Well, the formula for BMI is BMI = kg/m2 so you'll need a question for weight (in kg) and height (in meters). Then you just reference those fields in the calculation column of the XLS form (if you're using Survey123 Connect). Are you using connect, or the web form?
The calculation would look something like this:
${weight} div (${height} * ${height})
Best,
Katie
“The goal is not simply to ‘work hard, play hard.’ The goal is to make our work and our play indistinguishable.”
- Simon Sinek