BMI Calculator (Body Mass Index)
Calculate your BMI in metric (kg/cm) or imperial (lbs/ft) units. Includes BMI category, healthy weight range, and limitations of BMI as a health metric.
⚖️ BMI Calculator (Body Mass Index)
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What Is BMI and How Is It Calculated?
Body Mass Index (BMI) is a simple numerical measure of body weight relative to height. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the WHO as a population-level screening tool. BMI does not directly measure body fat — it is a proxy that works reasonably well at a population level but has well-documented limitations at the individual level.
📐 BMI Formula
Limitations of BMI
BMI overestimates adiposity in muscular individuals (athletes often register as "overweight" despite low body fat) and underestimates it in older adults who have lost muscle mass. It also does not account for fat distribution — abdominal (visceral) fat carries higher health risks than peripheral fat. Ethnicity also affects optimal BMI ranges; Asian populations tend to have higher cardiometabolic risk at lower BMI values.
Frequently Asked Questions
What is BMI and How is it Calculated?
Body Mass Index (BMI) is a numerical value calculated from your height and weight. It provides a simple screening tool to categorise body weight relative to height. While it doesn't directly measure body fat, it correlates well with more precise measures of body fat and is widely used in clinical and public health settings.
Imperial formula: BMI = (weight in lbs × 703) ÷ height in inches²
Metric formula: BMI = weight in kg ÷ height in metres²
BMI Categories — What Your Number Means
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency, osteoporosis risk |
| 18.5 – 24.9 | Normal weight | Lowest risk for most adults |
| 25.0 – 29.9 | Overweight | Moderately increased risk |
| 30.0 – 34.9 | Obese Class I | High risk |
| 35.0 – 39.9 | Obese Class II | Very high risk |
| 40.0 and above | Obese Class III | Extremely high risk |
How to Calculate BMI Manually
Convert to metric (if using imperial)
Convert your weight to kilograms (lbs ÷ 2.205) and height to metres (inches × 0.0254).
Square your height in metres
Example: 5'9" = 1.753m. Squared: 1.753 × 1.753 = 3.073m².
Divide weight by height squared
Example: 165 lbs = 74.8 kg. BMI = 74.8 ÷ 3.073 = 24.3 (Normal weight).
Limitations of BMI — What it Doesn't Measure
BMI is a useful screening tool but has known limitations. It does not distinguish between muscle mass and fat mass — a heavily muscled athlete may have a "overweight" BMI despite having very low body fat. It also doesn't account for age-related muscle loss in older adults, or differences in fat distribution (abdominal fat is more harmful than fat in other areas).
For a more complete picture, consider pairing BMI with our Body Fat Calculator (US Navy method) and BMR Calculator.
BMI for Different Populations
The standard BMI thresholds were developed primarily from studies of European populations. For people of Asian descent, health risks begin at lower BMI values — some guidelines suggest an overweight threshold of 23.0 and obese at 27.5. For older adults (65+), a slightly higher BMI (23–29.9) may be associated with better outcomes than the standard "normal" range.
Healthy Weight Range by Height
The healthy BMI range of 18.5–24.9 corresponds to different weight ranges for different heights. For a 5'6" person, that's approximately 115–154 lbs (52–70 kg). For 5'10", it's 132–174 lbs (60–79 kg). Use the calculator above to find your exact healthy range.