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BMI Calculator

Body mass index from your height and weight, the arithmetic behind it, and the people it misjudges.

Updated . An estimate for general education, not medical advice.

Height

Your result

Fill in the fields and your result shows up here. Everything is worked out in your browser; nothing you type is sent to us.

How BMI is calculated

BMI is your weight in kilograms divided by the square of your height in meters. That is the whole formula. Nothing about your age, sex, muscle or where you carry fat goes into it.

Metric
BMI = weight (kg) ÷ height (m)²
US units
BMI = 703 × weight (lb) ÷ height (in)²

This page converts US units to metric with the same constants the Dalia app uses (1 lb = 0.453592 kg, 1 in = 2.54 cm) and rounds to one decimal, so the number here matches the one in the app.

What the ranges mean

For adults 20 and older, the CDC's categories are: under 18.5 underweight, 18.5 to under 25 healthy weight, 25 to under 30 overweight, and 30 or more obesity. The Dalia app uses the same cut points and calls the top range "High".

For people of Asian descent, a WHO expert consultation found that risk of type 2 diabetes and heart disease rises at lower BMIs, and set public-health action points of 23 and 27.5.

Children and teens are measured differently: their BMI is compared with others of the same age and sex on CDC growth charts, so the adult ranges above do not apply.

What BMI is good for

BMI is a quick screen. It needs only a height and a weight, which is why it is everywhere, and it describes groups of people better than it describes one body. The trend of your own weight over several weeks says more about you than a single reading does.

Who this is wrong for

  • Muscular people and athletes. The CDC notes that BMI cannot tell fat from muscle and bone, so someone with a lot of muscle can land in the overweight or obesity range. It also cannot say where fat sits on the body.
  • People of Asian descent, for whom the WHO consultation found health risk rising below the standard cut-off of 25.
  • Pregnancy. Weight-gain advice in pregnancy is set from your BMI before pregnancy (Institute of Medicine, 2009), so a BMI taken while pregnant does not fit these ranges. Your prenatal care team is the right guide.
  • Children and teens under 20, who need the CDC's BMI-for-age percentiles instead of the adult ranges.

Talk to a clinician when

  • Your BMI is under 18.5, or you have lost weight without trying.
  • Your BMI is 30 or more and you want help weighing up the options, including whether medication is appropriate for you.
  • You are pregnant, planning a pregnancy, or managing a condition where weight matters, such as diabetes or heart disease.

Read more

BMI explained: what it measures and who it gets wrong

Sources

  1. Adult BMI Categories. Centers for Disease Control and Prevention.
  2. Frequently Asked Questions About BMI. Centers for Disease Control and Prevention.
  3. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet, 2004;363:157-163.
  4. BMI Percentile Calculator for Child and Teen. Centers for Disease Control and Prevention.
  5. Weight Gain During Pregnancy: Reexamining the Guidelines. Institute of Medicine and National Research Council, 2009.

Other calculators

This calculator gives an estimate for general education. It is not medical advice and it is not a substitute for care from a qualified clinician. Report an error to info@daliahealth.com.