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Friction Loss Calculator

Estimate straight-pipe water head loss with Hazen-Williams using flow, inside diameter, roughness value, and pipe length.

Varies by pipe material and condition, e.g. 150 for new PVC.

Result

0.585 ft/100ft

100 GPM through a 4in pipe (C=150) loses about 0.585 ft of head per 100 ft

Quick Answer

A friction loss calculator uses flow, pipe bore, and a Hazen-Williams C value to estimate water head loss. At 100 GPM through a 4-inch pipe with C = 150, the loss is 0.585 feet per 100 feet. Across 500 feet, the loss is 2.926 feet.

Water Pipe Friction Loss Explained For Design Checks

A friction loss calculator estimates energy lost as water moves through a full pipe. This tool uses Hazen-Williams. The method is common in water and fire pipe work. More flow raises loss fast. A wider pipe cuts loss fast. The C value stands for the chosen pipe's smoothness. The tool shows loss per 100 feet. A length input also gives total straight-pipe loss. Plumbers and designers can use the result for an early check. The method is for water, not every fluid. A code job still needs the right pipe data. Fittings, height, outlet pressure, surge, and demand must also be checked. Some systems need approved software and a signed design. Use this answer as one part of a full pressure and flow check for the whole system.

The Hazen-Williams Formula For Water Pipe Head Loss

Fittings, bends, valves, and elevation changes must be calculated separately. The head loss result represents friction losses along straight pipe runs.

  • h100 = 0.2083 × (100 ÷ C)^1.852 × Q^1.852 ÷ d^4.8655.
  • Total Head Loss = h100 × (Pipe Length ÷ 100).

h100: feet of water per 100 feet of pipe.

Q: gallons per minute.

C: Hazen-Williams value.

d: actual inside diameter in inches.

Pipe length: straight run in feet.

How To Use A Friction Loss Calculator In Five Steps

Inputs

  • Flow: water flow in GPM.
  • Inside diameter: real pipe bore in inches.
  • C value: pipe roughness value for the design case.
  • Length: optional straight run in feet.

Steps

  1. Confirm the fluid is water.
  2. Enter the flow.
  3. Enter the inside diameter.
  4. Enter the C value.
  5. Add length for total loss.

Water Pipe Friction Loss Example With Full Working

Flow is 100 GPM, inside diameter is 4 in, C value is 150, and length is 500 ft.

  1. Calculate h100: 0.2083 × (100 ÷ 150)^1.852 × 100^1.852 ÷ 4^4.8655.
  2. Roughness ratio: (100 ÷ 150)^1.852 = 0.4719.
  3. Flow factor: 100^1.852 = 5,058.25.
  4. Diameter factor: 4^4.8655 = 849.8121.
  5. Friction loss per 100 ft: 0.2083 × 0.4719 × 5,058.25 ÷ 849.8121 = 0.5851 ft.
  6. Total loss: 0.5851 × (500 ÷ 100) = 2.9256 ft.

Loss is 0.585 ft per 100 ft and 2.926 ft across 500 ft, rounded to three decimal places.

When Hazen-Williams Fits A Water Pipe Design Check

Use this tool for an early water-pipe check when the job accepts Hazen-Williams. Use Darcy-Weisbach or the required job method for other fluids or wider heat ranges.

For nearby fluid velocity and flow calculations, see the engineering calculators hub and the glossary.

Assumptions

  • The fluid is water.
  • Flow is steady and the pipe is full.
  • Inside diameter is correct.
  • The C value fits the pipe and age.
  • Total loss covers straight pipe only.

Limitations

  • This calculator uses the Hazen-Williams formula, which applies to water flow only and is not accurate for other fluids.
  • Fittings and valves are not included.
  • Height and outlet pressure are separate.
  • Surge and trapped air are not modeled.
  • Code work may need a signed check.

In Practice

Nominal pipe size is a frequent source of error when the bore is smaller. Check the maker's size table because a small bore change can shift the result a lot.

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Frequently Asked Questions About Water Friction Loss

Why Does Pipe Diameter Change Head Loss So Much?

Pipe bore has a large power in the formula, so a small bore change can cause a large loss change. Use the real inside diameter. Do not use nominal pipe size; check the maker table for the exact pipe bore.

What Does The Hazen-Williams C Value Mean?

The Hazen-Williams C value shows pipe smoothness for the chosen design case. Pipe type, age, wear, and lining can change the value over time. Use a number from the job rule, maker, or design guide.

Does The Result Include Elbows And Valves?

The calculator result covers friction in the entered straight pipe length only. Elbows, tees, valves, meters, and entries add more loss. Add each item with an approved loss value, then check total head for the full run.

Is Hazen-Williams Valid For Oil Or Other Fluids?

Hazen-Williams is a water-only rule that does not directly use fluid viscosity in the calculation. Use Darcy-Weisbach or another required method for oil, gas, or other fluids. Follow the job rule when water heat or other conditions fall outside the accepted range.

How Is This Different From A GPM Calculator?

A basic GPM calculator finds volume moved over time, while this tool estimates pipe head loss. The friction check starts with known GPM and pipe data. Pipe sizing may need both results plus pressure and speed checks.

Sources

Last updated: . Reviewed for accuracy against the formula shown above.