Refractive index is the most accurate way to analytically determine the concentrations of aqueous urea solutions. The Urea Concentration Calculator calculates the concentration of urea in a solution from the difference in refractive index (deltaN) at the sodium D line according to the data of Warren and Gordon (Warren, JR and Gordon, JA (1966) On the refractive indicies of aqueous solutions of urea. J Phys Chem, 70:297-300). Inputs are the refractive index of a buffer aqueous solution and a similarly buffered solution containing urea. Outputs are deltaN and the molar concentration of urea. Aqeous urea solutions are commonly used in studies of protein denaturation.