Quantization is a very effective scheme for public digital image watermarking. Currently, there are lots of quantization-based public digital image watermarking methods. The quantization formulas used in these methods are in many different formats, whenas some formulas are better than the others and some formulas are equivalent. For the existing several quantization formulas in different formats, their equivalence is analyzed in strict theory and their optimality in statistics, and then the best quantization formula is obtained. Thus, current existing quantization-based image watermarking methods without the best quantization formula can be improved by using the best quantization formula.