Generally, a fire is a combustion that is out of control or at least unwanted. Examples are a fire in a sofa or in a house. How fast such a fire develops is determined by a number of things. One thing is how large the fire is at any given moment. If the fire is large, it develops and spreads faster than if it’s small. But, this is of course determined also by how much available fuel there is and air supply. If the fire is inside a room (a house) there is a limited amount of air and the fire might develope and spread a lot slower, eventhough there is a large amount of fuel.
Consequently, a large amount of the heat generated is used for creating more fire and thus making the it grow even bigger and faster. We often say that the fire develops exponentially – it grows faster and bigger the larger it is.
Simplified, we can talk about doubling time – 1, 2, 4, 8, 16 etc – which means that the fire is doubled in intesity/size over some specific periods in time. Of course, there is an upper limit. The maximum size of a fire outside, with unlimited amount of air, is determined by the amount of available fuel. We then say that the fire is fuel controlled. The size of a fire inside a building is in many cases determined by the availble amount of air, which in turn is determined by the size, number and location of openings (or vents). Such a fire is said to be ventilation controlled. In many cases, it is very important for the fire service to be aware of if the fire is ventilation controlled or fuel controlled. The difference might have an impact on risks and what measures to take against fire.