

With exponential growth, the population keeps growing forever with a constant doubling time. If you want to use this general model, consider whether you should use the linear (log) version of it instead. When you fit a model, you want the scatter to be about the same at all time points. This assumption is often more likely to be true of you fit the log of concentration rather than concentration.
Create an XY table. Enter time values into X and population values into Y. After entering data, click Analyze, choose nonlinear regression, choose the panel of growth equations, and choose Exponential (Malthusian) growth.
Consider whether you want to constrain Y0 to a fixed value.
Y=Y0*exp(k*x)
Y0 is the starting population in the same units as Y
k is the rate constant (inverse of the X time units)
DoublingTime is the time need for the population to double. It is calculated as ln(2)/k