OBJECTIVE This study was performed to develop a new formula to estimate the renal depth in both children and adults; then compare the new formula with previously published formulas. METHODS Renal depth and total thickness (T, cm) of the body at the level of the kidneys were measured by CT in 113 children and 246 adults. Their sex, age, height (H, cm), and weight (W, kg) were recorded. Multiple stepwise linear regression analysis were conducted, using data from children and adults together. The 359 cases were divided into 2 random groups, of which, the first group was used to derive a regressive formula, and the second was used to verify the formula and compare the formula with previously published formulas in different groups. RESULTS Multiple stepwise linear regression analysis showed that the important variable in estimating the depth of each kidney was the ratio of body weight (W, kg) to body height (H, cm) and the total thickness (T, cm) of the body at the level of the kidneys. The new formula was as follows: for right renal depth (cm) = 0.22 × T + 7.714 × W/H-0.331 (r = 0.95), and for left renal depth (cm) = 0.238 × T + 6.553 × W/H-0.618 (r = 0.95). It is better than the other four formulas in different groups, especially in children and W/H ≤ 0.30 (in adults) groups. CONCLUSIONS We first introduced T into renal depth estimation formula and established the new formula. It has a better performance than the other four formulas in different groups. The new formula provided reliable and accurate renal depth and may contribute to improving the methods used to estimate renal function from radionuclide renography.