Pipe Flow Rate & Pressure Drop Calculator
Calculate pipe flow rates, pressure drops, and minimum pipe sizes for domestic plumbing and heating systems. Based on standard hydraulic engineering principles.
Domestic Plumbing Guide
Pipe Sizing for UK Domestic Plumbing
UK domestic supply pipe sizes: 15mm (½ inch): single shower, single tap supply. 22mm (¾ inch): kitchen cold and hot, boiler cold feed, multi-outlet supply. 28mm (1 inch): main header pipe serving multiple 22mm branches, large power shower. 35mm and above: commercial or high-demand applications. Copper pipe external diameters: 15mm OD = 13.6mm ID; 22mm OD = 20.2mm ID; 28mm OD = 26.2mm ID (the listed size is always external diameter for copper). MDPE blue pipe for underground mains: 25mm supplies
Flow Rates and Velocity Limits
Maximum recommended water velocity in domestic pipes: 3 m/s maximum (above this, noise and erosion occur). 1-2 m/s optimal for most domestic applications. Typical domestic flow demands: kitchen tap 12-15 L/min. Shower 8-15 L/min. Bath fill 15-25 L/min. Toilet cistern 5-10 L/min. A standard 22mm copper pipe at 3 bar inlet pressure can typically supply 40-50 L/min — more than enough for most single-outlet applications but potentially limiting when multiple outlets run simultaneously in larger homes.
Pressure Drop in Systems
The Hazen-Williams equation: V = 0.8492 × C × R^0.63 × S^0.54. Where C = pipe roughness coefficient (copper = 130), R = hydraulic radius (pipe radius for full pipes), S = head loss per unit length. In practice for domestic systems: pressure drop in copper pipe is approximately 10-30 mbar per metre of run at normal flow rates. Fittings add equivalent lengths: an elbow ≈ 0.7m equivalent straight pipe; a 90° tee ≈ 1.5m; a gate valve (fully open) ≈ 0.5m. Rule of thumb: add 50% to the measured pipe l
When to Call a Plumber
Plumbing regulations in England and Wales: all hot water and heating work must be carried out by a Gas Safe registered engineer (gas systems) or competent person (water regulations). Notifiable work under Water Supply (Water Fittings) Regulations 1999: new water supply, new toilet or shower with a spray, rainwater harvesting, garden irrigation. You can carry out your own minor plumbing (replacing a tap washer, unblocking a drain) but should engage a WaterSafe-approved plumber for any work on the
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