EngineeringFormulaFluid Mechanics
Water Pressure Calculator
Find the water pressure from a tank or column height, the height needed for a target psi, or the pressure at an upper-floor fixture.
Water pressure at a tap comes from height and from the supply. Each foot of water column adds 0.433 psi (9.79 kPa per meter), regardless of tank size or pipe length. This water pressure calculator covers the three questions people actually ask: how much pressure a tank or water tower at a given height delivers, how high a tank must sit to reach a target psi, and how much pressure is left at an upper-floor fixture when you know the pressure at the meter or gauge. Results are static (no-flow) gauge pressure in psi, bar, kPa and feet of head.
h · Vertical distance, tank water level to outlet
Solution
Pick a mode and enter the height or pressure to get the water pressure.
P = ρ g h
Formula Sheet
- PWater Pressure
- hWater Height
- ρWater Density
- gGravity
- P_sSupply Pressure
- ΔhFixture Height
Variables & Units
| Symbol | Variable | Description | Common Units |
|---|---|---|---|
| P | Water Pressure | Gauge pressure of the water at the outlet. | psi, bar, kPa |
| h | Water Height | Vertical height of the water column above the outlet. | ft, m |
| ρ | Water Density | 998.2 kg/m³ for fresh water at 20 °C, 1025 kg/m³ for seawater. | kg/m³ |
| g | Gravity | Standard gravity, 9.80665 m/s². | m/s² |
| P_s | Supply Pressure | Gauge pressure at the supply gauge, meter or pump. | psi, bar, kPa |
| Δh | Fixture Height | Vertical height of the fixture above the supply gauge; negative if below. | ft, m |
How to Use This Calculator
- 01Pick a mode: Pressure from Height (tank or tower above the outlet), Height from Pressure (how high the tank must be), or Pressure at Upper Fixture (supply gauge pressure and the fixture height above it).
- 02Enter heights as the vertical distance only. Measure from the water surface in the tank down to the outlet, not along the pipe.
- 03Choose fresh water or seawater. Use fresh water for tanks, plumbing and wells.
- 04In Upper Fixture mode, enter the supply pressure from your gauge and how far the fixture is above that gauge. Enter a negative height if the fixture is below the gauge.
- 05Read the result in your unit, plus the equivalent in bar, kPa and feet of head, and the status band that tells you whether the pressure is usable.
How the Formula Works
Water at rest gains pressure with depth because each layer carries the weight of the water above it. For a fluid of constant density, the gauge pressure at the bottom of a column is P = ρ g h. Only the vertical height h matters. The shape and volume of the tank do not change it.
For fresh water at 20 °C, ρ = 998.2 kg/m³ and g = 9.80665 m/s², so each meter of height adds 9.789 kPa (1.42 psi) and each foot adds 0.4327 psi. Seawater is about 2.7% denser, so it adds slightly more.
To find the height needed for a target pressure, the same relation is rearranged: h = P / (ρ g). A 20 psi target needs 46.2 ft (14.1 m) of water column above the outlet.
For an upper fixture, the elevation costs pressure: P_fixture = P_supply − ρ g Δh. Every 10 ft you go up removes about 4.3 psi from the supply pressure. If the result is zero or negative, the supply cannot push water that high.
All values are gauge pressure (above atmospheric) and static, meaning with no water flowing. Once water flows, pipe friction and fittings reduce the pressure at the tap.
Worked Example 01
Rooftop tank 30 ft above a shower
Known
- Water height: 30 ft (9.144 m)
- Water: Fresh, 998.2 kg/m³
Formula
P = ρ g h
Substitution
P = 998.2 × 9.80665 × 9.144 = 89,512 Pa
Result
12.98 psi (89.5 kPa)
30 ft of height gives about 13 psi. That is below 15 psi, so the shower will run weakly. A tank this high still needs a pump or much more height to reach 20 psi or more.
Worked Example 02
Tank height for 20 psi
Known
- Target pressure: 20 psi (137,895 Pa)
- Water: Fresh, 998.2 kg/m³
Formula
h = P / (ρ g)
Substitution
h = 137,895 / (998.2 × 9.80665) = 14.09 m
Result
46.2 ft (14.09 m)
To deliver 20 psi by gravity alone, the water surface has to sit about 46 ft above the outlet. This is why gravity-fed showers need tall towers or pumps.
Worked Example 03
Second-floor bathroom, 60 psi at the meter
Known
- Supply pressure: 60 psi
- Fixture height above gauge: 20 ft (6.096 m)
Formula
P_fixture = P_supply − ρ g Δh
Substitution
P = 60 psi − 998.2 × 9.80665 × 6.096 Pa = 60 − 8.65 psi
Result
51.3 psi
Going up 20 ft removes 8.65 psi of static pressure. 51.3 psi is still in the typical household range, before friction losses while water is flowing.
Applications
- 01Checking whether a rooftop or hillside tank can feed a shower or garden hose by gravity
- 02Sizing the height of a water tower or tank stand for a target psi
- 03Estimating pressure at a second-floor bathroom from the pressure at the meter
- 04Converting a height of water (head) to psi, bar, or kPa
Fresh-Water Height and Pressure (Static, Gauge)
| Height (ft) | Height (m) | Pressure (psi) | Pressure (kPa) |
|---|---|---|---|
| 10 | 3.05 | 4.33 | 29.8 |
| 20 | 6.10 | 8.65 | 59.7 |
| 30 | 9.14 | 12.98 | 89.5 |
| 46.2 | 14.09 | 20 | 137.9 |
| 69.3 | 21.13 | 30 | 206.8 |
| 92.4 | 28.17 | 40 | 275.8 |
| 115.5 | 35.22 | 50 | 344.7 |
| 138.6 | 42.26 | 60 | 413.7 |
What Static Water Pressure Means at the Tap
| Static pressure | Typical meaning |
|---|---|
| Under 15 psi (1.0 bar) | Very low: weak or no flow at most fixtures |
| 15 to 40 psi (1.0 to 2.8 bar) | Low to fair: fine for basic taps, weak showers and appliances |
| 40 to 60 psi (2.8 to 4.1 bar) | Typical household range |
| 60 to 80 psi (4.1 to 5.5 bar) | High but within the usual code limit; a regulator protects fittings |
| Over 80 psi (5.5 bar) | Above the usual code limit; a pressure-reducing valve is normally required |
Assumptions
- 01Pressure is gauge pressure (above atmospheric) and static, with no water flowing.
- 02Fresh water uses 998.2 kg/m³ (20 °C); seawater uses 1025 kg/m³. Density change with temperature is ignored.
- 03Standard gravity 9.80665 m/s² is used everywhere.
- 04The water column is continuous from the free surface to the outlet, and the tank is open to the atmosphere.
Where This Model Stops
- 01Does not include pipe friction, fittings, valves, or pump curves. Flowing pressure at the tap is lower than the static value shown.
- 02Does not calculate flow rate. A pressure result says nothing about gallons per minute without pipe size and length.
- 03Does not model closed or pressurized tanks, air cushions, or pumps. Add their pressure to the supply value yourself.
- 04The status bands are general guidance, not code compliance. Local plumbing code and fixture manufacturer requirements govern.
References
- [1]
IPC 604.8 and UPC 608.2 maximum water pressure
International Code Council; IAPMO
Basis for the 80 psi static pressure limit that normally requires a pressure-reducing valve.
- [2]
Standard acceleration of gravity, 9.80665 m/s²
BIPM / ISO 80000-3
Value of g used throughout the calculator.
- [3]
Density of water at 20 °C, 998.2 kg/m³
CRC Handbook of Chemistry and Physics
Fresh-water density used for the default fluid.
Frequently Asked Questions
How much water pressure does 1 foot of height give?
One foot of fresh-water column gives 0.433 psi. Equivalent values: 1 psi is 2.31 ft of head, and 1 meter of water is 9.79 kPa (0.098 bar, 1.42 psi).
How high does a tank need to be for a gravity-fed shower?
About 46 ft (14 m) for 20 psi. A tank only 2 m above the shower gives 2.8 psi, which is a trickle. Most gravity-fed systems below roof height need a pump or a booster to give a comfortable shower.
Does a bigger tank give more pressure?
No. Pressure depends only on the vertical height of the water, not on tank volume or shape. A larger tank lasts longer but delivers the same pressure at the same height.
What is a normal water pressure for a house?
Most homes run 40 to 60 psi static. Most plumbing codes (for example IPC 604.8 and UPC 608.2) require a pressure-reducing valve above 80 psi. Below about 30 psi, showers and appliances often perform poorly.
Why is the pressure at my tap lower than the calculator result?
The calculator gives static pressure with no flow. When water flows, friction in the pipe, fittings and valves lowers the pressure at the tap. The longer, narrower and more crowded the pipe, the bigger the drop.
How is this different from the Hydrostatic Pressure Calculator?
Both use P = ρgh. This page is built for practical water supply: psi and feet defaults, tank height and target pressure, pressure at an upper floor, and a usable-pressure guide. The Hydrostatic Pressure Calculator is the general tool for any fluid density, absolute pressure, and solving for density or depth.