Enter a pipe's inside diameter and the fluid's velocity to find the volumetric flow rate.
Flow Rate Calculator
LiveThe formula
A 2-inch pipe with fluid moving at 5 ft/s: area ≈ 0.0218 sq ft, flow = 0.0218 x 5 ≈ 0.1091 cubic ft/second, which converts to about 48.96 gallons per minute.
Step-by-step guide
- Measure the pipe's inside diameter, not its outside diameter - wall thickness matters.
- Determine or estimate the fluid velocity - this often comes from a flow meter reading or a pump/system specification.
- Read the flow rate in your preferred unit.
Inside diameter vs. nominal pipe size
Pipe is often labeled by a "nominal" size (like "2-inch pipe") that doesn't exactly match its true inside diameter - actual inside diameter varies by pipe schedule, material, and wall thickness. For an accurate flow calculation, measure the actual inside diameter directly or check your specific pipe's manufacturer specification rather than assuming the nominal label is exact.
Common mistakes
Frequently asked questions
How do I find the fluid velocity if I don't already know it?
A flow meter gives velocity or flow rate directly. Without one, velocity can sometimes be estimated from pump curves, system design specifications, or by timing how long it takes to fill a known volume at a discharge point.
Does pipe material affect the flow rate for a given velocity?
Not for this specific formula - flow rate at a given velocity depends only on the cross-sectional area. Pipe material and roughness matter for a separate calculation: how much pressure is needed to achieve that velocity in the first place.
Why does flow rate matter for pipe sizing?
Undersized pipe forces higher velocity for the same flow rate, which increases friction losses and pressure drop through the system - a key consideration when selecting pipe size for a new installation.
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