Water Pressure Drop
Estimate pressure loss in a pipe from length and flow.
About Water Pressure Drop
Friction in a pipe raises as flow rises and drops sharply as diameter shrinks. Converting that loss to psi shows what pressure actually reaches the fixture. Long, narrow or high-flow runs can drop pressure below a usable level, and this estimate justifies larger pipe or shorter runs.
Formula
Water Pressure Drop
Hazen-Williams style drop ≈ length x flow^1.85 / diameter^4.87
- head loss
- Energy lost to pipe friction
Worked example
With the default values below, Water Pressure Drop returns Velocity of 1.5. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Pipe lengthminimum 0 ft
- 50 ft
- Flow rateminimum 0 gpm
- 5 gpm
- Pipe inner diameterminimum 0.1 inches
- 0.75 inches
- Start pressureminimum 0 psi
- 50 psi
Results
- Velocity
- 1.5 ft/s
- Head Loss
- 239.1 ft
- Pressure Drop
- 103.55 psi
- Pressure at End
- 0.00 psi
Step by step
- 1Values used
- 2Water Pressure Drop
- 3Velocity= 1.5 ft/s
- 4Head Loss= 239.1 ft
- 5Pressure Drop= 103.55 psi
- 6Pressure at End= 0.00 psi
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Water Pressure Drop calculated?
Hazen-Williams style drop ≈ length x flow^1.85 / diameter^4.87 Friction in a pipe raises as flow rises and drops sharply as diameter shrinks. Converting that loss to psi shows what pressure actually reaches the fixture.
Why does Water Pressure Drop matter?
Long, narrow or high-flow runs can drop pressure below a usable level, and this estimate justifies larger pipe or shorter runs.
What values do I need to enter?
This calculator takes 4 inputs: Pipe length, Flow rate, Pipe inner diameter, Start pressure. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.