BACKGROUND: QT correction (QTc) formulae are widely used in clinical and research settings but often underperform, possibly due to demographic influences on the QT-heart rate (HR) relationship. To address this limitation, we developed an adaptive QTc (QTcAd) formula, which adjusts for demographic factors like age, and compared its efficacy to other standard formulae. METHODS: The QTcAd formula was tested across diverse age groups with different HR in both humans and guinea pigs. Using retrospective ECG data from 1819 pediatric patients at Children's National Hospital and 2400 subjects from the Pediatric Heart Network database, alongside in vivo (N=55) and ex vivo (N=66) guinea pig ECG recordings, we evaluated the formula's effectiveness. Linear regression fit parameters of QTc-HR (slope and RESULTS: In both human subjects and guinea pigs, the QTcAd formula (QTcAd=QT+(| CONCLUSIONS: The demography-based QTcAd formula showed superior performance across human and guinea pig age groups, which may enhance the efficacy of rate-corrected K.M.M. for cardiovascular disease diagnosis, risk stratification, and drug safety testing in children and adults.