A simple equation to predict the size of droplets formed during the breakup of cyclindrical liquid jets is obtained from instability analysis. This droplet-size equation applies to low-velocity, liquid-in-liquid and liquid-in-gas jets involving Newtonian or non-Newtonian fluids that follow power-law shear stress versus deformation rate relationships. The equation is tested by comparing the resultant theoretical predictions for droplet size with experimental data for seventeen Newtonian liquid systems and five power-law non-Newtonian/Newtonian liquid systems (power-law liquid jet in Newtonian liquid and Newtonian liquid jet in power-law liquid), as well as with numerical solutions to Tomotika’s equation. Good agreement is observed. The present analysis demonstrates clearly the dependence of droplet size on a modified Ohnesorge number.