What is BMI?
Body mass index (BMI) is a simple measure that compares your weight with your height. It is used by the NHS and the World Health Organization as a quick way to check whether an adult's weight is in a healthy range for their height.
BMI is a screening tool, not a diagnosis. It does not measure body fat directly, so it should be read alongside other measures such as waist size, and a doctor or practice nurse can put it into context.
The BMI formula
BMI = weight in kg ÷ (height in metres × height in metres)
So you divide your weight by the square of your height. The calculator converts your height from centimetres to metres first (175 cm = 1.75 m) and rounds the result to one decimal place.
The healthy weight range is found by turning the formula round: weight = BMI × height². The calculator uses BMI 18.5 and 24.9 as the lower and upper limits.
BMI categories for adults
- People of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background have a higher risk of conditions such as type 2 diabetes at a lower BMI. NHS and NICE guidance uses lower thresholds for them: 23 or above for an increased risk (overweight) and 27.5 or above for obesity.
- These categories are for adults. Children's BMI is compared with growth charts for their age and sex, so use the NHS child BMI calculator for under-18s.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obese (class I) |
| 35 to 39.9 | Obese (class II) |
| 40 or above | Obese (class III, severe) |
Worked example 1: healthy weight
Weight 70 kg, height 175 cm
Height in metres = 1.75; 1.75 × 1.75 = 3.0625
BMI = 70 ÷ 3.0625 = 22.9
Category: healthy weight
Healthy range for 175 cm: 18.5 × 3.0625 = 56.7 kg to 24.9 × 3.0625 = 76.3 kg
Worked example 2: overweight
Weight 95 kg, height 180 cm
1.80 × 1.80 = 3.24
BMI = 95 ÷ 3.24 = 29.3, in the overweight range
Healthy range for 180 cm: 59.9 kg to 80.7 kg
Reaching the top of the healthy range would mean losing about 14 kg, but even a 5% loss (about 5 kg) can bring health benefits.
Worked example 3: just inside the obese range
Weight 82 kg, height 165 cm
1.65 × 1.65 = 2.7225
BMI = 82 ÷ 2.7225 = 30.1, which is obese (class I)
Healthy range for 165 cm: 50.4 kg to 67.8 kg
Worked example 4: underweight
Weight 52 kg, height 168 cm
1.68 × 1.68 = 2.8224
BMI = 52 ÷ 2.8224 = 18.4, just underweight
Healthy range for 168 cm: 52.2 kg to 70.3 kg
Limitations of BMI
- Muscle weighs more than fat. Athletes and very muscular people can have a high BMI without excess fat.
- Older adults may have less muscle, so a 'healthy' BMI can still hide excess body fat.
- Pregnancy: BMI is not meaningful during pregnancy; your midwife uses your pre-pregnancy BMI.
- Where fat is stored matters. Fat around the waist is linked to higher health risks. A simple check is the waist-to-height ratio: keep your waist measurement to less than half your height.
- BMI cannot tell you about fitness, diet, blood pressure or blood sugar.
Converting stones, pounds, feet and inches
Many people in the UK know their weight in stones and height in feet. Use these conversions before entering your figures:
- Weight: kg = (stones × 14 + pounds) × 0.4536. For example, 11 st 4 lb = 158 lb × 0.4536 = 71.7 kg.
- Height: cm = (feet × 12 + inches) × 2.54. For example, 5 ft 9 in = 69 in × 2.54 = 175.3 cm.