The Arrhenius equation gives the dependence of the rate constant of a chemical reaction on the absolute temperature as
where
*  is the rate constant (frequency of collisions resulting in a reaction),
*  is the absolute temperature,
*  is the pre-exponential factor or Arrhenius factor or frequency factor. Arrhenius originally considered A to be a temperature-independent constant for each chemical reaction. However more recent treatments include some temperature dependence – see  below.
*  is the molar activation energy for the reaction,
*  is the universal gas constant.
Alternatively, the equation may be expressed as
where
*  is the activation energy for the reaction (in the same unit as kT),
*  is the Boltzmann constant.
The only difference is the unit of : the former form uses energy per mole, which is common in chemistry, while the latter form uses energy per molecule directly, which is common in physics.
The different units are accounted for in using either the gas constant, , or the Boltzmann constant, , as the multiplier of temperature .
The unit of the pre-exponential factor  are identical to those of the rate constant and will vary depending on the order of the reaction. If the reaction is first order it has the unit s, and for that reason it is often called the frequency factor or attempt frequency of the reaction. Most simply,  is the number of collisions that result in a reaction per second,  is the number of collisions (leading to a reaction or not) per second occurring with the proper orientation to react and  is the probability that any given collision will result in a reaction. It can be seen that either increasing the temperature or decreasing the activation energy (for example through the use of catalysts) will result in an increase in rate of reaction.
Given the small temperature range of kinetic studies, it is reasonable to approximate the activation energy as being independent of the temperature. Similarly, under a wide range of practical conditions, the weak temperature dependence of the pre-exponential factor is negligible compared to the temperature dependence of the factor ; except in the case of "barrierless" diffusion-limited reactions, in which case the pre-exponential factor is dominant and is directly observable.
With this equation it can be roughly estimated that the rate of reaction increases by a factor of about 2 to 3 for every 10 °C rise in temperature, for common values of activation energy and temperature range.
The  factor denotes the fraction of molecules with energy greater than or equal to .