Pipe Friction Loss Calculator
Calculates pressure loss (in PSI) due to pipe friction using the Hazen-Williams equation, given flow rate, pipe length, diameter, and material. Ideal for plumbers and engineers sizing water supply lines.
Last updated: September 2026
Formula below · 2 sources (epa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Friction loss is the reduction in water pressure caused by the resistance of pipe walls as water flows through them. This calculator uses the Hazen-Williams equation, a widely accepted empirical formula for pressurized water systems: 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 (higher C = smoother pipe), Q is the flow rate in GPM, and D is the internal pipe diameter in inches. The calculator scales this to your pipe length and converts feet of head to PSI (1 ft of water = 0.433 PSI): Pressure Loss (PSI) = h_f × (L / 100) × 0.433. Smoother materials like PVC (C ≈ 150) produce less friction loss than older steel (C ≈ 100) or very old cast iron (C ≈ 80). Fittings and valves add further loss not included here. The result helps verify that residual pressure at the end of a pipe run meets minimum fixture requirements; undersizing a pipe or underestimating friction loss leads to inadequate flow at fixtures.
How to use
Example: 20 GPM through 100 ft of PVC pipe (C = 150) with a 1-inch inside diameter. Step 1: (100 / 150)^1.852 ≈ 0.472. Step 2: 20^1.852 ≈ 256.7. Step 3: head loss per 100 ft = 0.2083 × 0.472 × 256.7 / 1^4.8655 ≈ 25.24 ft. Step 4: scale to length: 25.24 × (100 / 100) = 25.24 ft. Step 5: convert: 25.24 × 0.433 ≈ 10.93 PSI. That is far above the ~4 PSI per 100 ft design target, so 1-inch pipe is too small for 20 GPM. Trying D = 2 inches: 2^4.8655 ≈ 29.15, so the loss is 25.24 / 29.15 ≈ 0.87 ft, or about 0.37 PSI.
Frequently asked questions
What is the Hazen-Williams coefficient and how does it affect friction loss?
The Hazen-Williams C coefficient represents a pipe's smoothness—higher values mean less internal resistance and lower friction loss. PVC and copper pipes have C values around 130–150, while older galvanized steel may be as low as 100–120. Using the wrong C value can significantly underestimate or overestimate pressure loss. Always select the coefficient that matches both the pipe material and its age or condition, since older, corroded pipes have effectively lower C values.
How does pipe diameter affect friction loss in water supply systems?
Pipe diameter has a dramatic, nonlinear effect on friction loss—the denominator in the Hazen-Williams formula raises diameter to the power of 4.8655. This means doubling the pipe diameter reduces friction loss by a factor of roughly 2^4.87 ≈ 29 times. Even a small increase in diameter, such as going from 1 inch to 1.5 inches, can cut pressure drop by over 80%. This is why upsizing the pipe is often the most cost-effective solution when pressure loss is excessive.
When should I use the Hazen-Williams equation versus the Darcy-Weisbach equation?
The Hazen-Williams equation is best suited for turbulent flow of water at typical temperatures in pressurized distribution systems—conditions found in most residential and commercial plumbing. It is empirical and not valid for fluids other than water or for low-velocity laminar flow. The Darcy-Weisbach equation is more universally applicable and more accurate across a wider range of flow regimes and fluid types, but requires knowing the Darcy friction factor. For standard water piping design, Hazen-Williams is preferred for its simplicity and widespread code acceptance.