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Onboarding calculation

Body Mass Index Methodology

How onboarding calculates BMI from height and weight, and why BMI is only used as a first-pass screening signal.

Status: Used during onboarding before body-composition questions.
Last updated: June 15, 2026

Inputs and outputs

Inputs

  • Weight in kilograms
  • Height in centimeters

Outputs

  • BMI rounded to one decimal
  • Visual marker on the BMI range bar

Formula

height_m = height_cm / 100
BMI = weight_kg / (height_m * height_m)
shown_BMI = round(BMI, 1)
marker_percent = clamp(((shown_BMI - 14) / 22) * 100, 0, 100)

Calculation steps

  1. Convert height from centimeters to meters.
  2. Divide weight in kilograms by height in meters squared.
  3. Round the result to one decimal for display.
  4. Show the CDC adult healthy-zone reference of BMI 18.5 to 24.9, while telling the user that BMI does not estimate body composition.

Guardrails

  • BMI is not used as the final energy-needs model.
  • The onboarding flow immediately asks for body-composition context because two people with the same BMI can have different lean mass.

Sources

These formulas describe how Unflame estimates onboarding targets. They are planning estimates and should be reviewed against real-world trends, user preference, symptoms, and professional guidance where relevant.