BMI Calculator — Calculate Your Body Mass Index
Free, instant, and accurate. Get your BMI score, category, and personalised health advice in seconds.
<18.5 Normal
18.5–24.9 Overweight
25–29.9 Obese
30+
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📊 BMI Categories — Full Reference Table
The World Health Organisation (WHO) classifies BMI into the following categories for adults:
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency, weakened immunity, bone loss |
| 18.5 — 24.9 | Normal Weight ✅ | Lowest health risk — maintain with healthy lifestyle |
| 25.0 — 29.9 | Overweight | Increased risk of heart disease and type 2 diabetes |
| 30.0 — 34.9 | Obese Class I | High risk — significant lifestyle changes recommended |
| 35.0 — 39.9 | Obese Class II | Very high risk — medical consultation advised |
| 40.0 and above | Obese Class III | Extremely high risk — immediate medical attention needed |
What Is BMI and How Is It Calculated?
BMI stands for Body Mass Index. It is a simple numerical measure calculated from your height and weight that categorises whether you fall within a healthy weight range for your height. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and remains the most widely used screening tool for body weight classification worldwide.
Metric: BMI = Weight (kg) ÷ Height (m)²
Example: 70kg ÷ (1.75m × 1.75m) = 70 ÷ 3.0625 = BMI 22.9 (Normal)
Imperial: BMI = (Weight in lbs × 703) ÷ Height (inches)²
Limitations of BMI — What It Does Not Tell You
While BMI is a useful screening tool, it has important limitations you should know:
- Does not measure body fat directly — a heavily muscled athlete may have a high BMI despite very low body fat.
- Does not account for muscle vs fat distribution — two people with the same BMI can have very different body compositions.
- Age and gender affect results — older adults naturally have more body fat at the same BMI compared to younger people.
- Ethnicity matters — research shows Asian populations face increased health risks at lower BMI thresholds than the standard WHO classifications.
- Does not measure where fat is stored — visceral fat (belly fat) is far more dangerous than fat stored elsewhere, and BMI does not distinguish between them.
For a more complete picture of your health, use BMI alongside waist circumference measurements, body fat percentage, and blood work. A healthy waist circumference is generally below 94cm (37 inches) for men and below 80cm (31.5 inches) for women.
Frequently Asked Questions
BMI is a useful general screening tool for most adults aged 18–65 but is less accurate for athletes with high muscle mass, elderly people, pregnant women, and certain ethnic groups. It is best used as one indicator among several, not as the sole measure of health.
For adults aged 18–65, a healthy BMI is 18.5–24.9. For people over 65, a slightly higher BMI (around 22–27) may actually be protective, as some extra weight can help maintain bone density and reserves during illness. For children and teenagers, BMI is interpreted using age and sex-specific percentile charts rather than fixed categories.
Reducing BMI requires a combination of moderate caloric deficit through diet, regular aerobic exercise (walking, jogging, cycling), and strength training to preserve muscle mass. A safe, sustainable rate of BMI reduction is approximately 0.5–1 unit per month. Explore our related guides on belly fat, metabolism, and the Mediterranean diet for detailed strategies.
Yes — this is called "normal weight obesity" or "skinny fat." People with a normal BMI can have high body fat percentage and low muscle mass, carrying significant metabolic health risks. Regular exercise, adequate protein, and strength training are important regardless of BMI.
For most people, checking BMI every 1–3 months provides meaningful tracking without obsessive monitoring. More frequent daily weighing can create unnecessary anxiety. Focus on consistent healthy habits rather than the number itself.
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