Body mass index (BMI) calculator

Same weight, different height: 70 kg on four people

BMI divides weight by height squared, so height affects the result more than you might expect. The same 70 kg gives people of four different heights a completely different assessment:

• 160 cm: BMI 27.3 (overweight), normal weight at this height 47.4-64 kg.
• 170 cm: BMI 24.2 (normal), normal weight 53.5-72.3 kg.
• 180 cm: BMI 21.6 (normal), normal weight 59.9-81 kg.
• 190 cm: BMI 19.4 (normal), normal weight 66.8-90.3 kg.

Boundary values and rounding

The calculator rounds BMI to one decimal place and assigns the category from the rounded number. A couple of hundred grams can therefore move you into another category: at 180 cm and 80.8 kg the BMI is 24.9 (normal), but at 81 kg it is 25.0 (overweight). Likewise, 176 cm and 57 kg give a BMI of 18.4, which is underweight, even though the lower normal limit of 57.3 kg is only 300 grams away.

Body weight often swings by more than a kilogram during the day because of food, drink and fluid retention. If you are near a boundary, weigh yourself on several mornings under the same conditions and use the average. A change of one tenth in BMI is not a change in health.

Accurate height matters just as much. Many people enter a height from memory or from years ago, although height can shrink by a few centimetres with age. At 70 kg, 172 cm gives a BMI of 23.7, 170 cm gives 24.2 and 168 cm already 24.8. Measure your height in the morning, barefoot with your back against a wall, and enter it to the nearest centimetre.

Who may get a misleading BMI

BMI cannot tell muscle, fat and bone apart. These are situations where the result needs careful interpretation.

• Muscular people: 176 cm and 92 kg give a BMI of 29.7, the top of the overweight range, even though a strength athlete may carry little fat.
• Older adults: muscle mass declines with age, so a normal BMI can hide weak muscles and a relatively high body fat percentage.
• Children and teenagers: adult categories do not apply under 18; BMI is assessed against growth charts for age and sex.
• Pregnant women: the categories are not used during pregnancy. Recommended weight gain is based on pre-pregnancy BMI.
• People of Asian origin may face higher health risks at a lower BMI than people of European origin.

Waist circumference complements BMI

Where fat is stored also matters for health risk. Abdominal fat is linked to a higher risk of cardiovascular disease and type 2 diabetes, even when BMI is normal.

Measure your waist standing, after a relaxed breath out, midway between the lowest rib and the top of the hip bone. According to the World Health Organization, risk is increased from 94 cm in men and 80 cm in women, and substantially increased from 102 cm in men and 88 cm in women. A simple extra rule is to keep your waist below half your height.

How to use the result in practice

The upper limit of normal weight does not have to be your immediate goal. At 176 cm and 92 kg, for example, the upper normal limit is 77.4 kg, 14.6 kg away, but losing just 5-10% of body weight, roughly 4.6-9.2 kg, often improves blood pressure and blood sugar. The calorie calculator helps you find a realistic daily energy target. Judge progress as a trend: check your BMI about once a month under the same conditions rather than daily, because day-to-day swings hide the real direction.

Talk to your GP or practice nurse if your BMI is 30 or more, below 18.5, if your weight has changed noticeably over a few months without a clear reason, or if weight changes come with tiredness, thirst, breathlessness or other symptoms. BMI is a screening measure, not a diagnosis.