Creatinine excretion is widely used as a method to evaluate the adequacy of urine collection in different clinical settings. Many factors influence its elimination, such as protein intake, exercise, muscle mass, age, and sex, among many others. As 24-hour urine collections can be cumbersome, several equations have been developed to aid clinicians to correctly interpret results derived from them. In this review article, we report the factors that can modify creatinine excretion and we evaluate the accuracy of different published equations to estimate 24-hour urine creatinine excretion.