BACKGROUND: Although dosing and sizing of resuscitation drugs and equipment are mostly weight based, time is usually insufficient to weigh critically ill children. Many age-based weight estimation formulae for emergency use have been developed worldwide, but there is no specific formula for Japanese children. The aim of this study was therefore to develop and validate age-based formulae for estimating the bodyweight of children living in Japan. METHODS: A total of 370 980 measurements of bodyweight were obtained from 39 547 participants aged 12-155 months. They participated in a national survey, called the Longitudinal Survey of Babies in the 21st Century, which started in 2001. We created the new original weight estimation formulae for children living in Japan, called the Japanese Pediatric Assessment of Normal weight (JAPAN) formulae, using 75% of measurements that were randomly selected from the total measurements. To check the validity of the formulae, we applied the JAPAN formulae, the Park et al. formula for Korean children, and the commonly used Nelson formula for the remaining 25% of measurements. The mean absolute error and the root mean square error (RMSE) were calculated for each scale. RESULTS: The JAPAN formulae performed better than the two other formulae among Japanese children, with a mean absolute error and RMSE of 0.83 and 1.08 (kg), respectively. The performance of the Park et al. formulae for Korean children was poor for children living in Japan. CONCLUSIONS: The newly developed JAPAN formulae for age-based weight estimation are appropriate for children living in Japan.