Frequently Asked Questions

BMI is a quick screening measure for populations but is inaccurate for many individuals. It does not distinguish muscle from fat, so athletes and people with high muscle mass often classify as "overweight" despite low body fat. BMI also tends to misclassify older adults (who lose muscle) and certain ethnic groups whose body composition differs from the populations BMI was validated on. For these people, waist-to-height ratio or body composition tests are more informative.

Start from your TDEE (total daily energy expenditure) and create a modest deficit. A daily deficit of about 500 kcal typically produces roughly 1 lb (~0.45 kg) of weight loss per week, which is sustainable for most adults. Avoid dropping below your BMR (basal metabolic rate) for long stretches — very large deficits tend to lose lean mass, slow metabolism, and rebound. Use our calorie calculator to find your TDEE and a target deficit, and consult a registered dietitian if you have a medical goal.

BMR (Basal Metabolic Rate) is the energy your body burns at complete rest, just to keep vital functions working — heart, lungs, temperature, cells. TDEE (Total Daily Energy Expenditure) is BMR plus the calories you burn from activity, digestion, and movement throughout the day. TDEE is the number you compare against how much you eat to maintain, lose, or gain weight. BMR is the largest single part of TDEE for most people — typically 60–75%.

BMI is a screening tool, not a diagnostic one. A normal BMI does not guarantee good health, and a high BMI does not guarantee poor health — body composition, fat distribution, fitness, blood pressure, blood sugar, and cholesterol all matter more for predicting disease risk. For individuals, BMI should be interpreted alongside other measurements and a clinical assessment rather than treated as a verdict on health.

For most adults, the World Health Organization (WHO) defines a "normal" BMI as 18.5–24.9 kg/m². BMI below 18.5 is classified as underweight, 25–29.9 as overweight, and 30 and above as obese. These ranges are population-level averages and do not always apply to athletes, pregnant or breastfeeding people, older adults, or some ethnic groups. Always interpret BMI in context with a healthcare professional.

No single "ideal weight" number is reliable. The formulas used by these calculators (Devine, Robinson, Miller, Hamwi, etc.) were originally developed for other purposes such as drug dosing and give different answers for the same person. They ignore muscle mass, frame size, age, and body composition. Treat any ideal weight figure as a rough reference only, alongside BMI, body composition, and waist-to-height ratio, and discuss specific weight goals with your doctor.