How does fire develop?

On this page, you’ll obtain information to understand how fire develops and the evidence it may leave behind.

Safety note

At the Fire Safety Research Institute, part of UL Research Institutes, fire protection engineers and safety scientists study fire in a very controlled and scientific way. Do not attempt to recreate these tests yourself: instead, this pathway will take you through these experiments virtually. Your teachers can provide you with similar experiences that allow you to explore fire in a safe setting. If you are interested in performing similar tests yourself, consider pursuing a career as a safety scientist!

Asset: How does fire develop?

Fire development can be divided into four stages: ignition, growth, full development, and decay.

Let’s take a close look at the vocabulary that will help us communicate about the stages of fire development.



Ignition is when heat, oxygen, and a fuel source combine. The result of this chemical reaction is fire.

During the growth stage, the fuel will continue to burn because heat, oxygen, and fuel are available.

There are three methods of heat transfer that can cause a fire to grow and spread.

The methods of heat transfer are:

  • Conduction – The transfer of thermal energy within an object or between two objects that are in direct contact.
  • Radiation – The transfer of energy in the form of electromagnetic waves.
  • Convection – The transfer of thermal energy due to the circulation of a fluid such as a gas or a liquid.

All combustible materials are ignited. The fire will extinguish if heat, fuel or oxygen are limited.

Usually the longest stage of a fire. Oxygen, heat and/or fuel decrease until the fire goes out.

Real Lab Footage

Watch the video to observe how fire develops.

What to you notice as the fire progresses through the four stages?

Asset: Fire Development: Four Stages
Classroom Investigation 02

Heat Transfer & Ignition

Watch the video to observe how thermal energy transfers during fire development.

With proper adult supervision, you can complete this classroom investigation: Heat Transfer and Ignition Investigation Guide.


Asset: Heat Transfer and Ignition

Recall what you’ve learned about fire development from this page. What evidence might you observe in the burn scene that can indicate how the fire spread from the origin to other objects?

Heat transfer by conduction, convection, and radiation cause fire to grow and develop.

The amount of damage on an object can indicate how it was heated, and its location relative to the origin.

comparison of charred toaster and melted lightbulb

charred toaster versus melted lightbulb.

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How does fire behave?