What This Calculator Does
This tool sizes the internal diameter of a pipe needed to carry a given volume flow rate at a chosen mean flow velocity. It is based on the continuity equation from fluid mechanics and is universal — the physics applies to water, oil, air or any fluid, in any country, as long as you keep consistent units (cubic meters per second for flow and meters per second for velocity).
How to Use It
Enter the volume flow rate Q in cubic meters per second (m³/s) and the target flow velocity v in meters per second (m/s). The calculator returns the required internal diameter in millimeters and meters, plus the resulting cross-sectional flow area. Pick a velocity appropriate to your application — typical water supply lines use about 1–3 m/s to balance friction losses against pipe cost and noise.
The Formula Explained
The continuity equation states that the volume flow rate equals velocity multiplied by cross-sectional area: \(Q = v \cdot A\). For a round pipe the area is \(A = \dfrac{\pi D^2}{4}\). Substituting and solving for diameter gives $$D = \sqrt{\dfrac{4Q}{\pi v}}$$ Larger flow or lower velocity demands a bigger diameter; higher allowable velocity lets you use a smaller, cheaper pipe.
Worked Example
Suppose you need to move \(Q = 0.05\ \text{m}^3/\text{s}\) at a velocity of \(v = 2\ \text{m/s}\). Then \(4Q = 0.2\), and \(\pi v = 6.2832\), so \(\dfrac{4Q}{\pi v} = 0.031831\). The square root is $$D = \sqrt{0.031831} = 0.17841\ \text{m} \approx 178.4\ \text{mm}.$$ So you would select the next standard pipe size at or above roughly 180 mm.
FAQ
What velocity should I choose? For clean water, 1–3 m/s is common; pump suction lines are kept lower (around 0.6–1.5 m/s) to avoid cavitation.
Is this inner or outer diameter? The formula gives the internal (bore) diameter through which fluid actually flows — always select a pipe whose inner diameter meets or exceeds this value.
How do I convert other flow units? Divide L/s by 1000, or m³/h by 3600, to get m³/s before entering the value.
