The recommended shoe size for an 18-month-old child varies depending on the brand and the childs individual growth rate. However, according to the shoe size charts from BirdRock Baby, Robeez, and BlitzResults, the suggested shoe size for an 18-24 month old child is generally between 5.25 inches and 5.5 inches. It is important to measure the childs feet with a ruler or printable size chart and leave a little extra room for the childs feet to grow into their new shoes. It is also recommended to try on the shoes and confirm they fit with the "toe test". Keep in mind that every childs feet grow at different speeds, so it is best to measure the childs feet as close to the shopping trip as possible.