What Is BMI? How to Calculate It, What It Means and Its Limits

What is BMI?

Body mass index is a simple ratio between weight and height. It was developed in the 19th century to describe populations, and it became the standard way to classify body weight because it is quick, free and needs only a scale and a tape measure. Large studies link very low and high BMI values to higher risk of health problems such as type 2 diabetes and heart disease.

The key word is screening. The World Health Organization describes BMI as a surrogate marker of fatness and notes that extra measurements, such as waist circumference, help when assessing obesity. In 2025, an international Lancet commission went further and advised against using BMI alone to diagnose obesity in individuals, recommending that excess body fat be confirmed with other measures and that health and daily function be assessed as well.

How to calculate your BMI

In metric units, divide your weight in kilograms by your height in metres multiplied by itself. In imperial units, multiply your weight in pounds by 703 and divide by your height in inches squared. Both give the same number.

You can also work backwards to find the weight range that matches a healthy BMI for your height: multiply your height in metres squared by 18.5 and by 24.9. For someone 1.75 m tall, that range is roughly 56.7–76.3 kg.

• Metric: 70 kg ÷ (1.75 × 1.75) = 70 ÷ 3.06 = 22.9.
• Imperial: 154 lb × 703 ÷ (69 × 69) = 22.7.
• Weigh yourself in the morning, after using the toilet and before eating, for the most consistent readings.
• Measure height without shoes, standing straight against a wall.

WHO BMI categories for adults

The WHO categories below apply to adults of any age and both sexes, and are also used by the CDC in the United States. Keep in mind that health risk changes gradually across the scale; the cut-offs are convenient boundaries, not sudden jumps from healthy to unhealthy.

Being underweight carries its own risks, such as nutrient deficiencies and lower bone density, so a lower BMI is not automatically better. For children and teenagers, BMI is interpreted differently, using growth charts and percentiles for age and sex; the categories below and the calculators on this site are for adults only.

• Under 18.5: underweight.
• 18.5–24.9: healthy weight.
• 25–29.9: overweight.
• 30–34.9: obesity class 1.
• 35–39.9: obesity class 2.
• 40 or more: obesity class 3.

The limitations of BMI

Because BMI only uses weight and height, it can't tell what that weight is made of. Two people with the same BMI can have very different amounts of muscle and fat, and very different health risks.

This matters most at the individual level. BMI works reasonably well for spotting trends across large groups, but for one person it's a starting point for a conversation, not a verdict.

• Muscle: strength athletes and people who lift regularly can have a BMI in the overweight range with low body fat.
• Age: older adults tend to lose muscle, so they can carry more fat at the same BMI than younger adults.
• Sex: at the same BMI, women typically have a higher body fat percentage than men.
• Ethnicity: the link between BMI, body fat and health risk differs between populations.
• Fat distribution: BMI doesn't show whether fat is stored around the abdomen, which is linked to higher cardiometabolic risk.
• Pregnancy: BMI isn't meaningful for tracking weight during pregnancy.

Ethnicity-specific BMI cut-offs

A 2004 WHO expert consultation published in The Lancet found that Asian populations have different associations between BMI, body fat percentage and health risk than European populations. A large share of people in many Asian countries have a higher risk of type 2 diabetes and cardiovascular disease at BMIs below 25.

The consultation kept the international categories but identified additional public health action points of 23, 27.5, 32.5 and 37.5. Several Asian countries use lower thresholds, often around 23 for overweight and 25–27.5 for obesity. Some national guidelines, such as those in the UK, also recommend lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background. If this applies to you, a doctor can help interpret your result in context.

Waist circumference and waist-to-height ratio

Where you carry fat matters, not just how much you have. A 2020 consensus statement led by Ross and colleagues concluded that waist circumference adds information beyond BMI and recommended measuring it routinely: at any given BMI, a larger waist is linked to higher cardiometabolic risk.

Waist-to-height ratio is even simpler, because one cut-off works for most adults. In a 2012 meta-analysis, Ashwell and colleagues found it was better than both waist circumference and BMI at picking up cardiometabolic risk factors in men and women. The practical message is to keep your waist to less than half your height, or a ratio below 0.5.

• How to measure: stand relaxed, breathe out gently and measure midway between your lowest rib and the top of your hip bone.
• Example: at 170 cm tall, a waist under 85 cm gives a ratio below 0.5.
• Commonly used waist thresholds for higher risk in European populations are over 94 cm in men and over 80 cm in women, with substantially higher risk above 102 cm and 88 cm; lower values are often used for Asian populations.

What to do with your BMI result

If your BMI is in the healthy range and your waist is under half your height, you're in a good starting position; the focus is on staying active, eating well and sleeping enough. If your BMI is high but you're muscular and your waist is small, the number probably overstates your risk.

If your BMI and waist both point to excess fat, gradual changes to diet and activity can improve health markers even before large changes in weight. Our calorie calculator and ideal weight calculator can help you set realistic targets.