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The shock model of gamma-ray bursts (GRBs) has two free parameters: ϵB and ϵe. It has been shown that ϵB should range between few×10−3 and few×10−4. However, how to calculate the value of ϵe has remained an outstanding theoretical problem for over a decade. Here, we demonstrate that the Weibel theory inevitably predicts that ϵe≃√ϵB. The GRB afterglow data fully agree with this theoretical prediction. Our result explains why the electrons are close to equipartition in GRBs. This ϵe–ϵB relation can potentially be used to reduce the number of free parameters in afterglow models. [ABSTRACT FROM AUTHOR]