EngineeringFormulaVelocity (v)
Velocity is the rate of change of position with respect to time, in a stated direction.
In fluid mechanics it's the average bulk speed through a cross-section (Q = A v). In kinematics it's the speed of a moving object, related to acceleration by v = u + at. Base SI unit is meters per second (m/s); US customary work commonly uses feet per second or feet per minute (duct/conveyor velocity) instead.
A common mix-up: the velocity in Q = A v is the average bulk velocity across the whole cross-section, not the peak centerline velocity. Real flow profiles are faster in the middle and slower near a pipe or duct wall, so measuring or assuming a peak value instead of an average will overstate the flow rate.
Use this term page as a router: for moving objects, start with Velocity, Acceleration, or Kinetic Energy; for ducts and pipes, start with Flow Rate, CFM, Continuity Equation, or Reynolds Number.
Typical Velocities by Context
| Context | m/s | ft/s or ft/min |
|---|---|---|
| Human walking pace | 1.4 | 4.6 ft/s |
| Comfortable HVAC duct velocity | 2–4.5 | 400–900 ft/min |
| Residential water supply line | 1.5–3 | 5–10 ft/s |
| Highway driving speed | 30 | 67 mph |
| Speed of sound in air (20°C) | 343 | 1,125 ft/s |
Used In These Calculators
Related Terms
Frequently Asked Questions
Is velocity the same as speed?
No. Speed is a scalar - just magnitude, like "60 mph." Velocity is a vector - magnitude and direction. In the calculators on this site, velocity inputs are treated as magnitudes along a defined flow or motion axis, so the distinction rarely changes the arithmetic, but it matters for how a result should be interpreted.
Why do duct and pipe velocity limits exist if the formula works at any speed?
Q = A v is valid at any velocity, but real systems have practical ceilings: too fast and a duct gets noisy and wastes fan energy to friction, too fast in a pipe and erosion or water hammer become a risk. Those limits are a design choice layered on top of the physics, not a constraint the formula itself imposes.
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