Pipe SizingCalculator

BS EN 806-3 (Design for Water Supply) is the UK standard governing domestic hot and cold water installations. It requires pipe sizes to maintain adequate flow and pressure at all draw-off points while keeping velocity within limits that prevent erosion and noise. The standard is supplemented by BS 8558 and CIBSE Guide G, which provide detailed design charts. The equivalent length method used in this calculator is the approach endorsed by these standards.

CIBSE Guide G and BS EN 806 recommend keeping velocity in copper pipes below 2.0 m/s for hot water circuits and below 3.0 m/s for cold water under normal conditions. Many engineers use 2.5 m/s as a working limit for both. Exceeding this causes erosion corrosion — particularly at bends and fittings — creates noise, and can void manufacturer warranties on valves and cylinders. For microbore (10mm) pipe the limit is typically 1.5 m/s.

At a typical domestic operating pressure of 2 bar, a 15mm copper pipe (12.7mm internal diameter) carries approximately 14–18 L/min before velocity limits are breached. A 22mm pipe (20mm internal diameter) handles 30–40 L/min comfortably. The internal bore area of 22mm is nearly 2.5 times larger than 15mm. Most basin taps and shower valves specify a minimum 15mm supply; bath taps and combination showers generally require 22mm.

The simplified Darcy-Weisbach approach (CIBSE Guide G) has three steps: (1) Calculate the equivalent length by adding actual pipe length to fitting resistances — each 90° bend adds ~0.5m, each tee junction adds ~1.0m. (2) Find the pressure drop per metre for your pipe diameter at your design flow rate from BS EN 806 design charts. (3) Multiply drop per metre by equivalent length to get total drop in mbar. For domestic circuits, a total drop of 0.5 bar or less is the standard design target.

Fittings account for 20–50% of total pressure loss in a domestic pipe run. Each 90° elbow on 22mm copper has an equivalent length of roughly 0.5–1.0m; a tee junction drawing from the side branch adds 1.0–2.0m. Stop valves add 2–4m equivalent length each when half-open. Always count all fittings when sizing pipe — ignoring them produces an under-designed system that underperforms at the tap.

The sizing method is the same — the flow rate and equivalent length method applies to both. Hot water circuits have additional considerations: pipe material must be rated for sustained temperatures above 65°C (copper and CPVC are fine; PEX must be rated to at least 82°C for primary circuits); thermal expansion must be accommodated with expansion loops or flexible connections on longer runs; and insulation requirements under Part L of Building Regulations and BS 5422 apply to all hot water distribution pipes in habitable buildings.

Under Part L of the Building Regulations (England) and equivalent devolved requirements, all hot water distribution pipes must be insulated to BS 5422 levels wherever they run through unheated spaces (loft, garage, external void). For a 22mm copper pipe in an unheated space, 25mm thick class O insulation (e.g., Armaflex) is typically required. Cold water pipes in unheated spaces should be insulated against freezing, with minimum 25mm insulation and a minimum pipe depth below ground of 750mm for external runs per WRAS guidance.