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How to Calculate BMI: Formula, Categories, and What It Actually Means

By ClickSolveTools Editorial TeamUpdated 8 min read
Disclaimer: This article is for informational purposes only and is not medical advice. Consult a healthcare provider for guidance on your specific situation.

To calculate BMI, divide your weight in kilograms by your height in meters squared. If you use pounds and inches, divide your weight by your height in inches squared, then multiply by 703. For adults 20 and older, the CDC classifies a BMI under 18.5 as underweight, 18.5 to 24.9 as healthy weight, 25 to 29.9 as overweight, and 30 or higher as obesity.

BMI (body mass index) is a quick screening number, not a measure of body fat. It can't tell muscle from fat or show where you carry weight, so it works best alongside a waist measurement and your doctor's other checks. Below you'll find both formulas with worked examples, the official categories, a quick-reference chart, and the cases where BMI misleads. You can check any result with our BMI Calculator.

The BMI Formula

Metric Formula

BMI = weight (kg) / height (m)²

If you weigh 75 kg and stand 1.75 m tall:

BMI = 75 / (1.75 × 1.75) = 75 / 3.0625 = 24.5

Imperial Formula

If you work in pounds and inches, multiply by a conversion factor of 703:

BMI = weight (lb) / height (in)² × 703

For someone who weighs 165 lb and is 5 feet 9 inches (69 inches) tall:

BMI = 165 / (69 × 69) × 703 = 165 / 4,761 × 703 = 24.4

Where does 703 come from? One pound is 0.45359237 kg and one inch is 0.0254 m, so converting pounds per square inch to kilograms per square meter means multiplying by 0.45359237 / 0.0254², which is 703.07. Rounding it to 703 changes the result by less than 0.01%.

Adult BMI Categories

These are the CDC categories for adults 20 and older. They apply regardless of age or sex, and they line up with the World Health Organization definitions of overweight (BMI of 25 or more) and obesity (30 or more).

BMI Range CDC Category
Below 18.5 Underweight
18.5 to 24.9 Healthy weight
25.0 to 29.9 Overweight
30.0 to 34.9 Class 1 obesity
35.0 to 39.9 Class 2 obesity
40.0 and above Class 3 obesity (severe obesity)

Children and teens use a different system based on age and sex percentiles, covered further down.

Quick-Reference BMI Chart

Find your height in the first column and the closest weight across the top. Each value was calculated with the imperial formula above and rounded to one decimal place.

Height 120 lb 150 lb 180 lb 210 lb 240 lb
5'2" (62 in) 21.9 27.4 32.9 38.4 43.9
5'5" (65 in) 20.0 25.0 30.0 34.9 39.9
5'8" (68 in) 18.2 22.8 27.4 31.9 36.5
5'11" (71 in) 16.7 20.9 25.1 29.3 33.5
6'2" (74 in) 15.4 19.3 23.1 27.0 30.8

Where BMI Falls Short

BMI is popular because it only needs a scale and a tape measure. The CDC is clear about what it leaves out: BMI does not distinguish between fat, muscle, and bone, and it does not show where on the body fat is carried. That leads to four predictable blind spots.

1. It Can't Tell Muscle From Fat

Muscle adds weight, so a muscular person can land in the overweight or obesity range while carrying little fat. A 6 foot 5 inch (77 inch) athlete who weighs 260 lb has a BMI of 30.8, which falls in class 1 obesity no matter how lean they are. The CDC notes that highly trained athletes may have a high BMI because of muscle mass.

2. Age and Sex Change the Picture

At the same BMI, women tend to have more body fat than men, and older adults tend to have more body fat than younger adults, according to the CDC. Two people with a BMI of 25 can have very different amounts of body fat.

3. Risk Starts Lower for Some Ethnic Groups

A 2004 WHO expert consultation found that many Asian populations face higher health risks at lower BMI values than European populations and flagged 23 and 27.5 as public health action points. The UK's National Institute for Health and Care Excellence (NICE) now uses lower thresholds for people with a South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean family background: overweight from 23 to 27.4 and obesity at 27.5 or above.

4. It Ignores Where You Carry Fat

Fat around the abdomen is linked to a higher risk of heart disease and type 2 diabetes than fat on the hips and thighs. Two people with the same BMI can have very different waist sizes, and BMI treats them the same.

When BMI Is Useful

Measures to Pair With BMI

Waist Circumference

The National Heart, Lung, and Blood Institute (NHLBI) says a waist larger than 35 inches (about 89 cm) for women or 40 inches (about 102 cm) for men increases health risk. To measure, stand and wrap the tape around your middle just above your hipbones, then read it just after you breathe out.

Waist-to-Height Ratio

Divide your waist by your height, using the same units for both. NICE advises keeping your waist to less than half your height, a ratio under 0.5. Its guideline treats 0.5 to 0.59 as increased central fat and 0.6 or more as high, and the same cutoffs apply to both sexes and all ethnic groups for adults with a BMI under 35.

Waist-to-Hip Ratio

Divide your waist measurement by your hip measurement. The WHO expert consultation report on waist and hip measures lists a ratio of 0.90 or more in men and 0.85 or more in women as a sign of substantially increased risk of metabolic complications, and notes that these cut-offs come from earlier WHO reports. Our Waist-to-Hip Ratio Calculator does the division and shows the WHO category.

Body Fat Percentage

Body fat percentage measures composition directly instead of inferring it. DEXA scans are among the most accurate methods; bioelectrical impedance scales are convenient but less consistent; skinfold calipers depend on a trained measurer. For a free estimate from tape measurements, try our Body Fat Calculator, which uses the US Navy method.

Where BMI Came From

Belgian mathematician and statistician Adolphe Quetelet described the relationship between weight and the square of height in the 1830s while studying the characteristics of the average person in a population. It was a statistical tool, not a clinical one.

In 1972, physiologist Ancel Keys and colleagues compared several weight-for-height indexes in 7,424 men and found that weight divided by height squared was slightly better than the simple weight-to-height ratio at tracking body fat. They named it the body mass index, and it became the standard screen used by health agencies worldwide.

The debate is still moving. In January 2025, a Lancet Diabetes and Endocrinology commission of international experts recommended that obesity should not be diagnosed from BMI alone. It advised confirming excess body fat with at least one other measure, such as waist circumference, waist-to-hip ratio, or waist-to-height ratio, except at a BMI above 40, where excess fat can be assumed.

BMI for Children and Teens

For ages 2 through 19, the CDC compares a child's BMI with other children of the same age and sex and reports it as a percentile. A 10-year-old boy at the 85th percentile has a higher BMI than 85% of 10-year-old boys in the CDC reference data. The categories are:

Percentiles matter because normal body fat changes a lot during growth. On the CDC growth charts, a BMI of 22 sits in the healthy range for an 18-year-old but is above the 95th percentile, the obesity range, for a 7-year-old boy or girl. Our BMI Calculator applies the adult categories, so use the CDC's child and teen calculator for anyone under 20.

Tips for Using BMI Wisely

Frequently Asked Questions

Is BMI accurate for athletes and bodybuilders?

Not reliably. BMI cannot tell muscle from fat, and the CDC notes that highly trained athletes may have a high BMI because of their muscle mass. If you carry a lot of muscle, a waist measurement or a body fat estimate will tell you more than BMI.

What is a healthy BMI for adults?

The CDC defines healthy weight for adults 20 and older as a BMI of 18.5 to less than 25. NICE uses a lower upper limit of 23 for people with a South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean family background. A BMI in range is a good sign, but blood pressure, blood sugar, cholesterol, and waist size still matter.

Can I have a normal BMI and still be unhealthy?

Yes. Someone with a healthy-range BMI can still carry excess fat around the abdomen, which is linked to heart disease and type 2 diabetes. This is sometimes called normal weight obesity. Checking your waist against the NHLBI thresholds (35 inches for women, 40 inches for men) helps catch it.

Do BMI categories differ by ethnicity?

They can. A 2004 WHO expert consultation found higher health risks at lower BMI values in many Asian populations, and NICE in the UK now classifies overweight as 23 to 27.4 and obesity as 27.5 or above for people with a South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean family background. The CDC's US categories use the same cutoffs for everyone.

Is BMI being replaced?

Not replaced, but supplemented. In January 2025, a Lancet Diabetes and Endocrinology commission recommended against diagnosing obesity from BMI alone and advised confirming excess body fat with a waist-based measure or a direct body fat measurement. BMI remains the standard first screen used by the CDC and WHO because it is quick and needs no special equipment.

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