Last updated: 2026-06-16

The average reaction rate measures how fast a reaction proceeds over a finite time interval, expressed as a change in concentration per unit time. It is the first quantity introduced in chemical kinetics because it can be read directly from two concentration measurements. This calculator computes that average rate and normalises it by the stoichiometric coefficient, so the value describes the reaction as a whole rather than one particular reactant or product.

The rate expression

For a species $A$ with stoichiometric coefficient $a$ in the balanced equation, the reaction rate is:

$\text{rate} = \frac{1}{a}\,\frac{\lvert\Delta[A]\rvert}{\Delta t}$

Here $\Delta[A]$ is the change in concentration and $\Delta t$ is the elapsed time. Reactants are consumed and products are formed, so $\Delta[A]$ is negative for one and positive for the other; taking the magnitude keeps the reported rate positive.

Why divide by the coefficient

In the reaction $2A \rightarrow B$, the reactant $A$ disappears twice as fast as $B$ appears. If you tracked each species separately you would get two different numbers for the same reaction. Dividing every species rate by its coefficient resolves this, producing a single rate of reaction that is identical no matter which species is measured.

Worked example

Suppose the concentration of a reactant with stoichiometric coefficient one falls from $0.50\ \mathrm{M}$ to $0.20\ \mathrm{M}$ over 10 seconds:

The reaction consumes the species at an average of 0.030 moles per litre each second over that interval.

Average versus instantaneous

The average rate spans a measurable interval, whereas the instantaneous rate is the slope of the concentration–time curve at a single moment. Because most reactions slow as reactants are depleted, the average over an interval usually falls between the higher rate at the start and the lower rate at the end. To model how concentration changes continuously, move to the integrated rate laws: First-Order Integrated Rate Law Calculator, Second-Order Integrated Rate Law Calculator, and Zero-Order Integrated Rate Law Calculator.