Richmann's law, sometimes referred to as Richmann's rule, Richmann's mixing rule, Richmann's rule of mixture or Richmann's law of mixture, is a physical law for calculating the mixing temperature when pooling multiple bodies. It is named after the Baltic German physicist Georg Wilhelm Richmann, who published the relationship in 1750, establishing the first general equation for calorimetric calculations.

Through experimental measurements, Wilhelm Richmann determined that the following relationship holds when water of different temperatures is mixed:

It follows:

Here ${\displaystyle m_{1}}$and ${\displaystyle m_{2}}$ are the masses of the two mixture components, ${\displaystyle T_{1}}$ and ${\displaystyle T_{2}}$ are their respective initial temperatures, and ${\displaystyle T_{m}}$ is the mixture temperature.

This observation is called Richmann's law in the narrower sense and applies in principle to all substances of the same state of aggregation. According to this, the mixing temperature is the weighted arithmetic mean of the temperatures of the two initial components.

Richmann's rule of mixing can also be applied in reverse, for example, to the question of the ratio in which quantities of water of given temperatures must be mixed to obtain water of a desired temperature. Determining the quantities ${\displaystyle m_{1}}$and ${\displaystyle m_{2}}$ required for this purpose, given a total quantity ${\displaystyle M=m_{1}+m_{2}}$, is accomplished with the mixing cross. The corresponding formula, obtained from the above equation by rearrangement, is:

For the mixing ratio, this gives:

The physical background of the mixing rule is the fact that the heat energy of a substance is directly proportional to its mass and its absolute temperature. The proportionality factor is the specific heat capacity, which depends on the nature of the substance, but which was not described until some time after Richmann's discovery by Joseph Black. Thus, the validity of the formula is limited to mixtures of the same substance, since it assumes a uniform specific heat capacity. Another condition is that both components be uniformly warm everywhere and that there be no appreciable heat exchange with their other surroundings.