The accuracy of the spot urine analyte/creatinine ratio in estimating 24-hour excretion of the analyte is compromised because it is not adjusted for 24-hour creatinine excretion. The authors developed a model for conveniently estimating 24-hour creatinine excretion. The model was derived from 24-hour urine collections using multiple linear regression, including sex, weight, race, and age. The model was then evaluated in a validation cohort, assessing the correlation between estimated and measured 24-hour creatinine excretion and by comparing their correlation with muscle mass. Estimated creatinine excretion correlated strongly with measured creatinine excretion (r=0.80 in the entire cohort and 0.93 after eliminating patients with incomplete collections), and correlated at least as strongly as measured creatinine excretion with lean muscle mass (r=0.94 vs r=0.82, respectively). Adjusting spot urine analyte/creatinine ratios using the estimated 24-hour creatinine excretion by this convenient method can improve the accuracy of estimating 24-hour excretion of albumin, sodium, and other analytes.