Water Pressure & Flow Calculator
PVC / Plastic (Smooth) – C=150
Copper / Brass – C=140
Steel (New) – C=120
Cast Iron (Old) – C=100
Corroded Pipe – C=80
Fluid Velocity:
0 ft/s
Pressure Loss (PSI):
0 PSI
Head Loss (Feet of Head):
0 ft
Velocity Pressure:
0 PSI
How to Calculate Water Pressure from Flow Rate
Understanding the relationship between water flow rate and pressure involves fluid dynamics physics, specifically the concepts of friction loss and velocity. Unlike static calculations, determining pressure in a moving fluid system requires analyzing how the pipe material, diameter, and length resist the flow of water.
This calculator utilizes the Hazen-Williams equation, which is the standard method for calculating pressure drop (head loss) in water distribution systems and irrigation pipes.
Key Variables in the Calculation
- Flow Rate (GPM): The volume of water moving through the pipe per minute. Higher flow rates exponentially increase friction and pressure loss.
- Pipe Diameter: The internal width of the pipe. A smaller diameter forces water to move faster, significantly increasing friction and pressure drop.
- Roughness Coefficient (C-Factor): This represents the smoothness of the pipe interior. Plastic (PVC) is very smooth (C=150), while old iron is rough (C=100), causing more pressure loss.
The Formulas Used
To provide accurate results, we perform three distinct calculations:
1. Fluid Velocity
First, we determine how fast the water is moving inside the pipe:
Velocity (ft/s) = (0.4085 × Flow Rate) / (Diameter)²
2. Friction Head Loss (Hazen-Williams)
Next, we calculate the energy lost due to friction against the pipe walls:
Head Loss = 0.2083 × (100 / C)1.852 × (Flow Rate)1.852 / (Diameter)4.8655
This gives the loss per 100 feet of pipe. We then multiply by the total length of your pipe run.
3. Pressure Conversion
Finally, we convert "Head Loss" (measured in feet of water column) into PSI (Pounds per Square Inch):
PSI = Head Loss (ft) × 0.433
Practical Example
If you are pumping 10 GPM through 100 feet of 1-inch PVC pipe:
- The water velocity would be approximately 4.08 ft/s.
- The friction head loss would be roughly 7.2 feet.
- The resulting pressure drop would be about 3.1 PSI.
This means if your source pressure is 50 PSI, the pressure at the end of this 100-foot hose would be approximately 46.9 PSI (excluding elevation changes).