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Mechanical Engineering2D

Flow Rate Calculator

Calculate volumetric flow rate from pipe diameter and velocity, with unit conversions.

Inputs
Enter your values below
Result
Flow Rate
—
Enter values and click Calculate
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Formula
π × (diameter ÷ 2)² × velocity × 3600
Result unit: m³/h

Q = A × v = π(D/2)² × v. Diameter in m, velocity in m/s; ×3600 converts m³/s to m³/h.

Introduction

Calculate volumetric flow rate from pipe diameter and velocity, or convert between common flow units (m3/h, L/s, L/min, GPM, CFM). Enter any two of diameter, velocity and flow rate to size piping, check pump performance, or convert units for process engineering calculations.

How This Calculator Works

Q = A × v = π(D/2)² × v. Diameter in m, velocity in m/s; ×3600 converts m³/s to m³/h.

Step-by-step process:

  1. Enter your input values in the calculator above
  2. The engine converts all inputs to SI base units (meters, kg, Pa)
  3. The formula is evaluated: pi * (diameter/2)^2 * velocity * 3600
  4. Result is formatted with the appropriate unit and precision

Calculation Example

100mm pipe at 2 m/s carries 56.5 m³/h.

Inputs: diameter=100, velocity=2
Result: 56.5 m³/h

Flow Rate Formula Explained

Volumetric flow rate is the cross-sectional area times the average velocity:

Q = A x v = pi x D^2 / 4 x v

Where Q = flow rate (m3/s), D = pipe inner diameter (m), v = average flow velocity (m/s). Mass flow rate adds density: m_dot = rho x Q (kg/s).

Example: water at 2 m/s in a 100 mm ID pipe gives Q = pi x 0.1^2 / 4 x 2 = 0.0157 m3/s = 56.5 m3/h. The same velocity in a 200 mm pipe gives 4x the flow (226 m3/h) because area scales with diameter squared.

Flow Rate Unit Conversion Table

| Unit | Multiplier from m3/h |

|---|---|

| L/s | 0.277778 |

| L/min | 16.6667 |

| m3/s | 0.000277778 |

| US GPM | 4.40287 |

| Imperial GPM | 3.66615 |

| ft3/min (CFM) | 0.588578 |

| bbl/h (42 US gal) | 6.28981 |

Example: 100 m3/h = 100 x 4.4029 = 440 GPM = 100 x 16.667 = 1667 L/min. Most pump and pipe calculations can be done in any unit set as long as you are consistent.

Recommended Pipe Velocities

Economic velocity ranges used for preliminary pipe sizing:

| Service | Velocity (m/s) |

|---|---|

| Pump suction line (water) | 0.6 - 1.5 |

| Pump discharge line (water) | 1.5 - 3.0 |

| General process water | 1.0 - 3.0 |

| Viscous oils | 0.3 - 1.0 |

| Low-pressure steam | 15 - 30 |

| Compressed air / gas | 10 - 30 |

High velocity increases pressure drop, pump power and erosion; low velocity risks settling of solids and sluggish response. Use 1-3 m/s for liquids as the first-pass sizing rule.

Engineering Applications

  • •Flow measurement
  • •Pump sizing
  • •Pipeline capacity
  • •Process control
  • •Pipe sizing and line velocity checking
  • •Pump performance and system curve development
  • •Water treatment and cooling water flow audits
  • •Heat exchanger and process fluid balancing

Frequently Asked Questions

How do I convert to GPM?

1 m³/h = 4.403 GPM. Multiply the result by 4.403 for US gallons per minute.

What is typical flow for a 4-inch pipe?

A 4-inch (100mm) pipe at 2 m/s carries about 56 m³/h or 247 GPM.

How do I calculate flow rate from pipe diameter and velocity?

Use Q = pi x D^2 / 4 x v, where D is the pipe inner diameter and v the average velocity. For example, 0.1 m pipe at 2 m/s: Q = 3.1416 x 0.01 / 4 x 2 = 0.0157 m3/s = 56.5 m3/h.

What is the flow rate of a 2-inch pipe?

At the recommended water velocity of 2 m/s, a DN50 (2 inch) pipe with 52.5 mm inner diameter carries about 15.6 m3/h (68.7 US GPM). At 3 m/s the same pipe carries 23.4 m3/h - velocity, not diameter alone, determines flow.

How do I convert L/min to m3/h?

Multiply L/min by 0.06 to get m3/h (60 minutes per hour, 1000 L per m3). Example: 500 L/min = 30 m3/h. Conversely multiply m3/h by 16.667 to get L/min.

What is mass flow rate and how is it different from volumetric flow?

Mass flow rate (kg/s) = rho x Q, the product of density and volumetric flow. It is conserved exactly through a steady system, while volumetric flow changes with temperature and pressure - important for gases and hot liquids. Use mass flow for energy balances and compressor calculations.

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Engineering Disclaimer: Calculations are for reference and educational purposes only. Always verify results independently for engineering design. See full disclaimer.
Reviewed by: Industrial Engineering Team
References: ASME B31.3, ASTM A36, Perry's Chemical Engineers' Handbook