Baseline hydration status is typically determined through collecting 3 consecutive days of free-living nude body mass (BM). However, this method might not capture individuals in their euhydrated state. PURPOSE: To determine differences between euhydrated and free-living 3, 5, and 7 day hydration measures. METHODS: 27 males and 25 females (male age, 21±1years; female age, 20±1years; male BM, 79.38±13.31kg; female BM, 65.52±11.80kg) participated in this study. First morning nude BM, urine specific gravity (USG), urine color (UCOL), and urine osmolality (UOSMO) were assessed for 10 consecutive days. Participants arrived euhydrated the first 3 days (USG<1.020). The average of days 1-3 (euhydrated baseline), 4-6 (free-living 3 day baseline), 4-8 (free-living 5 day baseline), and 4-10 (free-living 7 day baseline) were calculated. Repeated measures ANOVA with LSD post-hoc comparisons were performed. Data are reported as mean±standard deviation (M±SD) and 95% confidence intervals (95%CI), p<0.05. RESULTS: Euhydrated baseline USG was lower (M±SD [95%CI], 1.009±0.003 [1.008, 1.009]) than free-living 3 day (M±SD [95%CI], 1.017±0.005 [1.016, 1.019]), 5 day (M±SD [95%CI], 1.018±0.005 [1.016, 1.019]), and 7 day (M±SD [95%CI], 1.018±0.005 [1.016, 1.019]), p<0.05. Euhydrated baseline UCOL was lower (M±SD [95%CI], 2±1 [2, 3]) than free-living 3 day (M±SD [95%CI], 5±1 [4, 5]), 5 day (M±SD [95%CI], 5±1 [1.016, 1.019]), and 7 day (M±SD [95%CI], 5±1 [4, 5]), p<0.05. Euhydrated baseline UOSMO was lower (M±SD [95%CI], 381.87±102.95 [353.21, 410.53] mOsmol) than free-living 3 day (M±SD [95%CI], 651.84±191.77 [598.45, 705.23] mOsmol), 5 day (M±SD [95%CI], 652.88±187.46 [600.69, 705.07] mOsmol), and 7 day (M±SD [95%CI], 659.24±182.33 [608.48, 710.00] mOsmol), p<0.05. Euhydrated baseline BM was lower (M±SD [95%CI], 72.60±14.32 [68.61, 76.59] kg) than free-living 3 day (M±SD [95%CI], 71.72±14.67 [67.64, 75.81] kg) but not different than 5 day (M±SD [95%CI], 72.24±2.02 [68.19, 76.28] kg) or 7 day (M±SD [95%CI], 72.05±2.02 [68.00, 76.09] kg), p<0.05. CONCLUSIONS: A free-living 3 day baseline is 1.21% lower than a euhydrated 3 day baseline BM. Therefore, a three-day euhydrated, 5 or 7 day free-living baseline BM may be useful to define a true hydration baseline, which is important when dehydration level is described by %BM loss.