Off Grid Living Tools · Water planning
Gravity-Fed Water Pressure Calculator
Will your elevated water tank deliver enough pressure? Compare static pressure with pressure remaining at your requested flow, including pipe friction and fittings. Find the tank water height needed for your target pressure without sizing a pump.
Plan pressure from the water level down
Hero image: AI-generated illustration of the gravity-feed concept, not an installation drawing.
Measure the vertical difference between the tank’s water surface and your outlet. Pipe length affects flowing losses; it does not create static pressure. A long downhill run can have plenty of elevation head yet lose much of it through a narrow pipe.
Enter the lowest expected water surface for a conservative operating case. Simple mode uses smooth plastic pipe, K = 5 and no separate device loss. Advanced mode lets you replace those assumptions.
Check your gravity water supply
Vented tank · One full pipe · Steady water flow at 20°C
Your pressure estimate
Calculate to see results.
Required elevation uses your original elevation reference. Required height refers to the water surface, not the tank base or tower height.
| Quantity | Estimate |
|---|
How gravity-fed water pressure is calculated
With a vented tank, elevation provides the energy. This tool applies Bernoulli’s equation from a large, nearly stationary tank surface to a pressure measurement point inside the supply pipe, just upstream of the fixture.
Static gauge pressure = ρgH
v = Q / A; Re = vD / ν
Pipe friction head = f(L/D)v²/(2g)
Minor-loss head = K v²/(2g)
Dynamic pressure = ρg[H − pipe friction − minor losses − device head − v²/(2g)]
Required water height = target pressure/(ρg) + losses + v²/(2g)
Internal calculations use metres, seconds and pascals; density 998.2 kg/m³, dynamic viscosity 0.001002 Pa·s and gravity 9.80665 m/s². Darcy friction factor: 64/Re up to Re 2,300; iterated Colebrook–White from Re 4,000; linear interpolation between the endpoint factors in the uncertain transition region. Zero flow has zero flow-related loss, including device loss.
The velocity head is shown separately, not hidden in K. Do not add an exit K = 1 at this same pipe measurement point: that would count the same energy twice. A free jet discharging into air has atmospheric pressure at its exit; the tool’s result describes available pressure upstream of a fixture, not pressure in the jet.
For the underlying pipe-friction and minor-loss approach, see the EPA EPANET network-model documentation. This calculator is a separate simplified model and does not run EPANET.
Worked gravity-water examples
Cabin tank: 10 metres of elevation
Water surface 10 m, outlet 0 m, 30 m of 25 mm inside-diameter pipe, 10 L/min, roughness 0.0015 mm and K = 5. Static pressure is approximately 0.979 bar (14.20 psi). At this flow, pipe and fitting losses are about 0.258 m and velocity head is 0.0059 m. Delivered pressure is about 0.953 bar.
A 1 bar target needs about 10.479 m of water height above the outlet, or about 0.479 m more than the current level. Use Reset example to reproduce this case.
US units: an ideal no-flow check
A water surface 20 ft above the outlet, with requested flow set to 0 US gpm, gives approximately 8.66 psi static and dynamic pressure. Pipe length, bore and K cause no loss at zero flow.
For 10 psi at zero flow, the required water height is about 23.11 ft. Once flow starts, friction, fittings, devices and velocity head increase the height required. This no-flow check is not a prediction of shower performance.
What to check before building
Use actual pipe bore and the full route length. Run the calculation at low tank level and peak simultaneous demand. A larger pipe can reduce flowing losses but cannot raise static pressure. Compare the result with each fixture’s specified minimum operating pressure and flow; the default 1 bar is an editable example, not a universal requirement.
Filters, backflow devices, heaters and regulators may need much more pressure than a gravity supply provides. A regulator cannot boost pressure. Check manufacturer flow curves and minimum inlet pressure. Elevated water is heavy: support, anchoring, overflow and access need separate design. Also check maximum static pressure at full tank and the lowest outlets against pipe and equipment ratings.
Gravity-fed water pressure FAQ
How much pressure does one metre of tank height provide?
At the assumed 20°C water density, one vertical metre provides approximately 0.0979 bar, 9.79 kPa or 1.42 psi of static pressure. One foot provides about 0.433 psi. Flowing pressure is lower.
Do I measure from the tank base or the water surface?
Measure from the water surface to the outlet using one elevation reference. Tank depth contributes pressure while the water is present. Test the lowest operating level, not only a full tank.
Does a bigger tank create more pressure?
Volume alone does not set pressure. A wider tank at the same water level has the same static pressure. A taller water surface increases pressure; stored volume determines how long you can sustain use.
Will a wider pipe improve gravity pressure?
It reduces velocity and flowing losses at the same flow, often substantially. It cannot increase the pressure available when all outlets are closed. Use inside diameter; nominal pipe sizes differ by material and wall thickness.
Can gravity run my shower or water heater?
Compare the calculated residual pressure at your required flow with the exact equipment specifications. Low-pressure fixtures may work with modest head, while other showers and heaters require more. The tool does not approve a fixture or predict its actual flow.
What does “requested flow cannot be sustained” mean?
The entered flow requires more energy than the elevation provides at a nonnegative gauge pressure at the measurement point. The negative head balance is a diagnostic deficit, not usable delivered pressure. Reduce requested flow, reduce losses or increase water height. Actual flow requires solving the fixture and system curves together.
Why is the device drop ignored at zero flow?
The entered drop describes resistance at the requested operating flow. Ordinary friction losses vanish at zero flow. Check valves with cracking pressure and controls with minimum inlet-pressure requirements need separate checks and are not captured by this simplified input.
Is this the same as a water pump sizing calculator?
No. This tool checks energy supplied by elevation alone. It does not choose a pump, motor, battery or solar array. If gravity cannot meet demand, first compare pipe size, flow and tank elevation; pump selection is a separate task.