Introduction: This study examines the relationship between relative strength ratio (RSR) and maximum number of repetitions (MNR) in the pull-up exercise. RSR, defined as the additional load for a 1RM divided by body weight, has been positively correlated with MNR in previous research. The study aims to establish a regression equation to predict one variable from the other in men and women. We hypothesized that a strong positive relationship between RSR and MNR would be found, with no significant sex-based differences, allowing for accurate MNR prediction from 1RM and body weight. Methodology: 69 men and 15 women performed an 1RM and MNR test one week apart after a familiarization session using a waist belt and weight plates. Their body composition was obtained through anthropometric assessments and skinfold measurements. Second grade polynomials were used to obtain the regression equations for the whole group and men–women separately. Potential differences in the regression models based on sex were assessed through the ANCOVA test. Subjects were divided in four quartiles of RSR (low, medium, high and very high) and differences in anthropometry, body composition and performance were assessed with either ANOVA or Kruskal–Wallis’ H tests. The goodness–of–fit for the prediction of MNR from RSR was assessed based on the mean absolute error (MAE) for the whole group and for each RSR subgroup. Results: A strong and positive relationship between RSR and MNR was found for the whole group (R² = 0.96), with no differences in the regression for men and women (p = 0.863). Further analyses comparing MNR and RSR based on muscle mass and lean body mass (both in kg) did not show a better fit (R² = 0.956 and 0.957, respectively). The goodness–of–fit analysis yielded a MAE of ~1 pull-up (MAE = 1.08) across the whole group of subjects ranging from 4 to 32 repetitions. More specifically, subjects in the low, medium and high RSR had a MAE of 0.5–0.8 repetitions, while subjects in the very-high RSR group showed a mean of ~2. However, no differences in the goodness–of–fit were found between groups (p = 0.472). No differences in weight nor in body mass index were found based on RSR groups, but subjects in the very high RSR group had significantly lower fat mass and body fat percentage (p < 0.001 in both) than the rest, and both variables were lower as RSR increased (p > 0.001). Conclusion: There is a strong relationship between RSR and MNR that allows one to be predicted from the other, with no differences between men and women. This will enable the estimation of MNR without performing tests to failure, thereby reducing the fatigue associated with these assessments.