What is the Hazen-Williams formula?

The Hazen-Williams formula is an empirical equation widely used in water distribution engineering to relate flow rate, pipe diameter, pipe length, head loss, and pipe roughness (C factor). It is accurate for water at temperatures between 40 F and 75 F (4 C to 24 C) and is not suitable for other fluids or highly turbulent flows.

What is the C factor (Hazen-Williams coefficient)?

The C factor represents the smoothness of the pipe interior. Higher C values mean smoother pipes and less friction. Typical values: PVC = 150, copper = 130-140, new steel = 120, cast iron = 100, old cast iron = 80. A smooth pipe carries more flow for the same pressure drop.

Why is the result rounded up to a standard size?

The formula gives the theoretical minimum diameter. In practice, pipes are manufactured in fixed nominal sizes (1/2", 3/4", 1", 1-1/4", etc.). Rounding up ensures the selected pipe can always deliver the required flow without exceeding the allowable head loss.

Head loss (also called friction loss) is the reduction in pressure due to fluid friction as water travels through the pipe. It is expressed in feet or meters of water column, or equivalently in psi or kPa. Allowable head loss is determined by your system pressure budget.

Does this account for fittings and valves?

No. This calculator covers straight-pipe friction loss only. Fittings, valves, bends, and elevation changes add additional losses. As a rule of thumb, add 10-20% to your pipe length to account for minor losses from fittings.

Pipe Size Calculator

Enter flow rate, pipe length, allowable head loss, and pipe material to calculate the minimum required pipe diameter using the Hazen-Williams formula.

Pipe Size Inputs

Unit System

Flow Rate

GPM

Pipe Length

Allowable Head Loss

Pipe Material (C Factor)

Fill in the inputs and click Calculate
to find the minimum pipe size.

Recommended Standard Pipe Size

Min. Calculated Diameter

Hazen-Williams C

Velocity (std pipe)

Recommended < 8 ft/s

Head Loss / 100 ft

in selected std pipe

Nearby Standard Pipe Sizes

Nominal	ID (in)	ID (mm)	Velocity (ft/s)	HL (ft/100ft)

Formula: Q = 0.2815 × C × d^2.63 × (h_f/L)^0.54 — Solved for d: d = (Q / (0.2815 × C × (h_f/L)^0.54))^1/2.63

Summary

This pipe size calculator uses the Hazen-Williams formula to determine the minimum required inside pipe diameter for a given water flow rate, pipe length, and allowable head loss. Supports inputs in US customary (GPM, feet) or metric (L/min, meters) units. Results are rounded up to the nearest standard pipe size and shown in both inches and millimeters. All calculations run entirely in your browser.

How it works

Select your unit system: US customary (GPM, feet, psi) or metric (L/min, meters, kPa).
Enter the design flow rate in gallons per minute (GPM) or liters per minute (L/min).
Enter the total pipe length in feet or meters.
Enter the allowable head loss in feet or meters (or pressure drop in psi/kPa).
Select the pipe material to set the Hazen-Williams C factor.
Click Calculate to see the minimum pipe diameter and the recommended standard pipe size.

Use cases

Sizing water supply lines for residential or commercial buildings.
Designing irrigation systems and selecting the correct pipe diameter.
Verifying that an existing pipe can handle a proposed flow increase.
Engineering fire-suppression system supply lines.
Estimating pipe diameters for HVAC chilled-water loops.
Teaching fluid mechanics and pipe flow to engineering students.
Checking contractor pipe specifications before construction begins.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu