What is BMI? How to Calculate Your Body Mass Index
BMI — Body Mass Index — is a simple number that helps assess whether your weight is in a healthy range for your height. It's one of the most widely used screening tools in healthcare worldwide, and takes just a few seconds to calculate.
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Calculate your BMI instantly — metric or imperial, with health category and ideal weight range.
What does BMI stand for?
BMI stands for Body Mass Index. It's a numerical value calculated from a person's weight and height. The concept was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has been the primary tool used by the World Health Organization (WHO) and healthcare professionals globally for over 150 years to classify weight status.
The BMI formula
The BMI formula is straightforward:
BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5
In imperial units (pounds and inches), the formula becomes:
BMI categories — what the numbers mean
The WHO classifies BMI into four main categories for adults 18 and over:
| BMI Range | Category | Health risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency risk |
| 18.5 – 24.9 | Normal weight | Minimal (healthy range) |
| 25.0 – 29.9 | Overweight | Moderate health risk |
| 30.0 – 34.9 | Obese Class I | High health risk |
| 35.0 – 39.9 | Obese Class II | Very high health risk |
| 40.0 and above | Obese Class III | Extremely high risk |
What is a healthy BMI?
A healthy BMI is between 18.5 and 24.9. This range is associated with the lowest risk of weight-related health conditions including type 2 diabetes, heart disease, and certain cancers. If your BMI falls in this range, maintaining it through regular physical activity and a balanced diet is the goal.
✓ Key facts about healthy BMI
- A BMI of 18.5–24.9 is considered the healthy range for adults
- The midpoint of the healthy range is approximately 21.7
- People in this range have the lowest statistical risk of major illness
- Diet and exercise are the primary ways to maintain a healthy BMI
Limitations of BMI — what it doesn't measure
While BMI is useful as a quick screening tool, it's important to understand its limitations:
- Muscle mass: Highly muscular athletes may have a high BMI but carry very little body fat. BMI cannot distinguish between fat and muscle.
- Body fat distribution: Someone with a "normal" BMI can still have dangerous visceral fat (fat around the organs) if most weight is carried around the abdomen.
- Age: As adults age, muscle mass naturally decreases and fat tends to accumulate. BMI doesn't account for this change in body composition.
- Ethnicity: Research suggests that people of Asian descent may face health risks at lower BMI thresholds than the standard WHO guidelines.
- Children and teens: BMI percentile charts (not standard BMI) should be used for people under 18, as they compare to others of the same age and sex.
The WHO itself acknowledges that BMI is a screening tool, not a diagnostic measure. A full health assessment by a doctor will consider BMI alongside other factors such as waist circumference, blood pressure, cholesterol levels, and family history.
How to calculate your BMI in 3 steps
- Measure your weight in kilograms (or pounds for imperial)
- Measure your height in metres (or inches for imperial)
- Divide weight by height squared: weight ÷ (height × height)
Or skip the maths — use our free BMI calculator to get instant results with your health category and ideal weight range.
What to do if your BMI is outside the healthy range
If your BMI falls outside the 18.5–24.9 range, it's not a cause for alarm — it's a signal to take stock and consider whether changes might benefit your health.
If you're underweight (BMI below 18.5): Focus on calorie-dense, nutritious foods. Speak with a doctor or dietitian to rule out underlying conditions and develop a healthy weight-gain plan.
If you're overweight (BMI 25–29.9): A moderate calorie reduction (300–500 kcal/day) combined with 150+ minutes of moderate exercise per week is an evidence-based starting point.
If you're in the obese range (BMI 30+): Consult a healthcare professional. Medically supervised weight loss programmes, dietary changes, and exercise plans are all effective options. Significant benefits appear with even modest weight loss of 5–10%.
Frequently asked questions about BMI
Medical disclaimer: This article is for informational purposes only. BMI is a screening tool, not a medical diagnosis. Always consult a qualified healthcare professional for personalised health advice.