Water Pressure Loss Calculator
Estimate Hazen-Williams pipe-friction loss, elevation loss, flow velocity and remaining pressure for a water line.
Default project preview
47.34 psi
Friction loss
6.16 psi
Elevation loss
6.5 psi
Total head loss
8.9 m
Flow velocity
4.08 ft/s
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Straight pipe length
Enter this project assumption in ft. The current imperial default is 150 ft.
Equivalent fitting length
Enter this project assumption in ft. The current imperial default is 30 ft. Minimum: 0.
Actual inside diameter
Enter this project assumption in in. The current imperial default is 1 in.
Water flow
Enter this project assumption in gal/min. The current imperial default is 10 gal/min.
Hazen-Williams C factor
Enter this project assumption in a unitless value. The current imperial default is 140.
Elevation rise
Enter this project assumption in ft. The current imperial default is 15 ft. Minimum: 0.
Available inlet pressure
Enter this project assumption in psi. The current imperial default is 60 psi.
How to read the outputs
Friction loss
pipe and equivalent fittings. Default preview: 6.16 psi.
Elevation loss
static elevation rise. Default preview: 6.5 psi.
Total head loss
friction plus elevation. Default preview: 8.9 m.
Flow velocity
inside pipe velocity. Default preview: 4.08 ft/s.
Effective pipe length
straight plus fittings. Default preview: 180 ft.
Flow rate
C factor 140. Default preview: 10 gal/min.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Hazen-Williams friction head = 10.67 × length × flow^1.852 ÷ (C^1.852 × inside diameter^4.871) in SI units. Elevation head is added before conversion to pressure.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
Use this calculator to estimate friction pressure loss for a pipe run from flow, inside diameter, length and editable roughness assumptions. It supports comparison, not final plumbing or fire-protection design.
Calculation steps
- 1Use actual inside diameter, not nominal trade size.
- 2Enter design flow and straight length.
- 3Add equivalent length or fitting losses consistently.
- 4Combine friction with elevation and equipment losses.
Worked numerical example
A long small-diameter pipe carrying high flow can lose many psi before elevation and fittings are included. Increasing inside diameter typically reduces friction sharply for the same flow.
Purchasing and rounding
Pipe size should be selected after checking velocity, pressure at fixtures, material limits and standard available sizes. The calculator does not create a bill of materials.
Interpret the result
Pressure loss is highly sensitive to diameter. A small diameter change can matter more than a modest change in length.
Page-specific assumptions
- Steady full-pipe flow.
- Fluid properties near the selected condition.
- Roughness or coefficient suits the pipe material.
- Minor losses are included separately or through equivalent length.
Common mistakes
- Using outside diameter.
- Ignoring elevation head.
- Mixing gpm and L/s.
- Applying one empirical coefficient outside its intended range.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Water Pressure Loss Calculator questions
Why use actual inside diameter?+
Small diameter changes have a large effect on friction loss, and nominal sizes do not equal the inside bore.
What is the Hazen-Williams C factor?+
It is an empirical roughness coefficient; smoother pipe uses a higher value, while ageing and deposits can reduce it.
Does this account for water hammer?+
No. Surge and transient pressure require separate analysis and appropriate controls.