PLUMBING
How to Calculate Water Pressure and Pipe Loss
Contractor method for estimating pressure loss along a water line by pipe size, flow and rise.
The pressure at the meter is not the pressure at the faucet. Friction along the pipe, fittings and the vertical rise all subtract pressure before the water arrives.
THE SHORT ANSWER
How do you calculate water pressure loss?
Add friction loss, elevation and fittings to the total drop. A 60 psi supply over 100 ft of 1-inch pipe at 8 gpm with 5 fittings and a 10 ft rise loses about 6.8 psi, leaving about 53 psi.
Start with the static pressure
Measure or use the supply pressure at the meter. That is the starting point before any loss.
Estimate the friction loss
Longer pipes, smaller diameters and higher flow all increase friction. A common friction model is the Hazen-Williams equation.
- Pipe length
- Pipe diameter
- Flow rate
- Pipe material
Add elevation and fittings
Every vertical foot of rise subtracts pressure, and each elbow, tee or valve adds a small friction loss.
Compute the remaining pressure
Subtract friction, elevation and fittings from the static pressure. If the result is low, the run needs a bigger pipe. Use the Water Pressure and Pipe Loss Calculator.