Water Supply Pipe Sizing Calculator
Determine the minimum pipe diameter needed to deliver a required flow rate under specified pressure and elevation conditions. Used by hydraulic engineers designing water mains, irrigation systems, and building reticulation.
About this calculator
This calculator sizes a water supply pipe using a form of the Hazen–Williams equation rearranged for diameter. The Hazen–Williams head loss formula is h_f = 10.67 × L × Q^1.852 / (C^1.852 × D^4.87), where h_f is head loss (m), L is pipe length (m), Q is flow (m³/s), C is the Hazen–Williams roughness coefficient (C = 150 for smooth new PVC), and D is internal diameter (m). Rearranging for D gives: D = [(Q^1.852 × L × 10.67) / (C^1.852 × h_f)]^(1/4.87). The available head h_f equals the pressure head minus elevation head: h_f = (P × 1000 / 9810) − Δz. A Hazen–Williams C of 150 is used for PVC; concrete pipes use C ≈ 130 and cast iron C ≈ 100. The result is the minimum internal diameter; the next larger nominal commercial size should be selected.
How to use
Example: flow rate Q = 2 L/s = 0.002 m³/s, pipe length = 200 m, elevation difference = 3 m (uphill), minimum pressure = 200 kPa, C = 150 (PVC). Step 1 — available head: h_f = (200,000 / 9,810) − 3 = 20.39 − 3 = 17.39 m. Step 2 — apply rearranged Hazen–Williams: D = [(0.002^1.852 × 200 × 10.67) / (150^1.852 × 17.39)]^(1/4.87). Numerator ≈ 0.000006 × 2,134 ≈ 0.01278. Denominator ≈ 8,347 × 17.39 ≈ 145,134. Ratio ≈ 8.8 × 10⁻⁸. D ≈ (8.8 × 10⁻⁸)^0.2053 ≈ 0.057 m = 57 mm. Select the next standard size: 63 mm or 65 mm nominal pipe.
Frequently asked questions
What Hazen–Williams C value should I use for different pipe materials?
The Hazen–Williams C coefficient quantifies a pipe's smoothness and hydraulic efficiency. New smooth PVC and HDPE pipes use C = 140–150. New ductile iron and steel pipes use C = 130–140. Concrete pipes range from C = 120–140 depending on lining and age. Old unlined cast iron can fall to C = 80–100 due to tuberculation and corrosion. For long-term design, it is conservative to reduce C by 10–20% from the new-pipe value to account for aging, biofilm, and scale buildup over the pipe's service life.
How does elevation difference affect pipe sizing for water supply systems?
When water must flow uphill, elevation difference consumes part of the available driving pressure, leaving less head to overcome pipe friction. This means the pipe must be larger to convey the same flow rate at the required delivery pressure. Conversely, when flow is downhill, gravity adds to the available head, potentially allowing a smaller pipe. In this calculator, the elevation difference is subtracted from the pressure head to find the net head available for friction losses. Always confirm that residual pressure at the delivery point meets the minimum service pressure required by building codes or the client brief.
What is the difference between nominal pipe size and calculated minimum diameter?
The calculated diameter is the theoretical minimum internal diameter needed to satisfy the hydraulic conditions. Nominal pipe size (NPS or DN) is a standardised commercial designation that approximates the outside diameter — the actual internal diameter depends on the pressure class (wall thickness) of the specific pipe selected. Always look up the internal diameter of the chosen nominal size and wall class, then verify that it equals or exceeds the calculated minimum. It is common practice to select the next larger standard size above the calculated value to provide a hydraulic safety margin and accommodate future demand growth.