Biot Number Calculator with Characteristic Length

Three types of heat transfer mechanisms.
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Biot numbers and characteristic lengths of bodies are very important in heat transfer calculations. Because they decide whether you can use lumped system analysis method to calculate the total heat transfer. Thus, we prepared calculators to make the calculations easy. So, you can use these calculators to make your biot number, and characteristic length calculations.

Biot Number Calculator

First of all, you will face Biot number calculations lots of the time. And you will probably calculate in lots of heat transfer calculations. So, you can use the Biot number calculator below to easily calculate the Biot number to see if you can use lumped system analysis in heat transfer calculations.

Biot Number and Characteristic Length Calculator

Result: The result that you can use lumped system analysis will appear here…

Firstly, the use of the Biot number calculator is very simple to use. You just need to enter these parameters from your heat transfer problem;

  • Volume: This is the volume of the body that you want to calculate heat transfer. Unit: ft3 or m3.
  • Surface Area: The total surface area of your heat transfer body. Unit: ft2 or m2.
  • Heat Transfer Coefficient: Heat transfer coefficient of the convection heat transfer between the body and the environment. Unit: W/(m2°C) or Btu/(hr-ft2°F).
  • Thermal Conductivity: This is the thermal conductivity of the material of your body. Unit: W.m-1K-1 or BTU.h-1.ft-1.℉-1

After that, after entering these values inside the brackets of the Biot number calculator, click on the ‘Calculate!’ button to see these results;

  • Characteristic Length: Characteristic length value which will be used in the calculation of the Biot number. Unit: m or ft.
  • Biot Number: The Biot number will appear here. Biot number is dimensionless.

In the result section, the result of whether you can use lumped system analysis or not will appear here.

What is this Number?

Before learning about the Biot Number, we need to delve into the lumped system analysis. Also, we are using the lumped system analysis to calculate the total heat transfer between bodies and the environment. Lumped system analysis is a very convenient tool to calculate the total heat transfer but conduction and convection in one formula. But on the other hand, if we can not use lumped system analysis, heat transfer calculations will be very hard.

There is a rule about lumped system analysis. All the temperatures inside the body are assumed same. But in real life, there is no kind of thing like this. For example, think about you baking bread in the oven. If you cut the bread in half, the temperature distribution is not the same. So, we can not use lumped system analysis here.

Biot number defines the usability of lumped system analysis up to a deviation level such as 15%. So, for Biot numbers below 0.1, you can use lumped system analyses, but for Biot numbers higher than 0.1, you can not use lumped system analysis. Biot number calculates this.

How to Calculate the Biot Number?

You can calculate the Biot number with this formula;

Biot number formula that is used in Biot number calculator.

So, the Biot number uses the thermal conductivity and heat transfer coefficient to see that you can use lumped system analysis. Also, the proportion above assesses the situation. Here, ‘Cl’ is the characteristic length of the body which is simply the division of volume with the surface area.

According to the Biot number, if a body is small and has very good thermal conductivity, we can use the lumped system analysis method to calculate the whole heat transfer for it. So, the Biot number has a very good logic for the lumped system analysis.


So, it will be very useful for you to use the Biot number calculator other than performing calculations by hand. You can use it in complex heat transfer calculations without facing any problems.

Above all, Mechanicalland does not accept any responsibility for calculations made by users in calculators. A good engineer must check calculations again and again.

Also, you can find out much more calculators like this Biot number calculator in Mechanicalland! Take a look at the other engineering calculators available in Mechanicalland!

Finally, do not forget to leave your comments and questions below about Biot number and characteristic length calculators.

Your precious feedbacks are very important to us.

FAQs About Biot Number

What are Biot numbers and their significance?

Biot number is the indicator of whether you can use lumped system analysis in the heat transfer calculations of bodies. Because lumped system analysis makes the heat transfer calculations very convenient. So, the significance of the Biot number emerges from this.

How to calculate the Biot number? 

The calculation is the Biot number is very simple. You just need to know the characteristic length, thermal conductivity, and heat transfer coefficient in the heat transfer problem. So, put these values into the Biot number formula and you will see the result.


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