BMI Calculator — Body Mass Index with Formula
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Free BMI Calculator — Body Mass Index with Healthy Weight Range
Calculate your Body Mass Index (BMI) instantly in metric or imperial units. See your BMI value, WHO category, healthy weight range for your height, and BMI Prime — with the formula shown. For adults 18 and older.
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How to Use This BMI Calculator
- Choose your unit system — Metric (kg and cm) or Imperial (lbs, feet, and inches).
- Enter your age — BMI categories apply to adults 18 and older. For children, BMI is interpreted differently using age and gender percentiles.
- Select your gender — used for context only; the BMI formula is the same for all genders.
- Enter your weight and height — results appear instantly. No button press needed.
- Read your results — your BMI value, WHO category, BMI Prime, and healthy weight range for your exact height are all shown together.
What Is BMI? — Simply Explained
BMI (Body Mass Index) is a number calculated from your weight and height. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and adopted by the World Health Organization (WHO) as a standard screening tool to classify weight status in adult populations.
BMI does not measure body fat directly — it estimates it. A person with a BMI in the "Normal" range is statistically more likely to have a healthy level of body fat. A BMI outside the normal range indicates a higher statistical risk for chronic diseases like type 2 diabetes, heart disease, and hypertension.
It is a population screening tool — not a clinical diagnosis. Your doctor will look at BMI alongside other factors like waist circumference, blood pressure, cholesterol, and activity level before drawing any conclusions about your health.
BMI Formula — Metric & Imperial
📏 Metric Formula
BMI = Weight (kg) ÷ Height (m)²
Example: 70 kg, 175 cm (1.75 m)
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
🇺🇸 Imperial Formula
BMI = 703 × Weight (lbs) ÷ Height (in)²
Example: 154 lbs, 5'9" (69 inches)
BMI = 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7
BMI Categories — WHO Classification
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | Below 18.5 | Increased risk — nutritional deficiency, bone loss, immune issues |
| Normal Weight | 18.5 – 24.9 | Lowest statistical risk for weight-related chronic diseases |
| Overweight | 25.0 – 29.9 | Moderately increased risk — especially with other risk factors |
| Obese Class I | 30.0 – 34.9 | High risk — consult a healthcare provider |
| Obese Class II | 35.0 – 39.9 | Very high risk |
| Obese Class III | 40.0 and above | Extremely high risk — medical intervention often recommended |
⚠️ Important: BMI Limitations
BMI is a useful population screening tool but has well-known limitations for individuals. It does not account for:
- Muscle vs. fat: Athletes and bodybuilders often have high BMI despite very low body fat. BMI cannot distinguish muscle weight from fat weight.
- Fat distribution: Where you carry fat matters. Abdominal fat carries higher risk than hip/thigh fat — BMI does not show this.
- Age and gender: Older adults tend to have more body fat at the same BMI as younger adults. Women generally have more body fat than men at the same BMI.
- Ethnicity: Some populations (particularly Asian adults) face elevated health risks at lower BMI thresholds than the WHO standard ranges suggest.
- Bone density: People with higher bone density may have a higher BMI without excess body fat.
BMI is a starting point — not a diagnosis. Always consult a qualified healthcare provider for a full health assessment.
What Is BMI Prime?
BMI Prime is your BMI divided by 25 (the upper limit of the normal range). It tells you how far you are from the ideal BMI boundary.
- BMI Prime below 0.74 = Underweight
- BMI Prime 0.74 – 1.00 = Normal Weight (prime of exactly 1.0 means BMI of exactly 25)
- BMI Prime above 1.00 = Overweight or Obese
BMI Examples
Weight: 80 kg | Height: 180 cm
BMI = 80 ÷ (1.80²) = 24.7
✅ Normal Weight
Weight: 65 kg | Height: 165 cm
BMI = 65 ÷ (1.65²) = 23.9
✅ Normal Weight
Weight: 95 kg | Height: 180 cm
BMI = 95 ÷ (1.80²) = 29.3
⚠️ Overweight — but likely muscle
Weight: 154 lbs | Height: 5'9"
BMI = 703×154÷(69²) = 22.7
✅ Normal Weight
Frequently Asked Questions
What is a healthy BMI for adults?
According to the WHO, a BMI between 18.5 and 24.9 is considered normal or healthy for adults. A BMI below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese. These are population-level guidelines — individual health depends on many factors beyond BMI.
Is BMI accurate for athletes and muscular people?
No — BMI is often inaccurate for athletes. Because muscle weighs more than fat, highly muscular people frequently have a BMI in the "Overweight" or "Obese" range despite having very low body fat. For athletes, body fat percentage measured by DEXA scan or skinfold calipers is a more accurate health indicator than BMI.
Can BMI be used for children?
Not with adult categories. For children and teens (ages 2–19), BMI is calculated the same way but interpreted differently using age and gender-specific percentile charts. The WHO and CDC publish separate growth charts for this purpose. This calculator is for adults 18 and older only.
What does BMI Prime mean?
BMI Prime is your BMI divided by 25. A BMI Prime of 1.0 means your BMI is exactly 25 — the upper boundary of the normal range. A prime below 1.0 means you're within or below the normal range; above 1.0 means overweight or obese. It makes it easy to see how far you are from the ideal boundary with a single decimal number.
What is a healthy weight for my height?
The calculator shows your healthy weight range (BMI 18.5 to 24.9) for your exact height. For example, a person 175 cm tall has a healthy weight range of approximately 56.7 kg to 76.3 kg. Enter your height above to see your personalised range.
Does this calculator store my data?
No. All calculations happen entirely within your browser using JavaScript. Your weight, height, age, and gender are never sent to any server, logged, or stored anywhere. Everything disappears when you close or refresh the tab.
How is BMI different from body fat percentage?
BMI is calculated only from weight and height — it estimates body fatness indirectly. Body fat percentage directly measures the proportion of your body that is fat. BMI is free, instant, and widely used for population studies. Body fat % requires a DEXA scan, hydrostatic weighing, or skinfold measurement — more accurate but harder to access. BMI is a useful screening tool; body fat % is more precise for individuals.
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