Flow Rate and Velocity Calculator
Determine the volumetric flow rate based on pipe diameter and fluid velocity.
millimeters (mm)
inches (in)
centimeters (cm)
meters (m)
meters/second (m/s)
feet/second (ft/s)
miles/hour (mph)
Please enter valid positive numbers for both fields.
Calculated Flow Rates
Cubic Meters per Hour
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Liters per Minute
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US Gallons per Minute (GPM)
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Cubic Feet per Second
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Understanding Flow Rate and Velocity
In fluid dynamics, calculating the flow rate and velocity is essential for designing efficient systems, whether you are engineering a municipal water supply, sizing an HVAC duct, or simply optimizing a home irrigation system. This Flow Rate and Velocity Calculator simplifies the complex relationship between the size of a pipe, the speed of the fluid, and the volume being moved.
The Fundamental Relationship: Q = A × v
The core principle behind this calculator is the continuity equation for incompressible fluids. The formula connects Volumetric Flow Rate (Q), Cross-Sectional Area (A), and Velocity (v):
Q = A × v
- Q (Flow Rate): The volume of fluid that passes through a cross-section per unit of time. Common units include cubic meters per hour (m³/h), liters per minute (L/min), and gallons per minute (GPM).
- A (Area): The cross-sectional area of the pipe or channel. For a round pipe, this is calculated using the internal diameter: A = π × (Diameter / 2)².
- v (Velocity): The average speed at which the fluid particles are moving along the pipe, typically measured in meters per second (m/s) or feet per second (ft/s).
Why Velocity Matters in Pipe Sizing
You might wonder, "Why not just increase velocity to get more flow?" while keeping the pipe small. However, velocity is a critical constraint in engineering:
- Erosion: Extremely high velocities can erode pipe materials, especially if the fluid contains particulates.
- Noise: In plumbing and HVAC, high velocity creates turbulence and noise (water hammer in pipes or whistling in ducts).
- Pressure Drop: Friction increases exponentially with velocity. Doubling the speed roughly quadruples the friction loss, requiring much larger pumps and more energy.
Conversely, if velocity is too low (e.g., in sewage lines), solids may settle out of the fluid, causing blockages. This is why calculating the precise balance between diameter and velocity is crucial.
How to Use This Calculator
Our tool simplifies the math by allowing you to input the two known variables to find the flow rate. Here is a step-by-step guide:
- Enter Pipe Diameter: Measure the internal diameter of your pipe. Do not use the outer diameter, as wall thickness does not contribute to flow area. Select your preferred unit (mm, inches, etc.).
- Enter Fluid Velocity: Input the estimated or measured speed of the fluid. In many design scenarios, you will aim for a specific target velocity (e.g., 1.5 to 2.5 m/s for industrial water lines).
- Calculate: Click the button to instantly convert these metrics into various flow rate units suitable for different industries (GPM for US plumbing, m³/h for industrial pumps).
Common Pipe Velocity Standards
While specific applications vary, here are general rules of thumb for maximum water velocities to avoid issues:
- General Water Supply: 1.5 to 2.5 m/s (5 to 8 ft/s).
- Pump Suction Lines: 0.6 to 1.5 m/s (2 to 5 ft/s) – kept lower to prevent cavitation.
- Drain Lines: Gravity flow usually aims for self-cleansing velocities around 0.6 to 1.0 m/s.
Example Calculation
Imagine you have a 2-inch (approx 50mm) pipe and water is flowing through it at 2 meters per second.
- First, convert diameter to meters: 50mm = 0.05m.
- Calculate Area: A = π × (0.025)² ≈ 0.001963 m².
- Calculate Flow Rate: Q = 0.001963 m² × 2 m/s = 0.003926 m³/s.
- Convert to L/min: 0.003926 × 60,000 ≈ 235.6 L/min.