Pipe area
A = πD² / 4
D is the internal pipe diameter in metres.
Existing engineering calculator
Pipe Flow Rate Calculator
Calculate flow rate, average velocity or internal pipe diameter from the continuity equation.
Calculator inputs and result
Select the quantity to calculate, then enter the two known values in the available units.
Engineering knowledge
Continuity, pipe area and average velocity
For a steady incompressible flow, volume flow rate equals cross-sectional area multiplied by average fluid velocity. The relation is a useful first check for pipe sizing and velocity estimates, but it does not calculate pressure loss, pump head or compressible-flow effects.
Related calculation knowledge
Use a pressure-loss method and actual pipe internal diameter when making system design decisions.
Explore Fluid Mechanics and Piping →Formula and inputs
Flow rate
Q = A × V
V is average fluid velocity in metres per second.
Diameter
D = √(4Q / πV)
Use the result as a preliminary hydraulic sizing value.
Assumptions and limitations
- The calculation assumes a circular pipe and an average velocity across its area.
- It excludes friction loss, fittings, elevation, viscosity, density changes and transient effects.
- Confirm nominal pipe size, schedule and actual bore from the applicable piping specification.
Frequently asked questions
Is this a pressure-drop calculation?
No. It only uses continuity. Add a suitable friction-loss calculation to determine pressure or head loss.
Which diameter should be entered?
Use the actual internal diameter, not simply the nominal pipe size.
Engineering disclaimer
Educational and preliminary engineering-reference use only.Verify pipe dimensions, fluid properties, operating conditions and applicable standards before project use.