Rewards are rhetorical arguments that have a positive nature since they use the argument that something positive will happen to the opponent if he accepts to do the proponent requirement. During a persuasive negotiation more than one reward can be generated and the proponent has to choose one to send to his opponent. Since one measure that could help him in this choice is the strength each reward has, the goal of this work is to propose a model for calculating rewards strength. We propose two ways for calculating the strength of rewards depending on the kind of negotiation the agent is participating. The first proposal is to be used when the agent negotiates only with one opponent, and the second when the agent negotiates with more than one opponent. Our model also considers that strength calculation should be done in two different moments:(i) before a reward is sent, this calculation is called initial, and (ii) when the proponent receives an answer for a reward that was sent, this calculation is called experience-based.