Free Online Calculators
Pipe diameter + velocity → flow rate & Reynolds number
This pipe flow calculator computes the flow rate in L/min (Q = A × v) and the Reynolds number from inner diameter and flow velocity. The Reynolds number shows whether the flow is laminar (below 2,300) or turbulent (above 4,000). Choose water or air at 20 °C as the medium; pressure drop is not calculated.
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Whether you are sizing a water line, an irrigation pipe or a round ventilation duct, two numbers come first: how much flows through the pipe and whether the flow is laminar or turbulent. This pipe flow calculator asks for three inputs: the inner diameter in mm, the flow velocity in m/s and the medium (water or air at 20 °C). It returns the flow rate in L/min, the Reynolds number and the flow type.
The formulas are Q = π · d²/4 · v (converted to litres per minute) and Re = v · d / ν, with a kinematic viscosity ν of 1.004 × 10⁻⁶ m²/s for water and 1.516 × 10⁻⁵ m²/s for air. Below Re 2,300 the calculator shows laminar, above 4,000 turbulent, and in between the transition zone. It does not calculate pressure drop and does not ask for pipe length, roughness or pressure.
A Reynolds number below 2,300 means laminar flow, where the fluid moves in smooth, parallel layers. Above 4,000 the flow is turbulent, with higher friction losses. Between the two values the flow is transitional and hard to predict.
Example: water at 20 °C in a 25 mm pipe at 0.5 m/s gives 14.73 L/min and a Reynolds number of 12,450, firmly turbulent. Push the same flow rate through a 100 mm pipe and the velocity falls to about 0.031 m/s; the Reynolds number drops to 3,113, which the calculator classifies as transition, not laminar. With the default values, 50 mm and 2 m/s, you get 235.62 L/min and Re 99,602. Switch the medium to air and the flow rate stays the same while Re falls to 6,596, because air's kinematic viscosity is about 15 times that of water.
The flow rate grows with the square of the diameter: doubling the diameter at the same velocity quadruples the flow. To find a size for a required flow, enter a typical design velocity, often 0.5 to 2 m/s for domestic water lines, and change the diameter until the flow rate matches. Always enter the inner diameter; the nominal or outer diameter overstates the cross-section.
For pump selection you also need the pressure drop, which this calculator does not output. It is usually calculated with the Darcy-Weisbach equation, Δp = f · (L/D) · ρv²/2, where the friction factor f depends on the Reynolds number shown here and on pipe roughness.
It predicts whether flow in a pipe is laminar or turbulent. That classification decides which friction factor applies in a pressure drop calculation: 64/Re for laminar flow, roughness-dependent correlations for turbulent flow.
With an inner diameter of 25.4 mm and a velocity of 1.5 m/s, the calculator gives 45.60 L/min, or about 0.76 litres per second. The actual velocity depends on supply pressure and losses in the line.
Yes, for round pipes and ducts with water or air at 20 °C. A 160 mm duct at 4 m/s carries 4,825 L/min, about 290 m³/h. Other fluids, temperatures or high-pressure compressed air have different viscosities and are not covered.