Discharge Pressure Calculator: Convert Head to PSI
Use this tool to calculate the discharge pressure of a pump in psi (pounds per square inch) based on the total head in feet and the fluid's specific gravity. This is helpful for engineers, technicians, and system designers working with centrifugal pumps and industrial piping systems.
Understanding Pump Discharge Pressure: Converting Head to PSI
In fluid mechanics and pump system design, the term "discharge pressure" refers to the pressure a pump generates to move a liquid through a system. Discharge pressure is often required in PSI (pounds per square inch), but most pump performance data are given in feet or meters of total head. Therefore, converting head to pressure is essential when specifying pumps, designing piping systems, or selecting pressure-rated equipment.
What Is Total Head?
Total head is the vertical distance (in feet or meters) a pump must overcome to move liquid from its source to its destination. This includes:
- Static lift: Vertical distance from pump inlet to the highest point of discharge
- Friction loss: Caused by pipe length, diameter, fittings, valves
- Pressure head: If discharge point is under pressure (e.g., boiler, tank)
Formula to Convert Head to Pressure (PSI)
The discharge pressure (in psi) is calculated using the following equation:
PSI = Total Head (ft) × 0.4335 × Specific Gravity
- 0.4335 is the pressure (in psi) exerted by 1 foot of water at 60°F
- Specific Gravity (SG) accounts for fluid density (SG = 1.0 for water)
This equation assumes the pump is discharging to atmospheric pressure. If there is additional back pressure (e.g., pressurized tank), that must be added separately.
Example Calculations
Example 1: A pump is generating 100 ft of head for clean water (SG = 1.0):
Discharge Pressure = 100 × 0.4335 × 1.0 = 43.35 psi
Example 2: The same head for a brine solution with SG = 1.2:
Discharge Pressure = 100 × 0.4335 × 1.2 = 52.02 psi
Unit Conversion Reference Table
| Total Head (ft) | Pressure (PSI) @ SG=1.0 | Pressure (PSI) @ SG=1.2 |
|---|---|---|
| 10 ft | 4.34 psi | 5.20 psi |
| 25 ft | 10.84 psi | 13.01 psi |
| 50 ft | 21.68 psi | 26.02 psi |
| 100 ft | 43.35 psi | 52.02 psi |
| 150 ft | 65.03 psi | 78.03 psi |
Common Applications and Target Pressure Ranges
| Application | Typical Discharge Pressure (PSI) | Notes |
|---|---|---|
| Residential Booster Pump | 30–60 psi | Standard municipal supply systems |
| Industrial Washdown System | 60–90 psi | Requires consistent spray pressure |
| RO Membrane System | 100–200 psi | High-pressure pumps required |
| Boiler Feed Pumps | 100–400 psi | Includes steam pressure considerations |
| Fire Pump Systems | 100–175 psi | NFPA compliance required |
Frequently Asked Questions
What is the difference between head and pressure?
Head is a measure of energy in feet of height the pump can lift a fluid. Pressure (psi) is a force per area. They are convertible based on fluid density.
What is specific gravity and why is it important?
Specific gravity (SG) is the ratio of a fluid's density to water. Heavier fluids like oil or brine require more pressure for the same head. SG affects pump load and pressure calculation.
Can I use this for fluids other than water?
Yes. Simply enter the correct specific gravity. For example, SG of seawater ≈ 1.03, sulfuric acid ≈ 1.8, milk ≈ 1.03. Check chemical data sheets.
What happens if I ignore SG in the calculation?
You may significantly underestimate the required pump pressure, leading to poor flow performance or pump failure due to overloading.
Does pipe diameter affect pressure?
Pipe diameter does not change the conversion between head and pressure directly but affects friction loss, which in turn increases total head.
Can I reverse the calculation (PSI to Head)?
Yes. Use the formula: Head (ft) = PSI / (0.4335 × SG). This is useful when you know system pressure and want to determine pump head requirement.
System Protection Tips
- Ensure pump is pressure-rated beyond calculated discharge pressure
- Install pressure relief valves for safety
- Use gauges to monitor discharge and suction pressures
- Consider pressure transducers with alarms for automation
- Regularly inspect seals, gaskets, and hoses
Installation & Design Notes
- Use appropriately rated pipe (e.g., Schedule 40, 80)
- Limit elevation changes and friction bends where possible
- Ensure proper NPSH margin on the suction side
- Install check valves to prevent backflow
- Protect instrumentation from pressure surges
Conclusion
Calculating discharge pressure from head is a fundamental skill in pump system engineering. This knowledge ensures that you choose the correct pump, avoid system failures, and maintain operational safety. Whether you're designing a residential booster system or sizing industrial equipment, this tool helps bridge the gap between theoretical design and real-world performance.