A new cometary law... well, new ''cometary law'', in this graph that I have made, I establish a correlation, between the absolute nuclear magnitude, and the effective radius of a comet in kms, with the proposed formula, Knowing only the absolute nuclear magnitude, we can make an approximate estimate of the size of the nucleus of a comet. I have based it on the work of Paradowski 2020, several curiosities, the first, the best fit is an exponential equation to the curve that presents, x It is the absolute magnitude and y, the cometary nucleus radius, another is the strong correlation index obtained from r=0.9993...well it is one more step, and a bridge, to new research and astrophysical studies that are carried out, in my opinion, It adjusts much more to reality, using RN(1,1,0), than the visual absolute magnitude, m0 or H, as it is called in other research.