The article proposes the method of estimating heat input to materials by measuring the cross-sectional area of volumes melted during fusion or pressure welding. In addition, the article describes methods presently used to estimate linear energy involving the fixing of wattmeters to arc power sources and referring this energy to the linear dimensions of welds. The article justifies the necessity of changing the approach to methods of calculating linear energy by the development of new welding methods and the launching of new materials sensitive to heat. The introduction of numerous impulse and hybrid (laser-based) welding methods contests the conventional methods of linear energy calculation (illustrated by examples). The article proposes a manner of calculating heat input during spot welding.