Water Pressure Calculator — Is My Pressure Sufficient?

This calculator estimates the effective water pressure at a fixture by starting with the available pressure (from gravity head or mains supply) and subtracting friction losses through the pipe run.

The formulas

Gravity pressure (bar) = Head height (m) × 0.0981 (1 m of water head = 0.0981 bar; ≈0.1 bar is the common site rule of thumb)
Equivalent pipe length = Actual length + (Elbows × 0.6m)
Friction loss uses the Hazen-Williams equation: h_f = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87), with flow Q in m³/s, internal diameter d in m, length L in m and roughness C = 130 for new copper. The head loss in metres is converted to bar at 0.0981 bar/m.
Effective pressure = Available pressure − Friction loss

Internal bores follow BS EN 1057 copper tube: 15 mm OD = 13.6 mm bore, 22 mm = 20.0 mm, 28 mm = 26.2 mm, 35 mm = 32.6 mm, 42 mm = 39.6 mm, 54 mm = 51.6 mm. Because friction scales with d^4.87, stepping up one pipe size roughly halves to quarters the loss.

Fixture minimum pressures

• Basin/bath tap: 0.1 bar
• Kitchen tap: 0.5 bar
• Mixer shower: 0.1 bar
• Thermostatic shower: 0.5 bar
• Power shower: 1.0 bar
• Electric shower: 1.0 bar
• Rain head shower: 1.5 bar

Note: This is a simplified calculation. Real-world pressure depends on many factors including pipe condition, simultaneous demand, and water company supply pressure. For accurate results, use a pressure gauge at the fixture location.