How to calculate flow rate?

Flow Rate Calculator with Pressure and Diameter

How to calculate fluid flow rate?

Flow is measured as a volume or mass per unit time. This calculator let you calculate the flow rate of any liquid or gas fluid starting from different linear, volumetric flow rate, mass flow rate or normal flow rate.

Using this calculator you can solve questions such as:

How to convert mass flow rate to volumetric flow rate?

Where can I find a easy velocity to mass flow rate converter?

How to convert volumetric flow rate to mass flow rate?

What is the difference between flow and pressure?

Flow measurement is one of the most important operations inside a chemical or process plant. Through the flow measurement we can know the quantity of raw material consumed, intermediate products, energies used (steam, water, fuels) and processed products. Flow is defined as the mass or volume of fluid flowing through a pipe or conduit per unit of time.

Optimum performance of some equipment and operations requiere specific flow rates. The cost of many liquids and gases are based on the measurement flow through a pipeline making it necessary to accurately measure and control the rate of flow for accounting purposes.

Proper installation of any flowmeter in fluid systems is essential in relation to piping layout, streamline-flow conditions, and accesibility to the device and its associated instruments and connections

How does this calculator work?

You simply fill in the cells with a white background and press the RESULTS button. The cells Tagname, Plant, Area and Notes are not necessary to perform the calculation but they are necessary if you want to export the obtained calculations to a results spreadsheet.

The State cell allows you to select the state of matter. Depending on the state of the matter, the necessary cells are enabled or disabled to perform the calculation.

Below the calculation section you will find a link to download a spreadsheet with the results of your operations.

Identification Data


Fluid & Pipe Data

Density [Kg/m3] (rho)
Temperature [ºC] (T)
Pressure In [bar a] (P1)
Molecular Weight (MW)
Pipe Diameter [mm] (D)
Starting data:
Velocity in Pipe [m/s]

Pipe Area [m2] (A)
Velocity in Pipe [m/s] (vp)
Volumetric Flow [m3/h] (qv)
Mass Flow [Kg/s] (qm)
Mass Flow [Kg/h] (qmh)
Normal Flow [Nm3/h] (qn)
Normal Conditions 20 °C and 101.325 kPa

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Flow Rate Calculator - Results Spreadsheet Sample Document
Results Spreadsheet Sample Document

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Information and Definitions


Tagname of the instrument. This is the identifier of the field device, which is normally given to the location and function of the instrument.

Flow Rate Calculator - Tagname

Plant, Area and Notes

Information Referred to the physical installation of the instrument. Plant and Process Area where the instrument is installed. Notes about the instrument.


Fluid Name or Composition. Fluid is called a type of continuous medium formed by some substance whose molecules have only a weak force of attraction.

Flow Rate Calculator - Fluid

A fluid is a set of particles that are held together by weak cohesive forces and the walls of a container; The term encompasses liquids and gases.


State of the matter. It could be Liquid, Gas or Steam.

Temperature (T)

Operating Temperature of the fluid in Celsius units. The flowing temperature is normally measured downstream from the orifice and must represent the average temperature of the flowing stream in degrees Celsius. Temperature has two effects on volume. A higher temperature means a less dense gas and higher flows, but when this higher flow is corrected to base temperature, the base flow is less.

Pressure In (P1)

Considering the direction of the fluid, we define P1 as the pressure (gauge or absolut) existing in the pipeline before the restriction orifice.

Orifice Plate Calculator

Pressure has two effects on volume. The higher pressure makes the gas denser so less volume flows through the meter. However, when the volume is expanded to base pressure, the volume is increased.

Density (rho)

Density is the relation of mass and volume.The density of a material varies with temperature and pressure. This variation is typically small for solids and liquids but much greater for gases.

Flow Rate Calculator - Denstity (rho)

...where rho is density, m is mass and V is volume.

Molecular Weight (MW)

Mass of a molecule of any pure substance, the value of which is the sum of the atoms that make it up.

Pipe Diameter (D)

Inside diameter of the pipe. All process calculations are based on the volume of the pipe which is the function of internal diameter of the pipe. As per standards, any pipe is specified by two non-dimensional numbers Nominal Diameter (in Inches as per American Standards or mm as per European standards) and Schedule (40, 80, 160,...). The outer diameter of the pipe is the diameter of outer surface of the pipe.

Velocity in Pipe or Velocity Flow Rate (vp)

Velocity is a measure of speed and direction of an object. When related to fluids it is the rate of flow of fluid particles in a pipe. The average rate flow is used in flow calculations. The units of flow are normally feet per second (fps), feet per minute )fpm), meters per second (mps), and so on. This calculator can be used to measure the steam pipe velocity calculator.

Flow Rate Calculator - volumetric flow rate units

...where V is Velocity in Pipe or Velocity Flow Rate, Q is Volumetric Flow Rate and Area is Pipe Surface.

Volumetric Flow Rate (qv)

Volumetric flow is often deduced by knowing the cross sectional area of the fluid. Most volumetric flow equipment measures the velocity and calculates the volumetric flow based on a constant cross seciotnal area. Volumetric flow is usually represented with Q letter. The volumetric flow rate units are usually m3/h or m3/s. Below you will find the volumetric flow formula:

Flow Rate Calculator - volumetric flow rate units

...where Q is Volumetric Flow Rate, rho is Density and W is Mass Flow Rate.

Volumetric Flow Conversion : mass flow Rate can be converted to volumetric flow rate using the above formula. Now, the difference between volumetric flow and mass flow rate can be easily understood observing the equation that relates both flows.

Mass Flow Rate (qm)

Mass of a substance which passes per unit of time. Mass flow in Kg/s units, flowing through the pipe. Mass flow is usually represented with W letter. Below you will find the mass flow formula:

Flow Rate Calculator - mass flow equation

...where W is Mass Flow Rate, rho is Density and Q is Volumetric Flow Rate.

Normal Flow Rate (qm)

Standard or normal conditions are used as reference values ​​in thermodynamics of gases. To specify the gas volume, Normal or Standard temperature and pressure conditions are generally used. The reason is very simple, the volume of a constant number of moles of gas depends on the measurements of temperature and pressure.

If you are interested to know more about this subject visit our article What is the difference between Actual, Standard and Normal Flows?

Flow Rate References

1. International Organization of Standards (ISO 5167-1). 2003. Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full — Part 1: General principles and requirements.

2. International Organization of Standards (ISO 5167-2) 2003. Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full -- Part 2: Orifice plates.

3. American Society of Mechanical Engineers (ASME). 2001. Measurement of fluid flow using small bore precision orifice meters. ASME MFC-14M-2001.

4. U.S. Dept. of the Interior, Bureau of Reclamation, 2001 revised, 1997 third edition, Water Measurement Manual.

5. Michael Reader-Harris (2015) Orifice Plates and Venturi Tubes.

6. Miller, R. W., Flow Measurement Handbook, 3rd ed., McGraw-Hill, New York, 1996.

7. American Gas Association, AGA Gas Measurement Manual, American Gas Association, New York.

8. Wikipedia

9. Corrosionpedia

10. Orifice Plates and Venturi Tubes (2015) - Michael Reader-Harris

11. EMERSON Fundamentals of Orifice Meter Measurement

12. Search Data Center

Any question regarding the Liquid or Gas Flow Rate Calculator?

If you have any question regarding the Liquid or Gas Flow Rate Calculator calculator, please don't hesitate to get in touch ... Anyway you can have a look at our frequently asked questions page, there you will find a solution to the most common questions.

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