Head Pressure Calculator
Calculation Results
Pressure in PSI: 0 psi
Pressure in Bar: 0 bar
Pressure in kPa: 0 kPa
How to Calculate Head Pressure
Head pressure, often referred to simply as "head," is a fundamental concept in fluid dynamics, engineering, and plumbing. It represents the internal energy of a fluid due to the pressure exerted on its container. Specifically, hydrostatic head pressure tells us how much pressure is generated at the bottom of a column of liquid due to the weight of that liquid acting on a specific area.
Whether you are designing a water tower, sizing a pump, or checking the integrity of a tank, understanding how to calculate head pressure is essential. This guide covers the formula, the variables involved, and practical examples.
The Head Pressure Formula
To calculate the pressure at the bottom of a column of liquid in PSI (Pounds per Square Inch) when you know the height in feet, the standard formula is:
Understanding the Variables
- Height (ft): This is the vertical distance from the surface of the liquid to the point where pressure is being measured. It is often called the "elevation head."
- Specific Gravity (SG): This is a dimensionless ratio comparing the density of the fluid to the density of pure water. Water has an SG of 1.0. Fluids heavier than water (like mercury) have an SG > 1, while lighter fluids (like gasoline) have an SG < 1.
- The Constant (0.433): This number represents the pressure in PSI exerted by a 1-foot high column of water. A column of water 1 inch square and 1 foot tall weighs 0.433 lbs.
Common Specific Gravity Values
The accuracy of your calculation depends heavily on the specific gravity of the fluid. Using the value for water (1.0) on a tank full of diesel fuel will result in an overestimation of pressure.
| Fluid | Approximate Specific Gravity (SG) |
|---|---|
| Pure Water (4°C) | 1.000 |
| Sea Water | 1.025 |
| Gasoline | 0.70 – 0.75 |
| Diesel Fuel | 0.82 – 0.95 |
| Crude Oil | 0.80 – 0.90 |
| Mercury | 13.56 |
Calculation Examples
Example 1: A Standard Water Tower
Imagine a water tower where the water level is 120 feet above the ground. You want to know the pressure at the base valve. Since it is water, the Specific Gravity is 1.0.
- Height = 120 ft
- SG = 1.0
- Formula: 120 × 1.0 × 0.433
- Result: 51.96 PSI
Example 2: A Tank of Diesel
You have a storage tank filled with 30 feet of diesel fuel. Diesel is lighter than water, with an approximate SG of 0.85.
- Height = 30 ft
- SG = 0.85
- Formula: 30 × 0.85 × 0.433
- Result: 11.04 PSI
Why is Head Pressure Important?
Pump Sizing: Pumps are rated by the amount of "head" they can overcome. If you need to pump water up 100 feet, you need a pump capable of generating at least 100 feet of head pressure (approx 43.3 PSI) just to reach the top, plus extra pressure to create flow.
Tank Safety: Engineers must calculate the pressure at the bottom of storage tanks to ensure the materials and welds can withstand the force exerted by the fluid.
Hydrostatic Testing: When testing pipes for leaks, they are often pressurized to a specific head equivalent to ensure they won't burst under operating conditions.
Converting PSI Back to Feet of Head
Sometimes you have a pressure gauge reading in PSI and need to know the equivalent height of the water column. You can rearrange the formula:
Height (ft) = PSI × 2.31 / Specific Gravity
Where 2.31 is the height in feet of a water column required to generate 1 PSI (which is the inverse of 0.433).