Water Flow Calculator
If you know how fast water moves through a pipe (from a flow meter or ultrasonic gauge), the flow rate follows directly from the pipe's inside area. This tool converts velocity into GPM and gallons per hour.
Inputs
Result
Planning estimate - not a substitute for a pressure-tested engineering design.
Formula
Flow (GPM) = 2.448 × velocity (ft/s) × (inside diameter in inches)². The constant converts ft/s through a circular cross-section into US gallons per minute.
Example
4 ft/s in 1-inch PVC (d = 1.049): 2.448 × 4 × 1.049² ≈ 10.8 GPM, about 647 gallons per hour. One hour of running moves ~647 gallons.
Assumptions
- Water fills the pipe completely and flows uniformly.
- Velocity is measured or specified at the same point as the diameter used.
Limitations
This tool is a planning aid for DIY design and pre-design work. It does not replace an on-site pressure and flow test, a catch-cup uniformity test, or local code and backflow requirements. For steep slopes, long pipe runs, large elevation changes or commercial-scale systems, have a qualified irrigation designer verify the plan before installation.
What This Means
Flow scales with the square of diameter - a 2-inch pipe at the same velocity carries 4× the water of a 1-inch pipe. That is why upsizing the main line is the cheapest capacity upgrade.
Next Step
Compare with your real supply using the GPM Calculator, or convert the flow into pressure loss with the GPM to PSI Calculator.
Sources
Frequently Asked Questions
How do I measure water velocity?
Insertion paddle-wheel or ultrasonic clamp-on flow meters report velocity directly; some municipal meters can be read for total volume instead.
Why does diameter matter so much?
Area grows with the square of diameter, so small size changes produce large flow changes at constant velocity.