Pulmonary function is important in neuromuscular weakness. In children, height determines normal values. Height measurement is unreliable when neuromuscular weakness or spinal deformity is present. The aim of this study was to accurately predict pulmonary function from a limb segment measurement that is precise and reproducible. Normal males (n = 1,144) and females (n = 1,199), 5.3 to 19.6 years old, were recruited from Melbourne schools. Height, weight, ulna, forearm, tibia, and lower leg lengths were measured using a Harpenden stadiometer and calipers, and electronic scales. Three maximal expiratory maneuvers were performed. Limb measurements were highly reproducible. Linear regression on log-transformed FEV1 and FVC was used to develop prediction equations from limb measurements and age. In males FEV1 = exp (0.071 x U + 0.046 x A - 1.269), r2 = 0.86; FVC = exp (0.77 x U + 0.041 x A - 1.285), r2 = 0.86 and in females FEV1 = exp (0.072 x U + 0.041 x A - 1.272), r2 = 0.84; FVC = exp (0.078 x U + 0.037 x A - 1.315), r2 = 0.83 (U refers to ulna length and A refers to age). Precision is similar to equations using height. Ulna measurement is accessible in wheelchair-bound children. Using ulna length to predict pulmonary function should facilitate respiratory assessment in children whose height is difficult to measure.