A leading risk factor for chronic disease is physical inactivity. In efforts to assess physical activity and inform designs for prevention, health professionals currently use inexpensive, but subjective validated scales, or objective, but expensive research-grade wearables. In the meanwhile, individuals increasingly use affordable consumer-friendly wearable devices that can objectively monitor behaviours while daily life unfolds. However, the relationships between their outcomes and the validated scales are yet to be calibrated. We report our results from a study on 31 seniors from Hungary and Spain (mean age 70.6 +/- 3.2). Our study quantified the relations between physical activity outcomes, as patient-reported through 53 answers (1.71 +/- 0.96 / person) on the International Physical Activity Questionnaire (IPAQ) with a 7-day recall period, and 5615 days (mean 181.1 +/- 179.2 days collected / person) technology-reported by Fitbit Charge 2. The wearables monitored daily life behaviours of physical activity and sleep for long durations (7 to 120 days). We found strong Spearman correlations between light and moderate IPAQ physical activity in the domestic activity domain, and light-fair intensity Fitbit physical activity (e.g., rS = 0.88, p <0.005). We also found negative moderate-strong correlations between Fitbit sedentary duration and all IPAQ physical activity domains and intensities (e.g., rS = 0.64, p <0.005). We obtained increasingly stronger relationships across all IPAQ domains and Fitbit intensities by monitoring physical activity beyond the scale recall period, quantifying physical activity relative to all activities of the day, and including sleep. Our findings inform the design of longitudinal observations and personalized, focused, and potentially effective interventions for physical activity in seniors.