Alexa will choose metric or imperial system automatically. But, please, don't mix both systems.
You can just ask for "Bodyshape Guide" and leave Alexa ask you. You: "Alexa, open Bodyshape Guide!" Alexa: "How tall are you?" You: "I'm 175 centimeters tall" Alexa: "How much do you weigh?" You: "60 kilograms" Alexa: "Here's your BMI: 19.59. This BMI is considered Normal for most cases. Congratulations!!!"
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