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 explains why outlets farther below the same cistern water surface have greater static pressure. Running flow also depends on the pipework, valves and outlet, as described below. 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 pressure with no water flowing. Once water flows, some of the head goes into moving the water and some is lost to friction in the pipes and fittings. So a static head calculation cannot tell you the running pressure or flow at a shower: check performance against the actual layout and the outlet's requirements.