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Background: Previous research studies have found that heart rate monitors that predict maximal oxygen consumption ( $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ ) are valid for males but overestimate $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ in females. Inaccurate self-reported physical activity (PA) levels may affect the validity of the prediction algorithm used to predict $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ . Purpose: To investigate the validity of the Polar M430 in predicting $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ among females with varying PA levels. Methods: Polar M430 was used to predict $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ ( $\mathrm{p}\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ ) for 43 healthy female study participants (26.9 ± 1.3 years), under three conditions: the participant’s self-selected PA category (sPA), one PA category below the sPA (sPA − 1), and one category above the sPA (sPA + 1). Indirect calorimetry was utilized to measure $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ ( $\mathrm{m}\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ ) via a modified Astrand treadmill protocol. Repeated-measures analyses of covariance using age and percentage of body fat as covariates were used to detect differences between groups. Bland–Altman plots were used to assess the precision of the measurement. Results: $\mathrm{p}\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ was significantly correlated with $\mathrm{m}\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ (r = .695, p < .001). The mean values for $\mathrm{p}\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ and $\mathrm{m}\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ were 44.58 ± 9.29 and 43.98 ± 8.76, respectively. No significant differences were found between $\mathrm{m}\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ , $\mathrm{p}\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ , sPA – 1, and sPA + 1 (p = .492). However, the Bland–Altman plots indicated a low level of precision with the estimate. Conclusions: The Polar M430 was a valid method to predict $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ across different sPA levels in females. Moreover, an under/overestimation in sPA had little effect on the predicted $\dot{\mathrm{V}}{\mathrm{O}}_2\text{max}$ .