Almighty Calculator

Hazen-Williams Friction Loss Calculator

Friction loss in psi per foot of pipe = 4.52 × Q^1.852 ÷ (C^1.852 × d^4.87), with flow Q in gpm and inside diameter d in inches.

gpm
in
1" Schedule 40 PVC is 1.049 in inside diameter.
ft
Total friction loss
25.73psi
Total friction loss (head)59.4 ft
Loss per 100 ft8.58 psi
Water velocity7.42 ft/s

Velocity above 5 ft/s can cause noise, water hammer, and erosion. Consider a larger pipe.

Show the math

Loss per foot = 4.52 × 20^1.852 ÷ (150^1.852 × 1.049^4.87) = 0.08577 psi/ft
Total loss = 0.08577 × 300 ft = 25.73 psi (59.4 ft of head)
Velocity = 0.4085 × 20 ÷ 1.049² = 7.42 ft/s

Rounded the same way as the result above.

How it works

Water pushed through a pipe loses pressure to friction. The Hazen-Williams equation is the standard empirical formula for water at ordinary temperatures in full pipes, and it is what pump and well-system tables are built on.

Enter flow, the pipe's inside diameter (not the nominal size), the length, and the material. The C factor rates how smooth the pipe wall is: higher is smoother. Add fittings and elevation change separately when sizing a pump.

hf (psi per ft) = 4.52 × Q^1.852 ÷ (C^1.852 × d^4.87) Total psi = hf × length in ft Head (ft) = psi × 2.31 Velocity (ft/s) = 0.4085 × Q ÷ d²

Worked example

20 gpm through 300 feet of 1-inch Schedule 40 PVC (1.049 inch inside diameter):

  1. Loss per foot = 4.52 × 20^1.852 ÷ (150^1.852 × 1.049^4.87) = 0.08577 psi/ft
  2. Total loss = 0.08577 × 300 ft = 25.73 psi (59.4 ft of head)
  3. Velocity = 0.4085 × 20 ÷ 1.049² = 7.42 ft/s
InputValue
Flow rate20 gpm
Pipe inside diameter1.049 in
Pipe length300 ft
Pipe material (C factor)PVC / CPVC / HDPE (C=150)
ResultValue
Total friction loss25.73 psi
Total friction loss (head)59.4 ft
Loss per 100 ft8.58 psi
Water velocity7.42 ft/s

Assumptions and limits

  • Valid for water near 60 °F, turbulent flow, and pipe diameters above about 2 inches; it is used more loosely for small pipe with slightly less accuracy.
  • Fittings, valves, and elevation are not included. Add equivalent length for fittings.
  • Use the true inside diameter. Nominal size differs by schedule and material.

Common questions

What C value should I use?

Use 150 for PVC and polyethylene, 140 for copper, 120 for new steel, and lower values for older, corroded iron.

How do I convert psi to feet of head?

Multiply psi by 2.31 to get feet of water; divide feet of head by 2.31 to get psi.

What is a good water velocity?

Keep supply lines at or below about 5 ft/s, and 8 ft/s as an absolute limit for short runs.

Sources

  • Hazen-Williams equation, US customary form: hf = 4.52 Q^1.852 ÷ (C^1.852 d^4.87) in psi per foot, with Q in gpm and d in inches.
  • C factors are typical published values and vary with pipe age and condition.

Updated 2026-09-30