OBJECTIVE: To compare the accuracy and efficiency of the respiratory rate (RR) RRate mobile application to the WHO ARI Timer. METHODS: Volunteers used both devices to measure RR from reference videos of infants and children. Measurements were compared using correlation, Bland-Altman analysis, error metrics and time taken. RESULTS: Measurements with either device were highly correlated to the reference (r = 0.991 and r = 0.982), and to each other (r = 0.973). RRate had a larger bias than the ARI Timer (0.6 vs. 0.04 br/min), but tighter limits of agreement (−4.5 to 3.3 br/min vs. −5.5 to 5.5 br/min). RRate was more accurate than the ARI Timer (percentage error 10.6% vs. 14.8%, root mean square error 2.1 vs. 2.8 br/min and normalized root mean square error 5.6% vs. 7.5%). RRate measurements were 52.7 seconds (95% CI 50.4 s to 54.9 s) faster. CONCLUSION: During video observations, RRate measured RR quicker with a similar accuracy compared to the ARI Timer.