# Friction Loss Calculator

Calculate pipe friction loss using the Hazen-Williams formula. Enter flow rate, pipe diameter, length, and material for PSI drop and head loss.

## What this calculates

When water flows through a pipe, friction between the water and the pipe wall causes pressure to drop. This calculator uses the Hazen-Williams equation to find the friction loss for any pipe run. A 100 GPM flow through 100 feet of 2-inch PVC pipe loses about 5.9 PSI, for example. Firefighters, plumbers, irrigation designers, and HVAC engineers all use this calculation.

## Inputs

- **Flow Rate** (GPM) — min 0 — Flow rate in gallons per minute.
- **Pipe Inside Diameter** (in) — min 0.25 — Internal diameter of the pipe (not nominal size).
- **Pipe Length** (ft) — min 0 — Total pipe run length.
- **C Factor (Roughness)** — options: PVC / CPVC / HDPE (150), New copper / New steel (140), Cement-lined ductile iron (130), New cast iron (120), Old cast iron / Corroded steel (100), Severely corroded pipe (80) — Hazen-Williams roughness coefficient. Higher = smoother pipe.

## Outputs

- **Friction Loss** (PSI) — Total pressure loss due to friction.
- **Friction Loss per Foot** (PSI/ft) — Pressure loss per linear foot of pipe.
- **Head Loss** (ft) — Friction loss expressed as feet of water head.
- **Flow Velocity** (ft/s) — Water velocity in the pipe.

## Details

**The Hazen-Williams formula:**

hf = (4.52 × Q^1.852) / (C^1.852 × D^4.8655)

Where hf is head loss per foot of pipe (in feet of water), Q is flow in GPM, C is the roughness coefficient, and D is the inside pipe diameter in inches.

**The C factor explained:**

The C factor represents how smooth the pipe interior is. Higher values mean less friction:
- **PVC/CPVC (150):** Very smooth, lowest friction loss
- **New copper (140):** Smooth metal surface
- **Cement-lined ductile iron (130):** Common in municipal water mains
- **New cast iron (120):** Slightly rougher than coated pipe
- **Old cast iron (100):** Decades of corrosion and mineral buildup
- **Severely corroded (80):** Old, unlined pipes in poor condition

**Flow velocity guidelines:**

The velocity output helps you check whether the flow rate is appropriate for the pipe size. Recommended maximum velocities:
- **Domestic plumbing:** 5-8 ft/s
- **Fire sprinkler mains:** up to 20 ft/s (during flow)
- **Municipal water mains:** 3-5 ft/s

Velocities above 10 ft/s cause noise, erosion, and water hammer. If the calculator shows a high velocity, consider upsizing the pipe.

**Firefighting application:**

Fire departments use a simplified friction loss formula for hose: FL = CQ²L, where C is a hose coefficient, Q is flow in hundreds of GPM, and L is hose length in 100-foot sections. This pipe calculator is more accurate for fixed piping systems.

## Frequently Asked Questions

**Q: What is friction loss in pipes?**

A: Friction loss is the pressure drop caused by water rubbing against the pipe walls and internal turbulence as it flows. The longer the pipe, the smaller the diameter, and the rougher the interior surface, the more pressure you lose. It is measured in PSI or feet of water head.

**Q: What C factor should I use?**

A: Use 150 for PVC, CPVC, or HDPE plastic pipe. Use 140 for new copper or steel. Use 130 for cement-lined ductile iron. For existing systems, the C factor decreases with age as corrosion and scale build up. If you are unsure about an old pipe, 100 is a conservative choice.

**Q: What is the difference between PSI and head loss?**

A: They measure the same thing in different units. PSI (pounds per square inch) is pressure, while head loss is measured in feet of water. One PSI equals 2.307 feet of water head. Head loss is convenient for pump sizing because pump curves are often plotted in feet of head.

**Q: When should I use Hazen-Williams vs. Darcy-Weisbach?**

A: Hazen-Williams is standard for water flow in pipes at typical temperatures (40-75°F) and moderate velocities. Darcy-Weisbach is more general and works for any fluid, any temperature, and any flow regime. For fire protection, plumbing, and irrigation with water, Hazen-Williams is the industry standard.

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Source: https://vastcalc.com/calculators/physics/friction-loss
Category: Physics
Last updated: 2026-04-08