Equation of Continuity

The Equation of Continuity is a statement of mass conservation

The Law of Conservation of Mass states that mass can be neither created or destroyed. Using the Mass Conservation Law on a steady flow process - flow where the flow rate do not change over time - through a control volume where the stored mass in the control volume do not change - implements that

  • inflow equals outflow

This statement is called the Equation of Continuity. Common application where the Equation of Continuity are used are pipes, tubes and ducts with flowing fluids or gases, rivers, overall processes as power plants, diaries, logistics in general, roads, computer networks and semiconductor technology and more.

Equation of continuity

The Equation of Continuity and can be expressed as:

m = ρi1 vi1 Ai1 + ρi2 vi2 Ai2 + .... + ρin vin Ain

    = ρo1 vo1 Ao1 + ρo2 vo2 Ao2 + .... + ρom vom Aom                            (1)


m = mass flow rate (kg/s)

ρ = density (kg/m3)

v = speed (m/s)

A = area (m2)

With uniform density equation (1) can be modified to

q = vi1 Ai1 + vi2 Ai2 + .... + vin Ain

    = vo1 Ao1 + vo2 Ao2 + .... + vom Aom                         (2)


q = flow rate (m3/s)

ρi1 = ρi2 = . . = ρin = ρo1 = ρo2 = .... = ρom

For a simple reduction (or expansion) as indicated in the figure above - the equation of continuity for uniform density can be transformed to

vin Ain = vout Aout                             (3)


vout = vin Ain / Aout                            (3b)

Example - Equation of Continuity

10 m3/h of water flows through a pipe with 100 mm inside diameter. The pipe is reduced to an inside dimension of 80 mm.

Using equation (2) the velocity in the 100 mm pipe can be calculated

(10 m3/h) (1 / 3600 h/s) = v100 (3.14 (0.1 m)2 / 4)


v100 = (10 m3/h) (1 / 3600 h/s) / (3.14 (0.1 m)2 / 4)

    = 0.35 m/s

Using equation (2) the velocity in the 80 mm pipe can be calculated

(10 m3/h) (1 / 3600 h/s) = v80 (3.14 (0.08 m)2 / 4)


v80 = (10 m3/h) (1 / 3600 h/s) / (3.14 (0.08 m)2 / 4)

= 0.55 m/s

Equation of Continuity Calculator

The calculator below is based on eq. 3b and can be used for fluids with uniform density - typical liquids like water, oil and more.

vin - input velocity (m/s, fpm)

Ain - input area (m2, sq.ft)

Aout - output area (m2, sq.ft)

Related Topics

  • Dimensions - Sizes and dimensions of pipes and tubes, and their fittings - inside and outside diameter, weight and more
  • Fluid Mechanics - The study of fluids - liquids and gases. Involves velocity, pressure, density and temperature as functions of space and time
  • Fluid Flow and Pressure Drop - Pipe lines - fluid flow and pressure loss - water, sewer, steel pipes, pvc pipes, copper tubes and more

Related Documents

  • Conservation of Mass - The Law of Conservation of Mass states that mass can neither be created or destroyed
  • Equations in Fluid Mechanics - Commonly used equations in fluid mechanics - Bernoulli, conservation of energy, conservation of mass, pressure, Navier-Stokes, ideal gas law, Euler equations, Laplace equations, Darcy-Weisbach Equation and more
  • Orifice, Nozzle and Venturi Flow Rate Meters - The orifice, nozzle and venturi flow rate meters use the Bernoulli Equation to calculate fluid flow rate using pressure difference through obstructions in the flow
  • Bernoulli Equation - Conservation of energy - non-viscous, incompressible fluid in steady flow
  • Sluice Gate Flow Measurements - Sluice gates are used for controlling and measuring flow rates in open channels and rivers, mainly in connection to hydro power plants
  • Technical Terms in Fluid Mechanics - Some of the most commonly used technical terms in fluid mechanics
  • Air Ducts Sizing - The ductwork of ventilation systems are often sized with either the Velocity, the Constant Pressure Loss (or Equal Friction Loss) - or the Static Pressure Recovery Methods
  • Mechanical Energy and Bernoulli Equation - The mechanical energy equation related to energy per unit mass, energy per unit volume and energy per unit weight involving head

Tag Search

  • en: equation continuity mass conservation
  • es: conservación de la masa continuidad ecuación
  • de: Gleichung Kontinuität Massenerhaltung

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