How this calculation works
Convert the internal diameter to metres and use the full circular flow area. Volume flow equals area times average velocity. Density converts that volume flow into mass flow.
Q = πD²v/4; v = 4Q/(πD²); mass flow = ρQA worked example
A fully filled 100 mm internal-diameter pipe at average velocity 2 m/s carries about 942.4778 L/min, or 56.5487 m³/h.
Good to know
Steady full-pipe continuity arithmetic. No pressure loss, roughness, compressible flow or allowable velocity is inferred. Use average flow velocity rather than an uncorrected centreline reading.
Reference: OpenStax: flow rate and continuity (opens in a new tab) ↗
A flow rate counts volume passing per time
Imagine a short fluid cylinder moving through a fully filled circular pipe. Its volume is flow area times travel distance. Divide by time and the distance-per-time becomes average velocity, giving Q = Av.
A 100 mm bore at 2 m/s carries approximately 942.4778 L/min. The millimetres must first become metres; forgetting that conversion creates an area error of one million.
The pipe Reynolds number calculator adds viscosity and density to interpret the same average velocity and internal diameter.
Diameter changes velocity through its square
At the same flow, halving diameter quarters area and multiplies average velocity by four. Doubling diameter quadruples area and quarters velocity. The comparison table holds flow constant so this relationship can be inspected directly.
The pipe Reynolds number calculator combines velocity with diameter and fluid viscosity. The Darcy Weisbach pressure loss calculator uses velocity to estimate losses from an independently supplied friction factor. Continuity alone does not give either quantity.
Nominal size and centreline readings need care
Use actual internal diameter rather than an external measurement or nominal trade size. Wall thickness and manufacturing conventions can make the internal flow area different from an assumed nominal circle.
The speed in Q = Av is averaged over the cross-section. A centreline velocity can differ from that average. This page also assumes the whole cross-section is filled; a partially filled pipe needs its actual wetted flow area.
A few questions, answered
Does this choose the correct pipe size?
No. It compares flow and velocity; acceptable pressure loss and operating constraints require additional information.
Can I enter mass flow directly?
The input is volume flow. Mass flow is displayed using your supplied density.
Why do two pipes with equal flow have different velocities?
Their internal areas differ, and the same volume per second must pass through each area.