Body Mass Index (BMI) is the world's most widely used weight screening tool — it's calculated by millions of people every month, used by doctors as a quick triage metric, and referenced in public health guidelines worldwide. It's also frequently misunderstood. The BMI Calculator gives you your result instantly in both metric and imperial units, along with a clear breakdown of what the number means — and what it doesn't.
The BMI Formula
BMI is a simple ratio of weight to height squared:
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = (weight (lbs) × 703) ÷ height (inches)²
For example, a person who is 175 cm (1.75 m) tall and weighs 75 kg has a BMI of: 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5 — in the normal range.
In imperial: a person 5'9" (69 inches) weighing 165 lbs: (165 × 703) ÷ (69 × 69) = 116,000 ÷ 4,761 = 24.4 — same result, slightly different rounding.
The Four BMI Categories
The World Health Organisation defines four adult BMI categories:
- Underweight: BMI below 18.5 — may indicate malnutrition, eating disorders, or underlying illness. Carries increased risk of bone loss, immune dysfunction, and fertility issues.
- Normal weight: BMI 18.5–24.9 — associated with the lowest risk of weight-related health conditions in population studies.
- Overweight: BMI 25.0–29.9 — increased risk of type 2 diabetes, hypertension, and cardiovascular disease, particularly when combined with other risk factors.
- Obese: BMI 30.0 and above — further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III (40+, sometimes called morbid obesity). Substantially elevated risk for multiple chronic conditions.
Metric vs Imperial: What to Enter
The BMI Calculator accepts both systems. In metric, enter height in centimetres and weight in kilograms. In imperial, enter height in feet and inches (separately) and weight in pounds. If you know your weight in stones (common in the UK), convert first: 1 stone = 6.35 kg, or 14 lbs.
How to Use the BMI Calculator
- Open the BMI Calculator.
- Select your unit system — metric (cm/kg) or imperial (ft, in/lbs).
- Enter your height and weight.
- Click Calculate to see your BMI, category, and healthy weight range for your height.
For example, if you're 5'6" and weigh 160 lbs: BMI = (160 × 703) ÷ (66²) = 112,480 ÷ 4,356 = 25.8 — just into the overweight category. The healthy weight range for someone 5'6" is approximately 115–154 lbs (BMI 18.5–24.9).
The Limitations of BMI
BMI is a population-level screening tool, not a diagnostic one. Several important limitations mean it shouldn't be interpreted in isolation:
Muscle Mass
BMI cannot distinguish between muscle and fat. A rugby player or bodybuilder may have a BMI of 28 — technically "overweight" — while having very low body fat and excellent cardiovascular health. Conversely, someone with a normal BMI can have high body fat and poor metabolic health ("skinny fat").
Age and Sex
BMI thresholds were derived from studies of white European adults. They may overestimate risk for Black populations and underestimate risk for some Asian populations, where higher health risks appear at lower BMI values. Some guidelines use a lower threshold of 23 for Asian adults.
Distribution of Fat
Where fat is stored matters as much as how much there is. Abdominal fat (high waist circumference) is more metabolically harmful than fat stored elsewhere, regardless of BMI. A waist-to-height ratio below 0.5 is a commonly used complementary metric.
Children
Standard BMI thresholds only apply to adults. For children and teens, BMI is expressed as a percentile relative to age and sex — the BMI Calculator calculates adult BMI; consult a paediatrician for child assessments.
What to Do With Your BMI Result
If your BMI is outside the normal range, it's a prompt for a conversation with a doctor — not a diagnosis. A clinician will consider your full picture: waist circumference, blood pressure, lipid profile, blood sugar, lifestyle factors, and family history. BMI is one data point, not the whole story.
FAQ
What is a healthy BMI for an adult?
18.5 to 24.9 is classified as normal weight for most adults. However, this range is a statistical average from population data, not a universal health guarantee. Individual factors like muscle mass, age, and ethnicity affect interpretation.
Can I have a high BMI and still be healthy?
Yes. Athletes and people with high muscle mass often have elevated BMIs despite excellent health. If your BMI is above 25 but your blood pressure, cholesterol, and blood sugar are normal and you exercise regularly, your overall risk profile may be much lower than BMI alone suggests.
Does BMI differ for men and women?
The categories are the same for both sexes, but the health implications can differ. Women naturally carry more body fat than men at the same BMI, which is physiologically normal. Some researchers argue for sex-specific thresholds, but standard clinical practice uses the same four categories.
How accurate is BMI as a health measure?
As a screening tool at a population level, it's useful. As a measure of individual health, it's imprecise. Combine it with waist circumference, blood work, and physical fitness assessments for a more complete picture.