| Head at a given speed (centrifugal, similar viscosity) | Not directly affected |
| Pressure represented by that head | Increases |
| Hydraulic and shaft power | Increases |
| Motor loading and starting torque | Increases |
| NPSH available, vapour pressure margin | Depends on the specific fluid’s vapour pressure, not SG alone |
Positive displacement pumps and specific gravity
A positive displacement pump’s flow is largely set by displacement per cycle and speed, so specific gravity does not change flow the way it might be assumed to. It still changes the differential pressure the pump generates for a given head requirement, and therefore the torque and power the drive must supply. Relief valve and pressure protection settings must also account for the actual fluid density, not a water-based assumption.
Common mistakes
The most frequent error is applying a water-based power figure to a denser fluid without recalculating, which can leave a motor undersized and tripping on overload. The reverse mistake also occurs: assuming a lighter fluid needs the same power as water and oversizing the drive unnecessarily. Confusing specific gravity’s effect on pressure with an effect on head is another common source of miscalculated system curves.
How Specific Gravity Affects Pump Selection FAQs
Does a heavier fluid reduce the flow a centrifugal pump produces?
Not directly. For fluids of similar viscosity, flow and head at a given speed are set by the pump’s hydraulic design, not fluid density. Density changes the power required and the pressure the head represents, not the flow rate itself.
How do I convert between head and pressure for a specific fluid?
Pressure equals density multiplied by gravitational acceleration multiplied by head. Always use the actual fluid density for the conversion, not water’s density, unless the fluid genuinely has an SG close to 1.
Does specific gravity affect NPSH?
Specific gravity alone does not determine NPSH margin. Vapour pressure, which does not track directly with density, has a much stronger influence. Check the vapour pressure of the actual fluid at the pumping temperature, not just its SG.
Do I need to recheck motor sizing if the fluid changes?
Yes, whenever the new fluid’s density is meaningfully different from the one used in the original sizing. Recalculate hydraulic power with the new density and confirm against the manufacturer’s power curve and the motor’s rated output.
This article explains general specific gravity effects on pump selection. It does not replace manufacturer-confirmed power curves or an application-specific engineering review.
Part of the pump selection guide. See also How Viscosity Affects Pump Selection, How Solids Affect Pump Selection and Why Oversizing Pumps Causes Problems.
About The Pump Expert
The Pump Expert provides independent, practical education for pump users, engineers and maintenance teams. TPE explains how pumps and pump systems behave so readers can make better technical decisions without supplier bias.
Last Updated on July 30, 2026 by TPE