The Borg Rating of Perceived Exertion (RPE) and Category Ratio (CR-10) scales are the most well-known and frequently used scales for quantifying subjective intensity during exercise. However, limited data exists comparing the intra-individual correlations among the Borg RPE and CR-10 scales. PURPOSE: To evaluate the intra-individual variability between the Borg RPE and CR-10 scales during incremental exercise. METHODS: 5 males (20.4±1.14 years) and 5 females (22.0±0.71 years) completed two graded exercise tests (GXTs) on an electronically braked cycle ergometer with a 48-hour interval in between. Each GXT included 2-minute stages with increments of 25 Watts/stage to volitional fatigue. Heart rate (HR), oxygen consumption (VO2) and power output (PO) were measured. Subjective responses were recorded at the end of each stage using Borg RPE and CR-10 scales with a randomized order between the GXTs. Pearson’s correlation with 95% Coefficient Intervals (95% CI) was used to examine the relationship between the Borg RPE and CR-10 scales. RESULTS: Maximal values during GXT were: VO2max = 48.0±8.13 ml/kg/min, HRmax = 191.0±6.65 bpm; Peak PO = 238.1±56.83 Watts; RPEmax = 18.7±0.95; CR-10max = 9.1±1.28). There was a significant (p<0.0001) and very strong correlation coefficient (r=0.94, 95% CI: 0.91-0.96). CONCLUSION: Results from this study showed that the Borg RPE and CR-10 scales are related in a highly regular and predictable way.