More attention has been paid to sustainable consumption and production (SCP) to explore the desirable linkage between consumers and producers for sustainability. To envision SCP, the authors have developed a workshop-based method for designing backcasting scenarios. The idea is to incorporate expertise, opinions and local knowledge that experts and stakeholders have into the scenario design process. While scenarios are generally described in narrative format, it is necessary to quantify described scenarios to examine how to bridge the gap between the current situation and predetermined goals for SCP. However, it is not easy to quantify backcasting scenarios because there are no systematized methods available in existing studies. This paper thus aims to develop a workshop-based process for undertaking a quantitative evaluation of backcasting scenarios in narrative format. To develop the quantification process, we conducted a literature review and held experimental workshops. Making scenario quantification feasible and efficient during the workshop, the scenario designers deal with the following two things before the workshop – (1) setting tentative input values and the rationales for them and (2) suggesting discussion points in quantification workshops. In a case study, we evaluated one of the described scenarios for Vietnam, inviting experts to discuss it. The SCP goal was assumed to halve CO2 emissions related to passenger cars in 2050 from the level of the Business-as-Usual (BaU) scenario. Through the workshop, the experts reached a consensus on the quantification results, which showed that the CO2 emissions were reduced 60% when half of car users shared a car with 10 users.