Objective To develop an age-based weight estimation rule in a Chinese population and to compare its performance with existing formulae.

Design Population-based observational study.

Setting Schools and kindergartens in Hong Kong.

Subjects Healthy Chinese children aged 1–10 years old on their last birthday.

Interventions Weight was measured to the nearest 0.2 kg.

Main outcome measures Linear regression was used to derive a simple formula relating weight to the child's age on his or her last birthday. The accuracy and precision of different age-based weight formulae was compared using coefficient of variation, Bland Altman plots, and by determining the proportion of children with estimates >30% outside the actual weight.

Results The Chinese Age Weight Rule is a simple linear formula that is more accurate than and at least as precise as any other age-based weight estimation rule: weight in kg=(3×age last birthday)+5. It is accurate and precise in children <7 years old, but all age-based weight estimates are imprecise in older children.

Conclusions The Chinese Age Weight Rule should be used in a Chinese population in preference to any other age-based weight estimation rule. Caution should be taken when using it in older children in whom other weight-estimation tools may be more appropriate.