Objectives Accurate determination of energy expenditure (EE) through accelerometry is relevant in the effectiveness of the PA programs. The mean amplitude deviation (MAD) is a good parameter to distinguish the intensity of physical activity. Here, the aims of the present study were twofold: a) to develop a new EE estimation equation using raw accelerometer data and heart rate, and b) to compare the oxygen consumption measured with the new equation developed and the most commonly used prediction equations in the literature in normal weight and overweight adults. Equipment and methods Twenty healthy adults (10 males and 10 females) wore a wearable device on the chest that integrates triaxial accelerometry and ECG signals. The test protocol consisted in 12 individualized intensities, 6 walking and 6 running speeds equally distributed. The correlation between MAD and measured oxygen consumption was investigated. Then, a new energy expenditure estimation equation was developed and compared with five formulas from the literature. Results Our results noted that MAD had a very high correlation (r = 0.937) with indirect calorimetry. The new equation developed had one of the two lowest mean absolute errors for both walking and running. Therefore, our equation appears to be suitable for both walking and running, for normal weight and overweight people. However, future studies should validate our new EE estimation equation with a wide range of population and field-based conditions.