The most popular way to select a size is to go by a child's height. The easiest way to check this is to measure the child from the top of head to the ground, standing up straight feet together with no shoes. For a baby lay them on a flat surface and measure top of head to heel. Alternatively measure a garment similar in style.
To measure the waist it should be taken at the natural waist line, a chest measurement from under the arms to the fullest part of the chest. If taking measurements from an existing garment lay it flat and measure across the chest or waist and then double the measurement.
Below is a rough guide to general sizing, but bear in mind different brands size slightly different. If unsure check using the individual Brands guide.
General:
Age Size in cm Weight
0-3 months 60cm 6kg/13lb
3-6 months 68cm 8kg/17.5lb
6-12 months 76cm 10kg/22lb
12-18 months 83cm 12kg/26.5lb
18-24 months 90cm 14kg/31lb
Age Height Chest Waist
2-3 years 98cm 54cm 50cm
3-4 years 104cm 56cm 51cm
4-5 years 110cm 57cm 53cm
5-6 years 116cm 58cm 54cm
6 -7years 122cm 60cm 55cm
7-8 years 131cm 64cm 56cm
8-9 years 134cm 66cm 58cm
9-10years 140cm 69cm 61cm