Relative growth rate (RGR) is growth rate relative to size - that is, a rate of growth per unit time, as a proportion of its size at that moment in time.
RGR is a concept relevant in cases where the increase in a state variable over time is proportional to the value of that state variable at the beginning of a time period.
its growth rate, then relative growth rate is If the RGR is constant, i.e., a solution to this equation is Where: A closely related concept is doubling time.
In the simplest case of observations at two time points, RGR is calculated using the following equation:[1] where:
= size at time two When calculating or discussing relative growth rate, it is important to pay attention to the units of time being considered.
[2] For example, if an initial population of S0 bacteria doubles every twenty minutes, then at time interval
is the number of twenty-minute intervals that have passed.
However, we usually prefer to measure time in hours or minutes, and it is not difficult to change the units of time.
is measured in hours, and the relative growth rate may be expressed as
[2] In plant physiology, RGR is widely used to quantify the speed of plant growth.
It is part of a set of equations and conceptual models that are commonly referred to as Plant growth analysis, and is further discussed in that section.