Math Emeritus Sci Advisor Thanks PF Gold P: 39,488 Adding Spheres: How to find the new radius? Two raindrops, each of radius r, have volume $(4/3)\pi r^3$ each. If they "coalesce", because mass is conserved, and the density of water is a constant, the volume will be $(8/3)\pi r^3$. Solve $(4/3)\pi R^3= (8/3)\pi r^3$ for the new radius, R.