Where A is the area of the cross-section, CB is the concentration of molecule B, Dr is the relative diffusion constant and Δt is the elapsed time. Now the problem left is how to pick the value of Δt. I have shown in the previous report that the 1 √∆𝑡 dependence of the collision rate with observation time is because of the fractal nature of collision that the molecules if zoomed in time reveal the finer collisions of the same molecules. For example, if we observe the two molecules collide in 1 ms, it could be that they have collided thousands of times in finer time scale, e.g. ns. Thus, it is critical to come up with an observation time that carries physical meaning for the specific system to calculate the reaction rate in a diluted solution, for which we can imagine the average distance of molecules A and B in the solution (Fig. ) with derivation shown in the supporting information.