Friction Loss Calculator
Estimate friction (head) loss for water flowing in a pipe using the Hazen-Williams equation. Enter the flow in GPM, the inside diameter in inches, the run length in feet, and pick the pipe material — you get head loss in feet, the pressure drop in psi, and loss per 100 feet.
Example: with Flow rate (GPM) 10 · Inside diameter (inches) 1 · Pipe length (feet) 100 · Pipe material (C-factor) PVC / plastic / PEX (C=150) → Head loss: 6.99 ft of head.
- Pressure drop3.03 psi
- Loss per 100 ft6.99 ft per 100 ft
Computed by the calculator below using its default values. Change any input to see your own numbers.
Friction loss is the pressure a pipe eats through wall drag. Hazen-Williams captures it for water with one empirical roughness number, the C-factor — higher C means a smoother pipe.
How Hazen-Williams estimates loss
Water dragging along a pipe wall loses energy, and that lost energy shows up as a pressure drop. The Hazen-Williams equation is the plumbing industry's workhorse for it: head loss rises with flow to the 1.85 power and falls steeply with diameter to the 4.87 power, all scaled by a roughness coefficient C. The strong diameter term is why bumping a line up one pipe size can slash friction loss dramatically.
The C-factor stands in for wall smoothness — about 150 for new plastic and PEX, 140 for copper, down to 100 for corroded cast iron. Because it is calibrated only for water near room temperature, Hazen-Williams should not be used for viscous fluids, air, or steam; those need the Darcy-Weisbach equation with a friction factor.
How it’s calculated
Head loss per 100 ft = 0.2083 · (100/C)^1.852 · Q^1.852 / d^4.8655, with Q in US GPM and d the inside diameter in inches (US Hazen-Williams form). Total head loss = per-100-ft × length/100. Pressure drop uses 1 ft of water = 0.43352 psi.
Hazen-Williams is empirical and valid only for water near 60 °F in full pipes; it ignores fittings, elevation change, and entrance losses. Add fitting equivalent lengths and confirm critical designs with a licensed engineer.
Hazen-Williams C-factors
| Pipe material | Typical C | Condition |
|---|---|---|
| PVC, PEX, plastic | 150 | New, smooth |
| Copper | 140 | New |
| New steel | 140 | Clean |
| Cast iron / concrete | 130 | Cement-lined |
| Welded steel | 120 | Aged |
| Cast iron | 100 | Old, tuberculated |
Source: AWWA / Hazen-Williams reference tables; values are typical design figures.
Common mistakes
- Entering nominal pipe size instead of true inside diameter — a small ID error is amplified by the 4.87 power.
- Using Hazen-Williams for hot water, glycol, oil, or air — it is calibrated for cool water only.
- Ignoring fittings; elbows, tees, and valves can add more loss than the straight pipe on short runs.
- Picking an optimistic C-factor for an old pipe whose interior has roughened over decades.
Frequently asked questions
What is the Hazen-Williams friction loss formula?
In US units, head loss per 100 ft = 0.2083 × (100/C)^1.852 × Q^1.852 / d^4.8655, with flow Q in GPM and inside diameter d in inches. C is the pipe's roughness coefficient.
What C-factor should I use?
About 150 for new PVC or PEX, 140 for copper and new steel, 130 for cement-lined pipe, and 100 for old cast iron. A lower C means a rougher pipe and more loss.
How do I convert feet of head to psi?
Multiply feet of water head by 0.4335 to get psi. So 10 feet of head loss is about 4.3 psi of pressure drop.
When should I not use Hazen-Williams?
Skip it for viscous fluids, air, steam, or hot water. Those need the Darcy-Weisbach equation, which accounts for viscosity through the Reynolds number. For critical work, have an engineer verify the design.