Pipe Pressure Drop Calculator (Hazen-Williams)

Include the equivalent length of fittings in the pipe length. Elevation gain costs 0.433 psi per foot of rise.

Important: Plumbing sizing is governed by the plumbing code adopted in your area (for example the IPC or UPC) and local amendments. This result is an engineering estimate, not a code determination.

Tapeline — Pipe Pressure Drop Calculator (Hazen-Williams)

Printed October 9, 2026

Flow rate10 gpm
Pipe inside diameter1.025 in
Pipe length (including fitting allowance)100 ft
Pipe material (roughness)Copper (C = 140)
Elevation rise (negative for drop)0 ft

Results update as you type.

gpm

Use the actual inside diameter, not the nominal size. ¾ in type L copper ≈ 0.785 in.

Loading animation…
Watch the step-by-step video

Watch it explained

A step-by-step video using your numbers: what to enter, what the result means, and a “what if” example. On-screen instructions follow each step as it happens.

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What the animation shows

The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.

Finished frame of the pipe pressure drop calculator (hazen-williams) animation for the default example
The last frame of the animation for the default example.

What you enter

  • Flow rate: 10 gpm.
  • Pipe inside diameter: 1.025 in. Use the actual inside diameter, not the nominal size. ¾ in type L copper ≈ 0.785 in.
  • Pipe length (including fitting allowance): 100 ft.
  • Pipe material (roughness): Copper (C = 140).
  • Elevation rise (negative for drop): 0 ft.

What happens, step by step

  1. 10 gpm rubbing on the pipe wall costs pressure every foot.
  2. 25 ft along, the gauge already reads 0.76 psi less.
  3. Level run: all the loss is friction, set by flow, length, bore and material.
  4. At the end: 3.02 psi lower than where it started.

The answer: total pressure loss is 3.02 psi.

How it’s calculated

  • Friction (psi) = 4.52 × L × Q^1.852 ÷ (C^1.852 × d^4.8704)
  • Q in gpm, d inside diameter in inches, L in feet
  • Elevation (psi) = 0.433 × rise (ft)

Worked example

10 gpm through 100 ft of 1 in type L copper (1.025 in ID), level run.

Inputs

Flow rate
10 gpm
Pipe inside diameter
1.025 in
Pipe length (including fitting allowance)
100 ft
Pipe material (roughness)
Copper (C = 140)
Elevation rise (negative for drop)
0 ft

Results

Total pressure loss
3.02 psi
Friction loss
3.02 psi
Elevation loss
0 psi
Velocity
3.89 ft/s

Load this example into the calculator

Assumptions and limits

  • Hazen-Williams applies to water near room temperature in turbulent flow. It is less accurate for hot water, very small pipes and very low velocities.

Sources: Hazen-Williams equation, US customary form (ASPE/Cameron Hydraulic Data).

Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.

Questions

How much pressure do I lose per floor?

About 0.433 psi per foot of height, so a 10 ft floor-to-floor rise costs about 4.3 psi before friction.

What C value should I use for PEX?

Smooth plastics are commonly modelled with C = 150. Fittings in PEX systems have significant restriction — add equivalent length for them.