Pipe Pressure Loss Calculator
Free pipe pressure loss calculator using the Hazen-Williams equation: head loss and PSI drop by pipe material, flow and diameter.
What does the Pipe Pressure Loss Calculator do?
It computes the friction head loss and pressure drop along a straight pipe using the Hazen-Williams equation, taking the flow rate, internal diameter, pipe material and length, and returning the loss in feet of head and psi.
- Inputs: flow rate, internal diameter, pipe material, pipe length.
- Output: head loss in feet and psi, plus velocity and friction slope.
- Method: the Hazen-Williams empirical equation with standard C factors.
Quick answer
100 gallons per minute through 100 feet of 4 inch PVC loses about 0.58 feet of head, or 0.25 psi, at a velocity of 2.55 feet per second. Smoother pipe holds a higher C factor and loses less head; older, rougher pipe loses more.
What This Calculator Really Does
Water moving through a pipe rubs against the pipe wall, and that friction converts pressure into heat and turbulence, so the pressure falls from one end of the pipe to the other. The Hazen-Williams equation is the standard empirical way to predict that fall for water at ordinary temperatures. It needs only the flow rate, the internal diameter and a roughness coefficient C that describes how smooth the pipe is. The result is usually quoted as head loss, the height of water that would produce the same pressure, because head is easier to work with than psi in pump and gravity systems.
The rules it applies
- C factors: PVC 150, copper 130, steel 120, cast iron 100. Higher is smoother.
- Velocity: flow in cubic feet per second divided by the internal area, in feet per second.
- Hydraulic radius: internal diameter divided by four for a pipe flowing full.
- Head to pressure: one foot of water equals 0.433 psi.
- Not modelled: fittings and valves (add equivalent length), elevation change, and very high velocities where the flow becomes fully turbulent.
Worked example with real numbers
100 gpm through 100 feet of 4 inch PVC with C = 150. Flow is 0.2228 cubic feet per second and the internal area is 0.0873 square feet, so the velocity is 2.55 feet per second. The hydraulic radius is 0.0833 feet and the friction slope works out at about 0.0058 feet per foot, giving a head loss of 0.58 feet over the 100 foot run. Converted to pressure that is 0.25 psi.
Reading the result
Keep the velocity in a sensible band, usually 2 to 5 feet per second for supply piping and up to 8 for some services; above that, noise and erosion accelerate and the head loss climbs steeply because it depends on velocity raised to a power. The single biggest lever is diameter: doubling the pipe diameter cuts the head loss by roughly a factor of thirty for the same flow, which is why undersized mains are the usual cause of a weak far-end tap. Remember to add the equivalent length of elbows, tees and valves to the straight length before trusting the total, and allow for elevation if the pipe rises.
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Frequently Asked Questions
How is the pipe pressure loss calculated?
With the Hazen-Williams equation: the velocity is found from the flow and internal area, the friction slope from velocity, diameter and the C factor, and the head loss is the slope times the length. The steps panel shows each figure.
What do I need to use the Pipe Pressure Loss Calculator?
The flow rate in gallons per minute, the internal diameter, the pipe material and the length. The standard Hazen-Williams C factors are built in.
What does the result from the Pipe Pressure Loss Calculator show?
The head loss in feet and psi, plus the velocity and the friction slope per 100 feet, so you can size the pipe.
What C factor does PVC have in Hazen-Williams?
150, the smoothest of the common materials. Copper is about 130, steel about 120 and older cast iron about 100, so the same pipe loses more head as it ages or changes material.
Is the Pipe Pressure Loss Calculator really free?
Yes — 100 percent free, no sign-up, and everything runs in your browser.
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