Pipe diameter and flow rate calculator

round, closed, full pipe diameter calculation

online since 2007

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Pipe diameter calculator

Easily calculate pipe diameter and flow rate using the continuity equation. Our pipe flow calculator helps determine the correct pipe size based on volumetric flow rate and velocity, making it ideal for both liquid and gas flow applications.

Suitable for engineering, HVAC, plumbing, and fluid dynamics.

How Does the Pipe Diameter And Flow Rate Calculator Works?

The calculator applies the continuity equation.

The continuity equation is a fundamental principle in fluid mechanics based on the conservation of mass. It states that for an incompressible fluid (constant density), the mass flow rate remains constant along a streamline. The equation is:

Q = A V

Where:

  • 𝑄 = Volumetric flow rate (m³/s)
  • 𝐴 = Cross-sectional area of the pipe (m²)
  • 𝑉 = velocity (m/s)

Since the cross-sectional area of a circular pipe is:

A = π D 2 4

We can rewrite the equation as:

Q = π D 2 4 V

Solving for Pipe Diameter (D):

D = 4 Q π V

where:

  • D = internal pipe diameter (m)
  • Q = volume flow rate
  • V = velocity (m/s)

Mass Flow Rate vs. Volume Flow Rate: Formula and Conversion Guide

The relationship between mass flow rate (m) and volume flow rate (Q) is given by:

m ˙ = ρ Q

Solving for Q:

Q = m ˙ ρ

The density (ρ) of a perfect gas is given by the perfect gas law, in the next chapter.

Perfect Gas Density Formula: How to Calculate Gas Density Using Pressure and Temperature

The density (ρ) of a perfect gas is given by the ideal gas law:

ρ = P R T

where:

  • ρ = Gas density (kg/m³)
  • P = Absolute pressure (Pa)
  • T = Absolute temperature (K)
  • R = Specific gas constant (J/kg·K), defined as 𝑅=𝑅𝑢/𝑀, where 𝑅𝑢 is the universal gas constant and M is the molar mass of the gas.
Calculate for:
Select values to input:
Flow rates and velocity
Pipe dimensions
Fluid properties

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Description

q - volumetric flow rate
Fluid flow rate in terms of units of volume per unit of time
ṁ - mass flow rate
Fluid flow rate in terms of units of mass per unit of time
D - pipe diameter
Internal circular pipe diameter
V - velocity
Flow velocity in terms of units of distance per unit of time
A - area
Internal pipe cross section area
ρ - fluid density
Mass per unit of volume
T - temperature
Fluid temperature for gas density calculation based on the ideal gas state equation
p - pressure
Fluid pressure at the start of the pipe for gas density calculation based on the ideal gas state equation
R - gas constant
Gas constant in terms of energy per unit of mass and temperature, for gas density calculation using ideal gas state equation

Calculation setup

Select value to calculate. You should enter not selected one.
D
pipe diameter
V
flow velocity
Choose a value to input. Enter the one you've selected; the other value will be calculated automatically.
q
volumetric flow rate
mass flow rate
Choose a value to input. Enter the one you've selected; the other value will be calculated automatically. Available only if the gas is selected fluid
ρ
fluid density
T
temperature
Select fluid type
liquid
for incompressible fluids
gas
for fluids where ideal gas state equation is used. Enables selection between density and temperature to input.
Select pipe shape
round pipe
full cross section fluid flow
rectangular duct
for rectangle channels and full cross section flow

When is This Flow Rate Calculator Applicable?

Using a pipe diameter and flow rate calculator makes it easy to determine pipe diameter, flow rate or velocity in the pipe. Our pipe diameter and flow rate calculator allows quick calculations for closed, round, and rectangular pipes, filled with liquid or a perfect gas.

If you're analyzing a system with multiple pipes, consider using a pipe network analysis calculator.

For accurate pipe diameter calculation, you must enter the flow velocity. If flow velocity is unknown, use the pressure drop calculator to determine flow rare or pipe diameter based on the pressure difference (head loss) between the pipeline's start and end.

How Does the Pipe Diameter and Flow Rate Calculator Work?

The pipe diameter calculator determines internal pipe diameter using the fundamental relation between flow rate, velocity, and cross-sectional area (Q = v·A).

To calculate pipe diameter

  • Enter the flow rate and velocity in the calculator.
  • Click the calculate button to get results.

To calculate flow rate

  • Enter the pipe diameter and velocity in the calculator.
  • Click the calculate button to get results.

To calculate flow velocity

Besides pipe diameter, the calculator can also determine:

  • Flow velocity for a given flow rate and internal pipe diameter.
  • Mass or volume flow rate conversions, using fluid density.

If working with perfect gases, you can convert volume flow rate based on pressure and temperature changes. This is useful for pipelines with pressure-reducing valves or varying conditions.

For example, if you know the volume flow rate of an ideal gas at standard conditions (p = 101325 Pa, T = 273.15 K), you can calculate its actual volume flow rate at real pipeline conditions, like p = 30 psi and t = 70°F.

When is the Flow Rate Calculator Not Applicable?

Although highly versatile, some limitations exist:

  • If flow velocity and flow rate are both unknown, use a pressure drop calculator instead.
  • For mass flow rate calculations, fluid density must be known. If density is unknown, mass flow rate calculations are not possible.
  • For gases, if pressure, temperature, and density do not follow the ideal gas law, the calculator cannot determine gas density.

What Information is Needed for Calculations?

  • To calculate pipe diameter, you must know flow velocity and flow rate. If using mass flow rate, fluid density must be known.
  • For gas flow, density is derived from gas constant, absolute pressure, and temperature using the perfect gas equation.
  • To calculate flow velocity, the required inputs are flow rate and pipe interior diameter.

How is the Calculation Performed?

The flow rate calculator uses the continuity equation to relate flow rate, flow velocity, and pipe diameter.

For gas flow, the ideal gas equation determines density based on gas constant, absolute pressure, and temperature.

This flow rate calculator is an essential tool for engineers and professionals working with liquid and gas pipelines. Whether determining pipe diameter, flow velocity, or flow rate conversions, it simplifies complex calculations efficiently.

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Services

  • Save/Open multiple results
  • Export to Word and Excel
  • Print results
  • Create list of custom fluid properties
  • Resistance factor K for valves/fittings
  • Pipe surface roughness selection
  • Pipe material selection
  • Gauge vs absolute pressure toggle
  • Compressible isothermal flow
  • Dry air isothermal flow
  • Gas offtake flow
  • Natural gas flow
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