Pipe Friction & Pressure Loss Calculator

Enter flow in gallons per minute, inside diameter in inches, pipe length in feet and a separately selected Hazen–Williams C coefficient. The default C=130 is an assumed scenario, not a manufacturer value for every pipe. Confirm the coefficient and applicability for the actual material, condition and water conditions.

Hazen-Williams applies to water at ordinary temperatures in turbulent flow, not to other liquids. Fittings, valves, elevation changes and pipe aging add loss. Different sources give slightly different C values.

The tool covers straight pipe, and elevation gain lowers pressure by about 0.433 psi per foot.

Pressure loss (psi) from flow rate

Flow rate	8
Pipe diameter	0.75
Pipe length	50
Roughness coefficient (C)	130
Pressure loss (psi)	5.3
Head loss (feet)	12.2

Common error: Choosing a smaller pipe to save cost on a long run. The pressure lost in the pipe can leave too little at the fixture and force a larger pipe anyway.

Pipe Friction & Pressure Loss: formula

hf = 0.2083 × (100 ÷ C)^1.852 × Q^1.852 ÷ d^4.8655 (Hazen-Williams, ft of head per 100 ft)

EPA EPANET 2.2 User Manual: pipe head-loss models

Pipe Friction & Pressure Loss: changing Flow rate

Pressure loss (psi): assumed Flow rate scenarios.
Flow rate	Pressure loss (psi)	Head loss (feet)
6	3.1	7.2
7	4.1	9.5
8 (example)	5.3	12.2
9	6.6	15.2
10	8	18.5

Pipe Friction & Pressure Loss: compare input ranges

Pipe Friction & Pressure Loss Calculator: Pressure loss (psi) with each input moved on its own, every other input held at the worked-example value.
Input	Tested from	To	Pressure loss (psi): low input	Pressure loss (psi): high input	Swing
Pipe diameter	0.675	0.825	8.8	3.3	5.5 (104%)
Flow rate	7	9	4.1	6.6	2.5 (46%)
Roughness coefficient (C)	117	143	6.4	4.4	2 (38%)
Pipe length	45	55	4.8	5.8	1.1 (20%)

Pipe Friction & Pressure Loss: individual input comparisons

Pipe Friction & Pressure Loss Calculator: change one input at a time; other values remain at the stated example.
Input	Value	Pressure loss (psi)	Head loss (feet)
Flow rate	6	3.1	7.2
	7	4.1	9.5
	8	5.3	12.2
	9	6.6	15.2
	10	8	18.5
Pipe diameter	0.6	15.7	36.2
	0.675	8.8	20.4
	0.75	5.3	12.2
	0.825	3.3	7.7
	0.9	2.2	5
Pipe length	40	4.2	9.8
	45	4.8	11
	50	5.3	12.2
	55	5.8	13.4
	60	6.4	14.7
Roughness coefficient (C)	104	8	18.5
	117	6.4	14.9
	130	5.3	12.2
	143	4.4	10.2
	156	3.8	8.7

Pressure loss (psi): Flow rate against Pipe diameter

Pipe Friction & Pressure Loss: Pressure loss (psi) at combined Flow rate and Pipe diameter settings.
Flow rate / Pipe diameter	0.6	0.75	0.9
6	9.2	3.1	1.3
7	12.3	4.1	1.7
8	15.7	5.3	2.2
9	19.5	6.6	2.7
10	23.7	8	3.3

Pipe Friction & Pressure Loss: values and definitions

Worked-example inputs and the results they produce for the Pipe Friction & Pressure Loss Calculator.
Quantity and definition	Value used
Flow rate — Flow rate through the pipe, in gallons per minute.	8
Pipe diameter — Inside diameter of the pipe, in inches.	0.75
Pipe length — Total length of the pipe run, in feet.	50
Roughness coefficient (C) — Enter a selected Hazen–Williams C coefficient. The default 130 is an assumed scenario, not a verified value for every material.	130
Pressure loss (psi)	5.3
Head loss (feet)	12.2

Pipe diameter: half and double

Pipe Friction & Pressure Loss scale comparison; assumed inputs.
Pipe diameter	Pressure loss (psi)
0.375	154.4
0.75	5.3
1.5	0.182

Water loses pressure as it moves through a pipe, and the loss rises steeply with flow and falls steeply with diameter. Eight gallons a minute through 50 feet of three-quarter inch pipe loses about 5.3 psi in this model, about 12.2 feet of head. Halving the diameter would multiply the loss by about 29, since it depends on the diameter to the power of about 4.87.

That is why a small pipe on a long run starves the fixture at the end.

Use the roughness coefficient for your pipe material and age, and add the losses from fittings and valves, which the straight-pipe formula leaves out.

See flow at a velocity on the flow rate calculator, the water in the pipe on the pipe volume page, and the effect of elevation on the drain slope and rafter pages. The duct sizing and static pressure calculators handle air, and the pressure converter changes units.

How does diameter enter this model?

With flow, length and C held fixed, the implemented head loss varies inversely with diameter to the power 4.8655. It is not an unconditional comparison between pipes carrying different flows.

Does elevation affect the pressure result?

The displayed result covers friction only. A separate elevation change alters static pressure; do not count that change as straight-pipe friction.

Pipe Friction & Pressure Loss Calculator