Is diameter inside or outside?
Use inside diameter (flow area).
Flow Rate Calculator
Estimate liquid volumetric flow from internal diameter and average velocity.
Calculate flow rate
Enter inside diameter and mean velocity, then choose the unit system.
About this calculator
How to use Flow Rate Calculator, plus the formulas and logic behind the results.
Tool description
The Flow Rate Calculator converts pipe diameter and velocity into volumetric flow for water and similar liquids, with metric and US outputs.
How to use
- Choose metric or imperial units.
- Enter inside diameter and average velocity.
- Select Calculate to view L/s, m³/h, and GPM.
Formulas & reference
A = πD² ÷ 4Q = A × vQ_m3h = Q_m3s × 3600Q_gpm = Q_m3s × 15850.323
Flow identity
- Q = A × v
- A = π × (D ÷ 2)²
- 1 m³/s = 1000 L/s = 15850.3 GPM (US)
Calculation logic
Inputs are converted to SI (meters and m/s). Area is the circular cross-section. Flow is area times velocity, then converted to common display units.
Assumes incompressible liquid and average velocity across the cross-section. Fittings and friction are not modeled.
Flow Rate Calculator FAQ
Common questions about this calculator and how to use the results.
What velocity should I assume?
Many water systems use roughly 1–3 m/s; follow your design guide.
Does viscosity matter here?
Not in this continuity equation—only diameter and velocity.
Can I use this for air?
Volumetric continuity still applies, but gas density/pressure effects need separate treatment.
What is GPM?
US gallons per minute.
Why is my result zero?
Diameter and velocity must both be greater than zero.