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BMI Calculator

Body mass index in either unit system, with the healthy range for your height — and a clear account of what BMI cannot tell you.

Enter a height and a weight.

Units
Estimate. A real-world figure based on the assumptions listed below, which you can change.

How this calculator works

BMI is weight divided by height squared. In metric that is kilograms over metres squared. In imperial the same ratio needs a conversion factor of 703 to turn pounds per square inch into the same units — the 703 is arithmetic, not a fudge.

The categories are the WHO and CDC standard for adults: under 18.5, 18.5 to 24.9, 25 to 29.9, and 30 or above. Those thresholds are conventions agreed for describing populations, not lines where something happens to a person.

The healthy range is also shown as a weight, because that is the more useful direction. Knowing your BMI is 25.8 tells you less than knowing the band for your height runs from 129 to 173 pounds.

What BMI cannot do is the part most calculators skip, so it is stated on every result. It compares mass to height and nothing else — it cannot distinguish muscle from fat, so a lean athlete and a sedentary person of identical build get the same number and it means opposite things. It says nothing about where weight sits, and abdominal fat carries risk that BMI is blind to.

It was devised in the 1830s by Quetelet as a way of describing populations, and he said explicitly that it was not for assessing individuals. That is still true.

The formula

Exactly what happens to your numbers, step by step.

  1. Metric

    BMI = kg ÷ m²

  2. Imperial

    BMI = 703 × lb ÷ in²

    703 converts the units; it is not an adjustment.

  3. Healthy weight range

    from 18.5 × m² to 24.9 × m²

A worked example

Someone 5 ft 10 in tall weighing 180 pounds.

What you enter

Height
5 ft 10 in (70 in)
Weight
180 lb

The working

Height squared
70² = 4,900 in²
BMI
703 × 180 ÷ 4,900 = 25.8
Category
Overweight (25–29.9)
Healthy range for this height
129–173 lb

BMI 25.8 — just above the healthy band

And this is exactly where BMI is least informative. At 25.8, a rugby player and someone who has not exercised in a decade get the same number, from the same height and weight, meaning entirely different things about their health. The figure is a starting point for a question, not an answer to one.

Assumptions

Every result here rests on these. Change your inputs and the result changes with them.

  • The adult WHO and CDC category thresholds apply.
  • Height and weight are as measured, with no allowance for clothing or time of day.
  • The healthy range spans a BMI of 18.5 to 24.9 for the height entered.

What this cannot tell you

  • BMI cannot tell muscle from fat. Anyone muscular reads high; anyone with low muscle mass can read normal while carrying more fat than the number suggests.
  • It ignores where weight sits, and that matters — fat around the abdomen carries considerably more metabolic risk than the same weight elsewhere. A waist measurement adds information BMI does not contain.
  • It does not apply to children or teenagers, who are assessed on age and sex percentile charts rather than fixed thresholds.
  • The thresholds are not universal. Several health bodies use lower cut-offs for people of South Asian descent, because cardiometabolic risk rises at a lower BMI in those populations.
  • It is not a diagnosis and this page does not give advice. A number outside the healthy band is a reason to talk to a clinician, who has access to things BMI does not measure.

Questions people ask

How is BMI calculated?

Weight divided by height squared — kilograms over metres squared in metric, or 703 times pounds over inches squared in imperial. Both give the same number for the same person.

What is a healthy BMI?

The standard adult band is 18.5 to 24.9, with 25 to 29.9 classed as overweight and 30 or above as obese. These are population conventions rather than individual thresholds, and the boundaries are not cliffs.

Is BMI accurate for athletes?

No, and this is its clearest failure. Muscle is denser than fat, so a well-trained athlete often reads overweight or obese while carrying very little fat. BMI measures mass against height and cannot see what the mass is made of.

Should I worry about my BMI?

It is one measurement among several and a poor one to act on alone. Waist circumference, blood pressure, blood glucose and lipids all say things BMI cannot, and a clinician can interpret them together. This calculator reports a number; it does not offer advice.

Does BMI work for children?

Not with these thresholds. Children and teenagers are assessed against age- and sex-specific percentile charts, because body composition changes substantially through growth. Use a pediatric chart rather than this.

Related calculators

Sources and review

Sources

  • WHO and CDC adult BMI categories: under 18.5, 18.5-24.9, 25-29.9, 30 and above

Methodology version 1.0.0 · Last reviewed