Background: Maximum repetitions to failure (RTF) and repetitions in reserve (RIR) can be estimated through fastest mean velocity (MVfastest) and mean velocity (MV), respectively. However, the impact of inter-repetition intervals (IRI) on these relationships in free-weight back squat and bench press exercises is unclear. Hypothesis: The IRI would affect RTF-MVfastest and RIR-MV relationships, with a higher goodness-of-fit using self-selected IRI (SSIRI) compared with 0 seconds (IRI0) and 3 seconds (IRI3). Study Design: Crossover study design. Level of Evidence: Level 3. Methods: Eighteen male participants completed 1 session per IRI configuration, consisting of 3 single sets of RTF (65%-75%-85% of the 1-repetition maximum) during the free-weight back squat and bench press exercises. Results: Individualized RTF-MVfastest and RIR-MV relationships were stronger than generalized (median R2 = 0.98 vs 0.65 and 0.84 vs 0.40, respectively). The goodness-of-fit of the relationships was stronger for SSIRI than for IRI0 during back squat (P < .01) and comparable between IRIs during bench press (P >= .28). During back squat, MVfastest values were higher for IRI0 than for IRI3 and SSIRI (eighth-fifteenth repetitions; P <= .07), whereas during the bench press, they were higher for IRI0 than for IRI3 (eleventh-fifteenth repetitions; P >= .28). Overall, MV values associated with each RIR were higher for IRI0 than for SSIRI (10 out of 18 comparisons) during back squat, and for IRI0 than for IRI3 and SSIRI (16 and 14 out of 18 comparisons) during bench press. Conclusion: These results highlight the importance of standardizing the IRI during set-to-failure to establish RTF-MVfastest and RIR-MV relationships, with SSIRI recommended as a more accurate and effective procedure. Clinical Relevance: This information may provide practitioners with a valuable tool to objectively quantify the level of effort being exerted during resistance training sets by measuring movement velocity in free-weight exercises.