Water Pressure Loss Calculator
Calculates friction-induced pressure drop along a water pipe using the Hazen-Williams equation. Use it when designing irrigation systems, fire suppression lines, or building water supply networks where adequate pressure at the endpoint matters.
Last updated: September 2026
Formula below · 2 sources (epa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
This calculator uses the Hazen-Williams equation, a widely adopted empirical formula for friction loss in pressurized water pipes. In US units it reads: h_f = 0.2083 × (100/C)^1.852 × Q^1.852 / d^4.8655, where h_f is the head loss in feet of water per 100 feet of pipe, C is the Hazen-Williams roughness coefficient (120 is used here, a conservative value for plastic or copper service pipe), Q is the flow rate in GPM, and d is the internal pipe diameter in inches. The calculator scales that per-100-ft loss to your pipe length (× length / 100) and converts feet of head to PSI (1 ft of water = 0.433 PSI): Pressure Loss (PSI) = h_f × (length / 100) × 0.433. A higher C value (smoother pipe) results in less friction loss. Because flow rate is raised to the power 1.852, doubling the flow rate increases the loss about 3.6 times. Fittings and valves add further loss that is not included here.
How to use
Example: 100-foot pipe, 1-inch inside diameter, carrying 10 GPM. Step 1: (100/120)^1.852 ≈ 0.714. Step 2: 10^1.852 ≈ 71.1; 1^4.8655 = 1. Step 3: head loss per 100 ft = 0.2083 × 0.714 × 71.1 ≈ 10.57 ft. Step 4: scale to length: 10.57 × (100 / 100) = 10.57 ft. Step 5: convert to PSI: 10.57 × 0.433 ≈ 4.58 PSI. The same 10 GPM through a 2-inch pipe loses only about 0.16 PSI per 100 ft, which shows how strongly diameter matters.
Frequently asked questions
What is the Hazen-Williams C coefficient and how does it affect pressure loss?
The Hazen-Williams C coefficient represents the smoothness of a pipe's interior surface. Higher C values mean smoother pipes and less friction loss — smooth PVC or new copper pipes have C ≈ 130–140, while older cast iron or corroded steel pipes may have C as low as 80–100. The coefficient appears in the formula as (100/C)^1.85, so lower C values directly increase calculated head loss. Selecting the correct C for your pipe material is critical for accurate pressure loss estimates.
How do I convert head loss in feet to PSI for plumbing pressure calculations?
Head loss calculated by the Hazen-Williams formula is expressed in feet of water column, which is a pressure unit. To convert feet of head to PSI, divide by 2.31 (since 1 PSI equals 2.31 feet of water at standard conditions). For example, 10.57 feet of head loss equals approximately 4.58 PSI. This calculator applies the conversion for you (× 0.433). This conversion is essential when comparing calculated losses against pump specifications or city supply pressure, which are typically given in PSI.
When should I use the Hazen-Williams equation versus the Darcy-Weisbach equation for pipe friction loss?
The Hazen-Williams equation is simpler and works well for water flowing in full turbulent conditions in pipes between 2 and 72 inches in diameter — typical for irrigation, fire protection, and building supply design. The Darcy-Weisbach equation is more physically rigorous and applies to any fluid, any flow regime, and any pipe size, making it preferred for engineering calculations involving non-water fluids, laminar flow, or very small pipes. For residential and commercial water system design, Hazen-Williams is industry standard and gives reliable results without needing fluid viscosity data.