Data loggers that measure acceleration are regularly used to quantify the behavioural energetics of animals. Calculating dynamic body acceleration (DBA) by smoothing acceleration data using a necessarily long-running mean of e.g. ≥1 s (the DBA method) is not appropriate for determining behaviour duration. In fact, the DBA method results in the elongation of behaviour durations (mean ± 95% CI behaviour duration elongation for scallops [n = 10 animals] and humans [n = 10 free-ranging aquatic biologists]: 508.5 ± 277.2 and 19.3 ± 1.1%, respectively) and consequently an overestimation of energy expenditure (mean ± 95% CI activity metabolic rate overestimation: 212.5 ± 48.0 and 6.12 ± 0.34%, for scallops and humans, respectively; overall metabolic rate overestimation: 91.1 ± 29.9 and 4.18 ± 0.20%, for scallops and humans, respectively). Behaviour duration elongation is of greatest significance when behaviour durations are short (minimum, mean and maximum single behaviour duration for scallops and humans recorded in this study: 0.12 and 0.37, 0.40 and 8.04, 0.84 and 185.73 s, respectively). Overestimation of the duration of behaviours naturally causes errors in the calculations of behavioural time-energy budgets for all species that move intermittently, as demonstrated for scallops, crabs, 'amphibious' signal crayfish and humans. Furthermore, behaviours of any duration should be accurately detected, quantified and when appropriate, the behaviour type should be identified. The R package BEnergetix can quickly calculate accurate invertebrate and vertebrate behavioural time−energy budgets from acceleration, metabolic rate and environmental data.