Pipe Pressure Drop Calculator
Friction Steals Pressure Before Water Ever Reaches the Fixture
Every foot of pipe a fluid travels through costs some pressure to friction against the pipe wall, and that loss compounds with flow rate, distance, and how rough the interior surface is. The Hazen-Williams equation is the standard way plumbing designers quantify that loss without modeling fluid mechanics from first principles — it condenses pipe material, diameter, length, and flow into a single pressure-drop figure.
The Formula
Total Pressure Loss = (Pressure Loss per 100 ft) × (Length / 100)
Here Q is flow rate in GPM, d is inside pipe diameter in inches, and C is the Hazen-Williams roughness coefficient — a higher C means a smoother pipe and less friction loss. This calculator uses C=150 for PVC/plastic, C=140 for copper, C=120 for new cast iron, and C=100 for aged steel or cast iron.
Where This Calculation Matters
- Long supply runs — confirming a fixture at the far end of a run still gets adequate pressure after friction losses.
- Pipe material selection — comparing how much less pressure drop a smoother material like copper or PVC produces versus aged steel over the same run.
- Booster pump sizing — total system pressure loss (this calculation plus elevation change) determines how much a booster pump needs to add.
- Diagnosing weak flow — when a fixture underperforms, comparing calculated versus observed pressure drop can flag a partially blocked or undersized pipe.
Worked Example by Pipe Material
20 GPM flowing through 100 ft of 1 in pipe:
| Material | C-factor | Pressure drop |
|---|---|---|
| PVC / Plastic | 150 | 0.11 psi |
| Copper | 140 | 0.12 psi |
| Cast Iron, new | 120 | 0.16 psi |
| Steel / Cast Iron, aged | 100 | 0.23 psi |
The exponent on flow rate (1.85) means doubling the flow rate through the same pipe more than triples the pressure drop, not just doubles it.
How to Use This Calculator
- Enter the flow rate in gallons per minute.
- Enter the pipe's inside diameter in inches.
- Enter the pipe length in feet.
- Select the pipe material to set the C-factor (PVC, copper, new cast iron, or aged steel/cast iron).
- Select Calculate to get the total pressure drop and the drop per 100 ft.
Related Calculations
To size a pipe from a required flow rate before estimating its pressure drop, use the Pipe Size Calculator. For elevation-based pressure separate from friction loss, see the Water Pressure Calculator.