Water Main Sizing Calculator — Pipe Size by Loading Units

Loading units represent the relative demand of each outlet on the water supply. They account for both flow rate and frequency of use. A WC has 2 LU while a bath has 10 LU because a bath demands more water when running. The total LUs are converted to simultaneous demand using a probability formula.

A 25mm MDPE service (design flow ≈ 0.6 L/s) is adequate for most single-bathroom homes. This calculator recommends 32mm once simultaneous demand exceeds about 0.6 L/s — roughly 30 loading units — or where a long run or low incoming pressure leaves too little residual. Common 32mm scenarios include 2+ bathroom houses, multiple showers running together, or incoming pressure below 2 bar. Your water company can confirm the available pressure and connection size.

There is no Building Regulations figure for tap pressure — Approved Document G only requires a supply of wholesome water to the kitchen sink. Pressure is set by the water company under the Guaranteed Standards Scheme: the statutory minimum is 0.7 bar (7m static head) at the boundary, and most companies aim for about 1 bar (10m head). BS 8558 gives a kitchen sink design flow of 0.2 L/s (about 12 L/min), minimum 0.1 L/s. Most appliances need at least 0.5 bar. If residual pressure is too low, a larger pipe, pressure booster or accumulator tank may be needed. See our Pump Station Calculator for pressure boosting options.

MDPE (Medium Density Polyethylene) blue pipe is the standard material for underground cold water supply in the UK. It is colour-coded blue to distinguish it from other services. Available in 25mm and 32mm for domestic use, it is flexible, corrosion-resistant and approved for potable water to BS EN 12201.

Every 1 metre of height above the water meter reduces available pressure by 0.1 bar (static head loss). A two-storey house with the meter at ground level loses about 0.3 bar to the first-floor taps. This must be accounted for alongside friction losses in the pipe.