The formula to calculate energy in joules is given by \( E = mc^2 \) in terms of mass-energy equivalence, where \( E \) is energy, \( m \) is mass, and \( c \) is the speed of light in a vacuum. However, in a more practical context, energy can also be calculated using \( E = Fd \), where \( F \) is force in newtons and \( d \) is distance in meters. Understanding these principles is crucial in fields like physics and engineering to quantify energy transfers and conversions.