About the Friction Loss Calculator
Water loses pressure as it flows through a pipe. It rubs against the pipe walls, and swirls through every elbow, tee and valve, turning pressure into heat. This friction loss decides whether a sprinkler at the end of a long lawn gets enough pressure, whether a shower upstairs runs strongly, and how big a pump needs to be. Too small a pipe can waste most of the available pressure on friction alone.
This friction loss calculator uses the Hazen–Williams formula, the standard method for water in pressure pipes. Enter the flow in gallons per minute, the inside diameter, the length and the pipe material, and it gives the pressure loss in psi, the equivalent feet of head, and the water velocity with a check against common design limits.
How to Use the Friction Loss Calculator
Enter the flow in gallons per minute.
Enter the inside diameter of the pipe in inches.
Enter the pipe length, choose the material, and add an equivalent length of fittings if you know it.
The Formulas
loss per foot (psi) = 4.52 × Q^1.852 ÷ (C^1.852 × d^4.87)
total loss = loss per foot × (length + fittings)
head (ft) = psi × 2.31
velocity (ft/s) = 0.4085 × Q ÷ d²Q is the flow in gallons per minute, d the inside diameter in inches and C the roughness coefficient.
Step-by-Step Example
10 GPM through 100 ft of 1-inch PVC.
per foot 4.52 × 10^1.852 ÷ (150^1.852 × 1^4.87) = 0.030 psi
total 0.030 × 100 = 3.0 psi
head 3.0 × 2.31 = 6.93 ft
velocity 0.4085 × 10 ÷ 1² = 4.09 ft/sThe C Factor
The Hazen–Williams coefficient C describes the smoothness of the pipe. Plastic pipes such as PVC and polyethylene are very smooth and stay that way, with C around 150. Copper is close behind at about 140. New steel is around 120, and old cast iron or steel, roughened by corrosion and scale, can drop to 100 or lower. A lower C means more friction. Older plumbing systems often lose much of their pressure because scale has both roughened and narrowed the pipes.
Diameter Matters Most
Friction loss depends on the diameter raised to the power of 4.87, so small changes in pipe size make a huge difference. Going from 1-inch to 1¼-inch pipe cuts the loss by about two-thirds. Doubling the diameter cuts it to about one-thirtieth. Always use the actual inside diameter: nominal pipe sizes are only labels, and the bore varies with the material and wall thickness. Schedule 40 PVC of nominal 1 inch has a bore of about 1.03 inches, while thick-walled pipe of the same nominal size is narrower.
Velocity Limits
Water moving too fast causes noise, erosion of fittings and pipe walls, and dangerous pressure spikes, called water hammer, when valves close quickly. Plumbing and irrigation designers generally keep velocity below about 5 feet per second, and below 8 at most. If the calculator shows high velocity, a larger pipe will both slow the water and reduce friction loss.
Fittings and Valves
Each elbow, tee, valve and meter adds resistance. Engineers express this as an equivalent length of straight pipe. For small pipes, a 90° elbow might equal 3 to 5 feet, a tee through the branch 5 to 8 feet, and a gate valve less than a foot. Add up the equivalent lengths of all fittings on the run and enter them in the fittings box. For runs with many fittings, they can add a third or more to the friction loss.
Elevation
Friction loss is only part of the pressure picture. Water also loses 0.433 psi for every foot it rises, and gains the same for every foot it falls. When sizing a pump or checking pressure at a fixture, add the elevation change to the friction loss, along with any pressure the fixture or sprinkler itself needs.
Choosing a Pipe Size
To size a new pipe, start with the flow you need and the pressure available, subtract what the fixtures or sprinklers require at the end, and allow for elevation. The remainder is the pressure you can afford to lose to friction. Try different diameters in the calculator until the total loss fits within that budget and the velocity stays under about 5 feet per second. Choosing the next size up gives headroom for future demand.
Hot Water and Other Fluids
The Hazen–Williams formula is designed for water at normal temperatures. It is less accurate for hot water, very small pipes and fluids other than water, where the Darcy–Weisbach method is used instead.
Irrigation Laterals
Sprinkler laterals carry less water toward their ends as each head takes its share, so the real loss is lower than a full-flow calculation suggests. A common approximation multiplies the full-flow loss by about 0.36 for a lateral with many evenly spaced outlets.
Understanding Your Result
The headline gives the total friction loss in psi.
The working line gives the loss per 100 feet and the pipe details.
The head line converts the loss to feet of head.
The velocity line gives the water speed and whether it is within limits.
When Should You Use This Calculator?
Use it when sizing irrigation mains and laterals.
Use it to check why a long water line has low pressure.
Use it when selecting a pump for a well, pond or booster system.
Common Mistakes
Using the nominal size. Enter the actual inside diameter.
Ignoring fittings. They can add a lot of loss.
Forgetting elevation. Rising water loses pressure too.
Letting velocity run high. It causes noise and water hammer.