This study investigated the relationship between changes in upon-waking body mass (BM) and changes in urine specific gravity (Usg) and urine color (Ucol) from 1 day to the next. Throughout the 5-day investigation, healthy adolescent Singaporean athletes (n = 66) had their upon-waking, bladder-voided BM measured. A small aliquot of the first bladder void each day was collected and analyzed for Usg and Ucol, the latter by both an investigator (IUcol) and individual participants (SUcol). Results revealed a significant inverse relationship between changes in BM and changes in Usg (p = .003) and Ucol (p = .001). On average, Usg and Ucol changed by ~0.003 units and ~1 color (across a 9-unit scale), respectively, with every 1% change in BM from 1 day to the next. There was a stronger relationship between Usg and IUcol (r = .82, p < .001) than between Usg and SUcol (r = .60, p < .001). These results suggest that the degree of fluid deficit may be predicted from the Usg measurements among moderately hypohydrated athletes. In addition, training athletes to interpret and use the Ucol chart is recommended.