BMI Calculator
Calculate your Body Mass Index and see where you fall on the BMI scale. Supports metric and imperial units.
What Is BMI and How Is It Calculated?
Body Mass Index (BMI) is a screening measure that relates your weight to your height. The formula is simple: BMI = weight (kg) ÷ height² (m²). For imperial units, it's weight (lb) ÷ height² (in²) × 703. It was developed in the 1830s by the Belgian statistician Adolphe Quetelet and remains the most widely used population-level indicator of weight status because it requires nothing more than a scale and a tape measure.
BMI Categories (WHO Classification)
| BMI Range | Category | General Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Elevated (nutritional deficiency, bone loss) |
| 18.5 – 24.9 | Normal weight | Lowest average risk |
| 25.0 – 29.9 | Overweight | Moderately elevated |
| 30.0 – 34.9 | Obesity Class I | High |
| 35.0 – 39.9 | Obesity Class II | Very high |
| 40 and above | Obesity Class III | Extremely high |
Worked Example
Someone who weighs 75 kg at 175 cm tall: height in metres is 1.75, so BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5 — at the top of the normal range. Their healthy weight range (BMI 18.5–24.9 at that height) is roughly 56.7–76.3 kg, which this calculator shows automatically.
What Is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper limit of the normal range). It expresses how far you are from the overweight threshold as a simple ratio: a BMI Prime of 0.98 means you're at 98% of the upper healthy limit, while 1.10 means you're 10% above it. It's a quick way to compare across people of different heights.
Where BMI Falls Short
BMI cannot tell muscle from fat. That creates predictable blind spots:
- Muscular athletes often register as "overweight" despite low body fat — a 90 kg rugby player at 180 cm has a BMI of 27.8.
- Older adults can have a "normal" BMI while carrying too little muscle and too much fat (sarcopenic obesity).
- Fat distribution matters: visceral fat around the organs carries more risk than fat on the hips, and BMI sees neither.
- Ethnicity: health risks appear at lower BMI values in some Asian populations, which is why several countries use a lower overweight cut-off of 23.
If your BMI result surprises you, cross-check it with our body fat percentage calculator, which uses circumference measurements, or read our guide on what BMI can and can't tell you. A waist measurement under half your height is another quick, independent check.
What To Do With Your Result
Treat BMI as a screening starting point, not a verdict. If you're outside the normal range, the practical next steps are estimating your daily calorie needs with our calorie calculator and setting a modest, sustainable deficit or surplus. Changes of 0.25–0.5 kg per week are the most sustainable in practice.
Frequently Asked Questions
Is a BMI of 24.9 really healthy but 25.1 unhealthy?
No — BMI risk is a continuum, not a cliff. The category boundaries are statistical conveniences for population research. A BMI of 25.1 with good fitness, normal waist size and good bloodwork is far less concerning than a BMI of 24 with high visceral fat and no exercise.
Does BMI work for children and teenagers?
Not with adult cut-offs. Children's BMI is assessed against age- and sex-specific growth percentiles because body composition changes rapidly during growth. This calculator is designed for adults aged 18 and over.
Why does my BMI say overweight when I lift weights?
Muscle is denser than fat, so muscular people often have a high BMI at low body fat. If you strength train regularly, a body fat estimate and waist measurement are far more informative than BMI alone.
How fast can I safely lower my BMI?
A sustainable rate is 0.25–0.5 kg per week, which moves BMI by roughly 0.1–0.2 points weekly for a person of average height. Faster loss usually sacrifices muscle, which worsens body composition even as BMI improves.