What is BMI?
Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has since become the standard screening tool used by doctors, dieticians, and public health organisations worldwide.
BMI does not measure body fat directly. Instead, it provides a quick estimate of whether a person's weight is likely to be associated with health risks. It is used as a first-step screening tool — not a diagnosis.
BMI is a population-level screening tool. It is useful for identifying trends across groups, but individual results should always be interpreted alongside other health indicators and professional advice.
The BMI Formula (Metric)
The standard metric formula uses weight in kilograms and height in metres:
Step-by-step process:
- Convert your height to metres if needed (e.g. 175 cm ÷ 100 = 1.75 m).
- Square your height in metres (1.75 × 1.75 = 3.0625).
- Divide your weight in kg by that result (70 ÷ 3.0625 = 22.86).
Imperial Formula (lbs & inches)
If you measure in pounds and inches, use this version of the formula:
Worked Examples
Weight: 70 kg | Height: 1.75 m
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625
Weight: 90 kg | Height: 1.68 m
BMI = 90 ÷ (1.68 × 1.68) = 90 ÷ 2.8224
Weight: 154 lbs | Height: 5 ft 9 in (69 inches)
BMI = (154 ÷ 69²) × 703 = (154 ÷ 4761) × 703
Skip the manual calculation — our free BMI Calculator gives instant results for both metric and imperial units.
Open BMI Calculator →BMI Categories Explained
The World Health Organisation (WHO) defines four main BMI categories for adults aged 18 and over:
Obesity Sub-classes
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | < 18.5 | Moderate |
| Normal weight | 18.5 – 24.9 | Low |
| Overweight | 25.0 – 29.9 | Increased |
| Obese — Class I | 30.0 – 34.9 | High |
| Obese — Class II | 35.0 – 39.9 | Very high |
| Obese — Class III (severe) | ≥ 40.0 | Extremely high |
BMI for Children & Teens
Standard adult BMI categories do not apply to children and teenagers (under 18). For children, BMI is calculated using the same formula but the result is then compared against age- and sex-specific growth charts using percentiles.
- Below 5th percentile — Underweight
- 5th to 85th percentile — Healthy weight
- 85th to 95th percentile — Overweight
- 95th percentile and above — Obese
Always use a paediatric BMI calculator for children, and consult a healthcare professional for an accurate interpretation.
Limitations of BMI
BMI is a useful starting point but it has well-documented limitations. Understanding these helps you interpret your result more accurately:
- Muscle vs fat — BMI cannot distinguish between muscle and fat. Athletes and bodybuilders often have a high BMI despite being very lean and healthy.
- Fat distribution — BMI ignores where fat is stored. Abdominal (visceral) fat carries higher health risks than fat stored elsewhere, but this is invisible to BMI.
- Age and sex — Body fat percentage naturally increases with age and differs between men and women, but BMI uses the same thresholds for all adults.
- Ethnicity — Research shows that health risks associated with BMI differ across ethnic groups. For example, the risk thresholds for South Asian populations are generally lower.
- Pregnancy — BMI is not an accurate health measure during pregnancy.
BMI is a screening tool, not a diagnosis. A high or low BMI does not automatically mean you have a health problem. Always consult a doctor or qualified health professional for a complete assessment.
How to Reach a Healthy BMI
If your BMI falls outside the healthy range, these evidence-based steps can help:
To reduce BMI (overweight / obese)
- Aim for a modest calorie deficit — 500 calories per day leads to roughly 0.5 kg per week loss.
- Prioritise whole foods, vegetables, lean protein, and complex carbohydrates.
- Get at least 150 minutes of moderate aerobic exercise per week (WHO recommendation).
- Reduce ultra-processed foods, sugary drinks, and alcohol.
- Seek guidance from a registered dietician or GP for a personalised plan.
To increase BMI (underweight)
- Increase calorie intake with nutrient-dense foods — nuts, whole grains, dairy, legumes.
- Eat more frequently — 5–6 smaller meals per day rather than 3 large ones.
- Incorporate strength training to build muscle mass alongside body weight.
- Rule out underlying causes — underweight can sometimes indicate a medical condition.
Sustainable changes made over months outperform extreme short-term diets every time. Focus on habits, not numbers.
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Open BMI Calculator →Frequently Asked Questions
Conclusion
BMI is a simple, free, and widely understood indicator of whether your weight falls within a healthy range for your height. It takes seconds to calculate and provides a useful starting point for health conversations.
That said, BMI is just one piece of the puzzle. Use it alongside waist circumference, blood pressure, blood tests, and professional medical advice for a full picture of your health. And remember — sustainable lifestyle habits matter far more than any single number.
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