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BMI explained: what it measures and who it gets wrong

Body mass index in plain terms: the formula, the ranges, a worked example and the people it misjudges.

EVEashan VagishOctober 15, 20263 min read

Last reviewed October 15, 2026

A gym with long racks of dumbbells and a few people training in the distance.

Body mass index (BMI) is your weight divided by your height squared, a quick screening number for weight status.[1, 2] For adults, the CDC calls under 18.5 underweight, 18.5 to under 25 healthy, 25 to under 30 overweight and 30 or more obesity.[1] It cannot tell muscle from fat, so it misjudges some bodies.[2]

Key takeaways

  • BMI is your weight divided by your height squared. Nothing else goes into it.[2]
  • For adults 20 and older, the CDC ranges are under 18.5 underweight, 18.5 to under 25 healthy weight, 25 to under 30 overweight, and 30 or more obesity.[1]
  • BMI cannot tell fat from muscle and bone, or say where fat sits on the body.[2]
  • The CDC calls BMI a screening measure, to be weighed with other factors.[1]
  • For Asian populations, a WHO consultation found that a substantial share of people are at high risk at BMIs below 25, and proposed extra action points of 23 and 27.5.[3]

How BMI is calculated

Body mass index (BMI)
Weight in kilograms divided by height in meters, squared. In US units, weight in pounds divided by height in inches squared, times 703.[2]

That is the whole formula. Age, sex and how much of your weight is muscle do not go into it. For adults 20 and older, the CDC uses the same categories regardless of age, sex or race.[1]

Worked example

BMI for one person

Take a woman who is 5 ft 5 in (165.1 cm) tall and weighs 154 lb (69.9 kg). The result is rounded to one decimal, as the Dalia app does.

  1. Height in meters, squared

    1.651 × 1.651

    2.726 m²

  2. Weight divided by height squared

    69.85 ÷ 2.726

    25.6

  3. Same answer in US units

    154 ÷ (65 × 65) × 703

    25.6

BMI

25.6, in the overweight range

At 5 ft 5 in, the healthy range of 18.5 to 24.9 works out to about 111 to 150 lb. The number says nothing about how much of her weight is muscle.

Run your own numbers in the BMI calculator, which shows the math and the healthy weight range for your height.

What the BMI ranges mean

BMICDC category
Under 18.5Underweight[1]
18.5 to under 25Healthy weight[1]
25 to under 30Overweight[1]
30 or moreObesity[1]
CDC BMI categories for adults 20 and older

The CDC splits 30 and above into three classes, with class 3 at 40 or more.[1] The Dalia app uses the same cut points and calls the top range "High".

The CDC describes BMI as a screening measure that should be considered with other factors when assessing someone's health.[1]

Who BMI gets wrong

  • People with a lot of muscle. BMI cannot distinguish fat from muscle and bone, so a muscular person can land in the overweight range.[2] Take a man who is 5 ft 10 in and 210 lb: his BMI is 30.1, which the CDC calls obesity, whether the weight is muscle or fat.[1]
  • Where fat sits. BMI cannot show where fat is located on the body.[2]
  • People of Asian descent. A WHO consultation found that the share of Asian people at high risk of type 2 diabetes and cardiovascular disease is substantial at BMIs below 25.[3] It proposed extra public health action points at 23 and 27.5.[3]
  • Pregnancy. Weight gain advice in pregnancy is based on BMI before pregnancy.[4] Ask your prenatal care team how to read your numbers.
  • Children and teens. Because they are still growing, their BMI is read as a percentile against children of the same sex and age, so the adult ranges do not apply.[5]

If you are working on weight, see what a calorie deficit is and how many calories to eat to lose weight.

When to talk to a clinician

References & further reading

  1. 1.Adult BMI categories, CDC
  2. 2.Frequently asked questions about BMI, CDC
  3. 3.Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies, The Lancet (WHO Expert Consultation), 2004
  4. 4.Weight gain during pregnancy, CDC
  5. 5.BMI percentile calculator for child and teen, CDC
  6. 6.Overweight and pregnant, NHS
EV

Eashan Vagish

Founder, Dalia Health

Eashan Vagish is the founder of Dalia Health, where he works on making heart and metabolic health easier to understand and track. He writes these guides to answer the health questions people actually ask, in plain language, with links to the research behind every claim.

About this guide: These guides are written by the Dalia Health team. Every factual claim links to its source, preferring peer-reviewed research and public-health bodies such as the NIH, CDC and WHO. Each guide shows when it was last reviewed. They are for general education and are not a substitute for medical advice. Read our editorial policy at daliahealth.com/editorial-policy, and report an error to info@daliahealth.com.

Medical disclaimer: This article is for general educational purposes only and is not medical advice. It is not a substitute for diagnosis or treatment from a qualified healthcare professional. Always talk to your doctor about your individual health, and seek immediate care for any urgent symptoms.

How we write, source and correct these guides: our editorial policy. Spot an error? Email info@daliahealth.com.

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