Using pressure equals density times gravity times height to turn a head in metres into pressure.
In a vented system the pressure at a tap depends on how high the water is stored above it. This example shows how to calculate that pressure from the head. The numbers are chosen for the worked example.
The formula. Static pressure p = density (rho) x gravitational acceleration (g) x height (h). For water, density is about 1000 kg/m3 and g is about 9.81 m/s2.
The situation. A cold water cistern surface is 15 metres above an outlet, so the head h is 15 m.
Step 1 — substitute. p = 1000 x 9.81 x 15.
Step 2 — calculate. 1000 x 9.81 = 9810; 9810 x 15 = 147,150 pascals (Pa).
Step 3 — convert to bar. 1 bar is 100,000 Pa, so 147,150 / 100,000 = about 1.47 bar.
Sense check. A handy rule of thumb is that 10 metres of water head is roughly 1 bar, so 15 m being about 1.5 bar fits. This is why a cistern higher in the building gives stronger flow at the taps below it, and why the lowest outlets see the most pressure. Use consistent SI units and confirm any required pressures against current guidance.
Static head is not the same as friction loss
The calculation above gives the no-flow pressure for the stated open cistern. Once water flows, some available head is associated with its velocity and some is lost overcoming pipe and fitting resistance. A static head calculation therefore does not by itself predict the running pressure or flow at a shower. Use the actual layout and outlet requirements when checking performance.