Enter the height of the water column, the (mass) density of water, and the acceleration due to gravity into the calculator to determine the pressure at the base of a water tower.
Water Tower Pressure Formula
Water tower pressure is the hydrostatic pressure created by the vertical distance between the water surface and the point where pressure is measured. In practical terms, the higher the water level above the outlet, the greater the available pressure.
The core hydrostatic relationship is:
P = \rho g h
When using US customary inputs with height in feet, density in lbm/ft3, gravity in ft/s2, and pressure in psi, the calculator can be expressed as:
P = \frac{h d g}{g_c \cdot 144}Variable Definitions
- P = pressure at the base or selected outlet point
- h = vertical height of water above that point
- ρ or d = liquid density
- g = gravitational acceleration
- gc = US customary conversion constant used when working with lbm and lbf
How to Use the Calculator
- Enter the height of water above the point where pressure is being evaluated.
- Enter the density of the liquid. For fresh water, the default value is often close to standard water density.
- Enter gravity or use the standard Earth value.
- Read the resulting pressure in psi, bar, kPa, or atm.
- If you already know pressure, you can use the same relationship in reverse to estimate the required tower height or liquid density.
Quick Pressure Reference for Fresh Water
For fresh water under normal Earth gravity, pressure rises by about 0.433 psi per vertical foot of water or about 9.81 kPa per meter.
1\ \text{psi} \approx 2.31\ \text{ft of water}1\ \text{m of water} \approx 9.81\ \text{kPa} \approx 1.42\ \text{psi}| Water Height | Approx. Pressure (psi) | Approx. Pressure (kPa) |
|---|---|---|
| 10 ft (3.05 m) | 4.3 | 29.9 |
| 25 ft (7.62 m) | 10.8 | 74.8 |
| 50 ft (15.24 m) | 21.7 | 149.5 |
| 100 ft (30.48 m) | 43.3 | 299.0 |
| 150 ft (45.72 m) | 65.0 | 448.5 |
| 200 ft (60.96 m) | 86.6 | 598.0 |
Example
If the water surface is 100 feet above the outlet, the pressure is approximately 43.3 psi for fresh water.
P \approx 0.433 \times 100 = 43.3\ \text{psi}What Affects Water Tower Pressure?
- Water height: Pressure is directly proportional to the vertical head. Doubling the height doubles the pressure.
- Liquid density: Denser liquids produce more pressure at the same height.
- Gravity: Changes in gravity change pressure, though this effect is usually negligible for normal Earth-based applications.
- Water level: As the tower drains and the water surface falls, outlet pressure drops.
Important Practical Notes
- Pressure depends on vertical height, not tank diameter or tank shape.
- A larger tank stores more water, but it does not create more pressure unless the water surface is higher.
- Actual pressure at homes, hydrants, or fixtures may be lower than the theoretical tower pressure because of pipe friction, valves, fittings, filters, and elevation changes along the distribution path.
- If the tank is sealed and the air space above the water is pressurized, that surface pressure adds to the hydrostatic pressure.
P_{total} = P_{surface} + \rho g hCommon Questions
Does a partially full water tower still provide pressure?
Yes. The system still produces pressure as long as the water surface remains above the outlet. The pressure is based on the current water level, not the maximum tower height.
Is this gauge pressure or absolute pressure?
For an open water tower, hydrostatic calculations are normally treated as gauge pressure, meaning pressure above atmospheric pressure. Absolute pressure includes atmospheric pressure as well.
P_{absolute} = P_{gauge} + P_{atm}Can the calculator estimate required tower height from a target pressure?
Yes. Rearranging the hydrostatic equation allows you to solve for the height needed to produce a desired pressure.
h = \frac{P}{\rho g}Why does tank width not matter?
At a given depth, pressure is determined by the weight of the fluid column above that point per unit area. Width changes stored volume, but not hydrostatic pressure at the same elevation.
