The dynamic equations for integral quantities relevant to turbulent flames are developed based on atheorem of geometric measure theory. The total area of level surfaces is considered in detail using mixture fraction, a quantity which is important for non-premixed combustion, to define the level surfaces. The level surface corresponding to its stoichiometric value is defined as the flame surface. It is shown that the rate of strain generated by the motion of the fluid, coupled with molecular diffusion and source terms of the scalar variable defining the level surface, determines the evolution of the surface properties. Direct numerical simulation (DNS) results obtained for a low-Reynolds-number turbulent non-premixed flame are used to evaluate the effects of strain rate and scalar dynamics on the surface area of level surfaces, including the flame surface.