What is the Fluid Velocity Calculator?
Calculate fluid velocity from volumetric flow rate with this free fluid velocity calculator. Use v = Q/A for circular pipes, rectangular open channels, annular gaps, or a known cross-section area. Apply the continuity equation across a pipe reducer to find upstream and downstream velocities. Convert between m/s, ft/s, km/h, and mph. Reynolds number and laminar/turbulent classification for water. Metric and imperial units. Runs instantly in your browser.
How to use the Fluid Velocity Calculator
- Choose Circular Pipe, Rectangular Channel, Annulus, Known Area, Continuity, or Velocity Converter.
- Select metric or imperial units (not needed for converter).
- Enter cross-section dimensions and volumetric flow rate in L/s, GPM, m³/s, or other units.
- For continuity mode, enter upstream and downstream pipe diameters.
- Review mean velocity, Reynolds number, and copy results with calculation steps.
Common use cases
- Finding mean velocity in a supply pipe from pump flow rate
- Checking velocity increase at a pipe reducer or contraction
- Estimating flow speed in an irrigation channel or ditch
- Converting field velocity readings between m/s and ft/s
- Hydraulics homework on continuity and Reynolds number
Frequently asked questions
- How do I calculate fluid velocity in a pipe?
- Mean velocity v = Q/A, where Q is volumetric flow rate and A is cross-sectional area. For a circular pipe, A = π(D/2)². Enter diameter and flow rate — the calculator returns v in m/s, ft/s, km/h, and mph.
- What is the continuity equation?
- For incompressible flow, Q = A₁v₁ = A₂v₂. When a pipe narrows, velocity increases in proportion to the area ratio. Continuity mode computes v₁ and v₂ from two diameters and a constant flow rate.
- How is velocity calculated in an open channel?
- Use rectangular channel mode: A = width × depth, then v = Q/A. Hydraulic radius R = A/P is shown for reference, with Reynolds number based on 4R.
- What Reynolds number indicates turbulent flow?
- Re = vL/ν where L is a characteristic length (pipe diameter or hydraulic diameter). Laminar Re < 2,300; turbulent Re > 4,000. Water at 20°C uses ν ≈ 1.004×10⁻⁶ m²/s.
- How is this different from the pipe flow calculator?
- This tool focuses on solving velocity from geometry and flow rate across multiple cross-section shapes, continuity at expansions/reducers, and velocity unit conversion. The pipe flow calculator adds Darcy–Weisbach head loss and Hazen–Williams pipe sizing.
