Sizes domestic water pipework using the BS EN 806-3 / BS 8558 / CIPHE loading-unit method —
aggregates fixture loading units to a design (simultaneous) flow rate, then picks the smallest copper, steel
or MLCP pipe meeting a velocity and pressure-drop limit.
Tender-stage only. Verify final design to project spec and a suitably qualified engineer.
For design guidance only. Always verify with a qualified engineer.
About this pipe sizing calculator
This free pipe sizing calculator sizes domestic water pipework by the loading-unit method from BS EN 806-3, BS 8558 and the CIPHE Design Guide. It is aimed at plumbing and building services designers who need to convert a schedule of fixtures into a design flow and then a pipe size. Enter how many of each fixture the run serves (basins, sink, bath, WC, shower, washing machine, dishwasher), the frequency-of-use band, the pipe material and run length, and your velocity and pressure-drop limits, and it returns the total loading units, the design flow and the smallest copper, steel or MLCP pipe that meets both limits. Everything runs in your browser. Nothing is uploaded.
How pipe sizing by loading units is calculated
Each fixture contributes loading units from the CIPHE Design Guide Table 15, banded by frequency of use; these are summed to a total. The total loading units convert to a design (simultaneous) flow rate by log–log interpolation through sourced worked-example nodes on the BS 8558/CIPHE design-flow curve. Each candidate pipe size is then tested with Darcy–Weisbach and a Colebrook–White friction factor: velocity is Q ÷ cross-sectional area and the pressure drop per metre follows from the friction factor and velocity head. The smallest DN tier whose velocity (CIPHE ≤1.5 m/s noise limit) and pressure-drop rate (CIBSE Guide B1 ≤200 Pa/m) both pass is selected.
Frequently asked questions
What does the frequency-of-use band change?
It sets how many loading units each fixture counts for. Low is a single person or small group, medium a typical office or dwelling, and high a peak-demand venue like a theatre or stadium, the same band is applied across every fixture. Higher frequency means more loading units, a higher design flow and potentially a larger pipe.
Which materials can it size, and what are the sizes?
Copper (BS EN 1057), medium galvanised steel (BS EN 10255) and MLCP, each with its own bore table across six sizes (roughly 15 mm up to 54 mm equivalent). Because the internal bores differ between materials, the same flow can land on a different nominal size depending on which you pick, the candidate table shows velocity and pressure drop for every tier.
What if even the largest pipe is undersized?
The tool tells you no tier meets both limits and selects the largest as a fallback. That usually means the run is carrying too much simultaneous flow, split the load across more than one riser or branch, relax the velocity or pressure-drop limit if the design allows, or check the run length. It is a tender-stage estimate; verify the final design with a qualified engineer.