A new equation of state for elastomer network under constrains due to fillers has been proposed and verified for two most common and industry important rubbers, natural rubber and styrene-butadiene rubber. A series of elastomer materials without and with reinforcing carbon black Was prepared. Analyses of the obtained data confirm that the most important parameter is extensibility parameter q, describing the effective network density for a given space distribution of chains. Conformational statistics changes with addition of fillers have been discussed.