Gravity Water Flow Rate Calculator
Calculate GPM and Velocity for Gravity-Fed Pipe Systems
Calculation Results
Note: High velocity detected (> 10 ft/s). Consider a larger pipe diameter to reduce friction and noise.
Understanding Gravity Water Flow
A gravity-fed water system relies on the force of gravity to move water from a higher elevation to a lower elevation. This calculator uses the Hazen-Williams Equation to estimate the flow rate, accounting for the vertical drop (Head), pipe diameter, and friction losses caused by the pipe material and length.
The Physics of Gravity Flow
The flow of water in a pipe is dictated by the balance between the potential energy (Head) and the energy lost through friction. Key factors include:
- Vertical Head: Every 2.31 feet of vertical drop creates approximately 1 PSI of static pressure. The more "head" you have, the more pressure is available to push water through the pipe.
- Pipe Diameter: Flow capacity increases exponentially with diameter. Doubling the pipe size can increase flow by more than four times.
- Friction Loss: As water rubs against the pipe walls, it loses energy. This is why a 1,000-foot pipe will have significantly lower flow than a 10-foot pipe of the same diameter.
- C-Factor: This represents the smoothness of the pipe interior. Smoother pipes (like PVC) have a higher C-factor (150) and allow more flow than rougher, older pipes.
The Hazen-Williams Formula
Q = 0.43 × C × d2.63 × (h / L)0.54
Where:
- Q = Flow rate in Gallons Per Minute (GPM)
- C = Pipe material roughness coefficient
- d = Inside diameter of the pipe in inches
- h = Vertical head (drop) in feet
- L = Total pipe length in feet
Practical Example
Imagine you have a water tank located on a hill 50 feet above your cabin. You run 200 feet of 1-inch PVC pipe (C-factor 150) to the cabin.
- Head: 50 ft
- Diameter: 1 inch
- Length: 200 ft
- Result: You would achieve approximately 22.5 GPM at the cabin.
Frequently Asked Questions
Does pipe length affect pressure?
Static pressure (when water is not moving) depends only on the vertical drop. However, "working pressure" or dynamic pressure decreases as the pipe length increases due to friction.
What is a good water velocity?
For most residential systems, a velocity between 2 and 5 feet per second (ft/s) is ideal. Velocities over 10 ft/s can cause "Water Hammer" and erode pipe fittings.