Estimated 24-hour urinary creatinine excretion (24 hrUCr) may be useful for converting spot urine analyte/creatinine ratio into estimated 24-hour urinary excretion of the evaluated analyte, and for verifying completeness of 24-hour urinary collections. We compared various published 24 hrUCr-estimating equations against measured 24 hrUCr in hospitalized hypertensive patients. 24 hrUCr was measured in 293 patients and estimated using eight formulas (CKD-EPI, Cockcroft-Gault, Walser, Goldwasser, Rule, Gerber-Mann, Kawasaki, Tanaka). We used the Pearson correlation coefficient, the Bland-Altman method, and the percentage of estimated 24 hrUCr within 15%, 30% (P30), and 50% of measured 24hUCr to compare estimated and measured 24 hrUCr. Differences between the mean bias by eight formulas were evaluated using the Friedman rank sum test. Overall, the best formulas were CKD-EPI (mean bias 0.002 g/d, P30 86%) and Rule (mean bias 0.022 g/d, P30 89%), although both tended to underestimate 24 hrUCr with higher excretion values. The Gerber-Mann formula and the Asian formulas (Tanaka, Kawasaki) were less precise in our study population but superior in an analysis restricted to subjects with highest measured 24 hrUCr per body weight. We found significant differences between 24 hrUCr-estimating equations in hypertensive patients. In addition, formula performance was critically affected by inclusion criteria based on measured 24 hrUCr per body weight.