BMI Calculator

What is BMI?

Body Mass Index (BMI) is a single number that estimates whether your weight is healthy for your height. It is calculated from your weight and height alone and is used worldwide as a quick screening measure for underweight, healthy weight, overweight and obesity. BMI does not measure body fat directly, but for most people it correlates well enough to be a useful first check.

How is BMI calculated?

The BMI formula is your weight divided by the square of your height:

• Metric: BMI = weight (kg) ÷ height (m)²
• Imperial: BMI = 703 × weight (lb) ÷ height (in)²

Worked example (metric)

A person weighing 70 kg at 1.70 m tall: BMI = 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2 → normal weight.

Worked example (imperial)

A person weighing 154 lb at 5 ft 7 in (67 in): BMI = 703 × 154 ÷ (67 × 67) = 108,262 ÷ 4,489 = 24.1 → normal weight.

BMI categories (WHO, adults 20+)

BMI range	Category	What it suggests
Below 18.5	Underweight	May indicate undernutrition or other health issues
18.5 – 24.9	Normal weight	Lowest risk range for most adults
25.0 – 29.9	Overweight	Increased risk of weight-related conditions
30.0 – 34.9	Obesity (class I)	High risk
35.0 – 39.9	Obesity (class II)	Very high risk
40.0 and above	Obesity (class III)	Extremely high risk

Healthy weight range by height

Because BMI depends on height, the healthy-weight band (BMI 18.5–24.9) translates to a different weight range for everyone. Common heights:

Height	Healthy weight (kg)	Healthy weight (lb)
1.55 m (5’1”)	44 – 60 kg	98 – 132 lb
1.60 m (5’3”)	47 – 64 kg	104 – 140 lb
1.65 m (5’5”)	50 – 68 kg	111 – 149 lb
1.70 m (5’7”)	53 – 72 kg	118 – 159 lb
1.75 m (5’9”)	57 – 76 kg	125 – 168 lb
1.80 m (5’11”)	60 – 81 kg	132 – 178 lb
1.85 m (6’1”)	63 – 85 kg	140 – 188 lb

How to read your result

Your BMI is a screening signal, not a diagnosis. A result outside the healthy range is a prompt to look closer — not a verdict. Pair BMI with other simple measures such as waist circumference (a marker of abdominal fat) and how you feel day to day. If your BMI is in the overweight or obese range, small, consistent changes to diet and activity move the number more reliably than crash diets.

Limitations of BMI

BMI is popular because it is simple, but that simplicity is also its weakness:

• Muscle vs fat — muscle is denser than fat, so athletes and very muscular people can have a high BMI while being lean.
• Body composition & fat distribution — BMI says nothing about where fat sits; abdominal fat carries more risk.
• Age & sex — older adults may have more body fat at the same BMI; women carry more fat than men at the same number.
• Ethnicity — some populations (e.g. South Asian) face higher risk at lower BMI values, so adjusted thresholds are sometimes used.
• Children & teens — under-20s should use age- and sex-specific BMI percentiles, not these fixed ranges.

Frequently used BMI facts

• Normal body temperature aside, 18.5–24.9 is the adult healthy band.
• A BMI of exactly 25 marks the start of “overweight”; 30 marks “obesity”.
• BMI was devised in the 1830s by Adolphe Quetelet and was originally called the Quetelet Index.

BMI is a general screening measure provided for information and education only. It is not medical advice or a diagnosis. For personal guidance, consult a qualified healthcare professional.