The unit mix-up that ruins a calculation

What to read before using any of this

• Read this

  The formulas disagree by 75 kcal (4.4%)

  Resting expenditure comes out between 1664 and 1739 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 1997-2087 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 291 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.

• 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.