BMI and Health Metrics: What They Mean and When to Use Them

Body Mass Index (BMI) is weight in kilograms divided by height in metres squared (kg/m²). It’s a simple screening tool used by many health organisations to put adults into broad categories: underweight, normal weight, overweight, and obesity.

It doesn’t measure body fat or health directly. It’s a rough proxy that’s useful for populations and quick checks, but it has clear limits for individuals.

You need your weight (kg or lb) and height (m or ft/in). The formula is weight ÷ height² in metric. Example: 70 kg and 1.75 m gives 70 / (1.75 × 1.75) ≈ 22.9, which falls in the “normal weight” range (18.5–24.9).

Standard adult categories: under 18.5 = underweight; 18.5–24.9 = normal weight; 25–29.9 = overweight; 30 and over = obesity. These are for adults only; children need age- and sex-specific charts.

Very muscular people can have a high BMI without high body fat. Older adults may have a “normal” BMI but low muscle mass. BMI also doesn’t account for where fat is stored (e.g. around the waist vs. hips). So one number doesn’t tell the full story.

Use BMI as a starting point, not a diagnosis. For personal health decisions, use it alongside other measures and, when in doubt, a healthcare provider.

BMR (basal metabolic rate) is the calories your body uses at rest. TDEE (total daily energy expenditure) adds activity: it’s an estimate of how many calories you burn in a day. They’re useful for rough calorie targets for losing, maintaining, or gaining weight.

Formulas like Mifflin–St Jeor use age, sex, weight, and height. Our BMR & TDEE calculator lets you enter these and choose an activity level to get an estimate. It’s for adults and is not a substitute for a dietitian or doctor.

Track trends over time (e.g. BMI or weight) rather than obsessing over a single reading. Pair metrics with how you feel, how you move, and what you eat. If you have a history of eating disorders or other health conditions, work with a professional instead of relying on calculators alone.

Use our BMI Calculator for metric or imperial input; it shows your BMI and category. Use the BMR & TDEE Calculator for calorie estimates. Both run in your browser and don’t store your data. For children or clinical use, rely on appropriate medical tools and advice.