BMI, and what it does not tell you

What to read before using any of this

• Read this

  The formulas disagree by 58 kcal (4.1%)

  Resting expenditure comes out between 1400 and 1458 kcal depending on which standard equation you use, and both are fitted to population averages rather than to you. After the activity factor the plausible range is 1680-1750 kcal. Any calculator that reports a single number to the calorie is implying precision that does not exist.

  What to do: Treat the range as the answer. Pick a point inside it, hold it for a few weeks, and adjust based on what actually happens to your weight — measured response beats any formula.

• Note

  The activity multiplier is the largest source of error

  The multipliers run from 1.2 to 1.9, so the choice between two adjacent bands moves the estimate by roughly 245 kcal — more than the disagreement between the formulas. Most people overestimate their own activity level.

  What to do: If you are between two bands, take the lower one and adjust upward from evidence.

• Note

  BMI cannot tell muscle from fat

  BMI is weight over height squared and nothing else, so a muscular person and an inactive person of the same weight and height get the same number. That is the well-known limitation and it is not a small one — it misclassifies athletes routinely.

  What to do: Supply a body fat percentage. It adds the Katch-McArdle estimate, which uses lean mass, and computes protein on lean mass instead of total weight.

• Read this

  This BMI falls where the cutoffs disagree

  The standard WHO cutoffs were derived largely from European populations. WHO publishes lower thresholds for South and East Asian populations, where cardiometabolic risk rises at a lower BMI. This value is in the normal range by the standard cutoffs and in the increased-risk range by those.

  What to do: Which set applies is a clinical question, not an arithmetic one.

• Note

  Not medical advice

  These are population-level estimates from formulas fitted to population averages. They are not medical advice and know nothing about your health, medication, pregnancy, or history. BMI in particular is not a measure of health. Talk to a clinician or a registered dietitian before acting on any of it.