📖 How To Use
How to Use This Water Pressure Calculator
Getting your gravity-fed water pressure takes under 30 seconds:
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Measure your tank's height above ground
This is the vertical distance from the ground (or a reference point) to the bottom of the water in your storage tank. For rooftop tanks, this includes the full building height plus any stand height.
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Measure your outlet height above ground
This is the height of the tap, showerhead, or irrigation outlet above the same reference point. For ground-floor fixtures, this is often 0.8–1.2 m. Leave it at zero for outlets at ground level.
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Select your fluid type
Fresh water is the default (998 kg/m³). Choose seawater or brine if you're working with saltwater systems — higher density means slightly higher pressure for the same head.
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Hit Calculate
Results appear instantly — PSI, Bar, kPa, Pascals, metres head, and feet head. Use the pressure rating guide to check if your system meets household, shower, or irrigation requirements.
Tip: Always measure from the bottom of the water in the tank (or the outlet pipe connection), not the tank lid. The water surface height drives the hydrostatic head — a near-empty tank delivers significantly less pressure.
📐 The Formula
Water Pressure Formula Explained
Gravity-fed water pressure is determined entirely by the vertical height difference between the water surface and the outlet — known as the hydrostatic head. Tank size and pipe diameter do not affect static pressure.
P = ρ × g × h
Where:
P = Pressure (Pascals)
ρ = Fluid density (kg/m³) — 998 for fresh water
g = Gravitational acceleration (9.81 m/s²)
h = Vertical head (metres) = Tank height − Outlet height
To convert between pressure units:
Bar = Pa ÷ 100,000
kPa = Pa ÷ 1,000
PSI = Pa ÷ 6,894.76
Metres head = Pa ÷ (ρ × 9.81)
Pressure per Metre of Head
For fresh water (998 kg/m³), every metre of vertical head generates approximately:
| Head (m) | kPa | Bar | PSI | Typical Use |
|---|
| 1 m | 9.79 | 0.098 | 1.42 | Gravity shower (minimum) |
| 3 m | 29.4 | 0.294 | 4.26 | Standard household fixtures |
| 5 m | 49.0 | 0.490 | 7.11 | Good shower pressure |
| 10 m | 97.9 | 0.979 | 14.2 | Excellent household pressure |
| 15 m | 146.8 | 1.468 | 21.3 | High-rise rooftop supply |
| 20 m | 195.7 | 1.957 | 28.4 | Commercial irrigation systems |
| 30 m | 293.6 | 2.936 | 42.6 | Multi-storey building supply |
❓ FAQ
Water Pressure Calculator — Frequently Asked Questions
How do I calculate water pressure from tank height?
Multiply the vertical height difference (in metres) between the tank's water surface and the outlet by the fluid density (998 kg/m³ for fresh water) and gravitational acceleration (9.81 m/s²). This gives pressure in Pascals. Divide by 100,000 for Bar, by 1,000 for kPa, or by 6,894.76 for PSI. Our calculator handles all conversions automatically.
What water pressure is needed for a good shower?
Most standard showerheads require at least 0.1 Bar (1.45 PSI), but 0.3–1 Bar gives a comfortable flow. Power showers and thermostatic mixing valves typically need 1–3 Bar. For a gravity shower from a rooftop tank, the tank bottom should be at least 3 metres above the showerhead for 0.3 Bar pressure.
Does tank size (volume) affect water pressure?
No. For gravity-fed systems, static water pressure depends only on the vertical height (head) of water above the outlet — not on tank diameter, volume, or shape. A 200-litre tank and a 10,000-litre tank at the same height produce identical pressure. Tank volume affects flow duration and flow rate sustainability, not pressure.
How high does my tank need to be for 1 Bar?
For fresh water, 1 Bar requires approximately 10.2 metres of vertical head. This means the bottom of the water in your tank must be 10.2 metres above the outlet. In practice, a rooftop tank on a 3-storey building (around 9–10 m) delivers close to 1 Bar at ground-floor fixtures — though pipe friction losses reduce actual flow pressure.
What is the difference between static pressure and dynamic pressure?
Static pressure is the pressure when no water is flowing — calculated purely from head height. Dynamic pressure (or residual pressure) is what you measure when water is flowing, which is always lower due to friction losses in pipes, bends, and fittings. This calculator gives static pressure. For dynamic pressure, you also need to account for pipe diameter, flow rate, and pipe material using Hazen-Williams or Darcy-Weisbach equations.
How do I convert PSI to Bar or kPa?
To convert PSI to Bar: divide by 14.5038 (e.g., 10 PSI = 0.69 Bar). To convert PSI to kPa: multiply by 6.89476 (e.g., 10 PSI = 68.9 kPa). To convert Bar to kPa: multiply by 100 (e.g., 1 Bar = 100 kPa). This calculator outputs all four units simultaneously so you never need to convert manually.
Does seawater generate more pressure than fresh water at the same height?
Yes, slightly. Seawater has a density of approximately 1,025 kg/m³ versus 998 kg/m³ for fresh water — about 2.7% denser. At 10 metres head, seawater generates roughly 100.5 kPa versus 97.9 kPa for fresh water. The difference is small but relevant for marine engineering, desalination, and coastal tank systems. Use the fluid type selector to account for this.