One-Size diapers are designed to fit babies from birth through potty training. Exact sizing will depend on the brand, but most one-size diapers fit babies from about 8lbs up to approximately 35lbs. Purchasing one-size cloth diapers is far more economical than purchasing a sized system of Smalls, Mediums, and even Larges, but because they will get heavily used for 2.5 to 3 years, they will incur more wear and tear. For this reason, we recommend having a rotation of about 24 diapers.