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Calculator Answers

BMI Calculator

Source: World Health Organization, Obesity and overweight fact sheet · Source verified August 19, 2026

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

Body mass index (BMI) is a value derived from the weight and height of a person, calculated as weight in kilograms divided by the square of height in meters (kg/m²). It is used by the World Health Organization and CDC as a population-level screening tool to classify individuals as underweight, normal weight, overweight, or obese. BMI does not directly measure body fat and may be inaccurate for athletes, the elderly, or pregnant women.

Enter your height and weight in imperial or metric units. We compute Body Mass Index, the standard category, and the healthy weight range that corresponds to BMI 18.5–25 at your height.

See also: TDEE Calculator, BMR Calculator, and Ideal Weight Calculator.

Quick Answer

Enter your height and weight to calculate your BMI. The result is classified as underweight (under 18.5), normal (18.5-24.9), overweight (25-29.9), or obese (30+) per WHO standards.

Units

lb
Height
ft
in

Standard BMI scale

  • < 18.5 — Underweight
  • 18.5–24.9 — Normal weight
  • 25.0–29.9 — Overweight
  • 30.0–34.9 — Obese (class I)
  • 35.0–39.9 — Obese (class II)
  • ≥ 40.0 — Obese (class III)
Body Mass Index

Overweight

25.8

180 lb · 5′10″

CategoryOverweight
Healthy range (BMI 18.5–25)129–174 lb
Weight (kg)81.6 kg
Height (cm)177.8 cm

BMI is a screening estimate from height and weight. It doesn't distinguish muscle from fat, doesn't adjust for age or frame, and isn't a diagnosis on its own. Use it as a starting point, not a verdict.

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Examples

5′10″, 180 lb

BMI ≈ 25.8 · Overweight

5′6″, 140 lb

BMI ≈ 22.6 · Normal weight

178 cm, 70 kg

BMI ≈ 22.1 · Normal weight

165 cm, 90 kg

BMI ≈ 33.1 · Obese (class I)

How it works

Formula · BMI = weight kg / (height m)^2, or BMI = 703 x weight lb / (height in)^2 using the rounded imperial factor

Body Mass Index is the ratio of weight to height squared. We convert both inputs to metric, compute the index, and look up the standard category band.

BMI · kg / m²

Imperial form · 703 × lb / in²

The healthy weight range we report is the kg band corresponding to BMI 18.5–25 at your specific height — handy if you want a target range rather than a single number.

What BMI measures, and what it does not

BMI is a weight-for-height ratio. It takes two measurements, divides one by the square of the other, and returns a single number. That is the whole of it: no other input is collected and none is inferred.

What it does not measure is body composition. The index cannot distinguish muscle from fat, cannot see where weight is carried, and cannot account for frame or bone density. Two people of the same height and weight receive the same BMI whatever the difference between them. It was built to screen populations, and the bands below are screening categories rather than findings about any individual.

The WHO classification bands

The scale below shows where the thresholds sit. A result of 27.3, for example, falls in the band labelled overweight, which is a statement about the number rather than about the person who entered it.

18.525303540

BMI band thresholds. The World Health Organization publishes the first three: under 18.5 is underweight, 18.5 to under 25 is normal weight, and 25 to under 30 is overweight, with obesity beginning at 30. The class divisions above 30 are conventional rather than WHO figures: 30 to under 35 is obese class I, 35 to under 40 is obese class II, and 40 and above is obese class III.

Under 18.5
Underweight
18.5 to under 25
Normal weight
25 to under 30
Overweight
30 to under 35
Obese, class I
35 to under 40
Obese, class II
40 and above
Obese, class III

Working it out by hand

Metric

Someone 175 cm tall weighing 70 kg:

  1. Convert height to metres: 175 cm is 1.75 m.
  2. Square it: 1.75 × 1.75 = 3.0625.
  3. Divide weight by that: 70 ÷ 3.0625 = 22.86, shown as 22.9.

At that height the band from 18.5 to 25 corresponds to roughly 56.7 kg to 76.6 kg, which is the healthy weight range the calculator reports.

Imperial

Someone 68 inches tall weighing 154 lb, using the imperial shortcut:

  1. Square the height in inches: 68 × 68 = 4,624.
  2. Multiply weight by the factor: 154 × 703 = 108,262.
  3. Divide: 108,262 ÷ 4,624 = 23.41, shown as 23.4.

The 703 is a unit conversion, not a constant of nature. One pound is 0.45359237 kg and one inch is 0.0254 m, so the exact factor is 0.45359237 divided by 0.0254 squared, which comes to 703.0696. Rounding it to 703 shifts the result by about two thousandths of a BMI point. This calculator does not use the shortcut at all: it converts imperial input to kilograms and metres first, then runs the metric formula, which is why its answer for the same person is 23.4154 before rounding rather than 23.4131.

Where BMI is a poor fit

  • Muscular builds. Muscle weighs more than fat for the same volume, so athletes and heavily trained people routinely produce numbers in the overweight or obese bands while carrying little fat. The index has no way to tell the two kinds of mass apart.
  • Older adults. Muscle mass tends to fall with age while fat mass rises, so a number that sits mid-band can describe quite different bodies at 25 and at 75.
  • Children and teenagers. Anyone under 18 is read against age- and sex-specific percentile charts instead of fixed thresholds. This calculator implements the adult scale only and does not support percentiles.
  • Pregnancy. Weight gain during pregnancy is expected and is tracked differently, so the adult bands are not a useful reading.
  • Population differences. The WHO describes BMI as a surrogate marker of fatness rather than a direct measure, and notes that measurements such as waist circumference add information it does not carry. Some countries apply lower cut-offs for their populations for that reason.

Common BMI misreadings

  • Reading a band as a verdict. The bands sort numbers, not people. Where a figure falls is the beginning of a question rather than the answer to one.
  • Treating a threshold as a cliff. A BMI of 24.9 and one of 25.1 differ by a rounding error in the inputs, not by anything meaningful. The thresholds are round numbers chosen for reporting convenience.
  • Chasing the number. BMI is an output of two measurements. It moves when they move, and it carries no information about how or why they moved.
  • Mixing up the formulas. Entering pounds and centimetres, or kilograms and inches, produces a number that means nothing. Set the unit toggle first, then enter both figures in the units it names.

Your height and weight stay in the page

The height and weight you enter are processed by this page in your browser. They are not sent to a server, not stored after you close the tab, and not visible to anyone else. There is no account and nothing to sign up for.

Sources

Neither cited page enumerates the obesity subdivisions this calculator reports. Splitting obesity into classes I, II and III at 35 and 40 is the standard clinical elaboration of the same scheme, and the calculator follows it, but the thresholds above 30 are not stated on the WHO pages linked here.

Estimate, not medical advice. BMI is a screening tool. It doesn't distinguish muscle from fat, doesn't apply to children, teens, or pregnant women, and isn't a diagnosis. If you have a medical condition, an eating disorder, or any nutrition concern, talk to a qualified healthcare professional rather than relying on a calculator.

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More tools from Health

TDEE CalculatorEstimate your Total Daily Energy Expenditure from sex, age, height, weight, and activity — with maintenance, cut, and bulk targets.BMR CalculatorEstimate your Basal Metabolic Rate — the calories your body burns at rest — using the Mifflin–St Jeor equation.Ideal Weight CalculatorEstimate a healthy weight range from height and sex using the Hamwi, Devine, Robinson, and Miller formulas — and the BMI healthy range.Body Fat CalculatorEstimate body fat percentage using the U.S. Navy circumference method, with the standard fitness category scale.

Frequently asked questions

Body Mass Index is a number computed from your height and weight: BMI = kg / m². It's used as a quick screening tool to flag whether someone's weight falls in a typical range for their height. It is not a diagnosis, and it doesn't measure body composition.

Careful about whose scale this is. WHO publishes the first three cuts and this page quotes them: "overweight is a BMI greater than or equal to 25; and obesity is a BMI greater than or equal to 30", with underweight below 18.5. The class divisions ABOVE 30 are not on the WHO pages this site links, and the Sources note has said so for a while; they are conventional and widely used rather than quoted here. So: under 18.5 underweight, 18.5–24.9 normal weight, 25–29.9 overweight, and then the conventional 30–34.9 obese class I, 35–39.9 obese class II, 40 or above obese class III. The same scale applies to adult men and women, though clinical interpretation varies by population.

Take your height in metres, square it, then multiply by 18.5 and 25 — that's the kg range corresponding to BMI 18.5–25. We compute that range for you in either pounds or kilograms. It's where most adults sit when their weight is described as “healthy” at their height.

BMI doesn't distinguish muscle from fat, doesn't account for frame size, and was originally derived from northern European populations. Athletes and very muscular people can land in the “overweight” or “obese” bands while having low body fat. Older adults can have a “normal” BMI with high body fat (sarcopenic obesity). Use BMI as one signal among several, not as a verdict.

No. BMI for children and teens uses age- and sex-specific percentile charts, not the adult scale. BMI is also not a useful screening tool during pregnancy. This calculator targets non-pregnant adults.

They are screening categories, not goals and not diagnoses. A BMI of 27.3 falls in the band the WHO labels overweight; that is a statement about where a weight-for-height ratio sits on a population scale, not a finding about the person it came from. Clinicians use the bands as one input alongside measurements the index cannot see.

Because muscle is denser than fat and the index cannot tell them apart. It divides total weight by height squared, so a heavily muscled person and a person carrying the same weight as fat produce the same number. That is why a rugby forward and a sedentary person of identical height and weight land in the same band despite very different body composition.

They are the same calculation in different units. Metric divides kilograms by height in metres squared. The imperial form multiplies pounds by 703 and divides by height in inches squared, where 703 is the rounded unit-conversion factor. This calculator converts imperial input to metric and runs the metric formula, so the two routes agree to within a rounding difference in the third decimal.

It converts pounds-per-square-inch into kilograms-per-square-metre. One pound is 0.45359237 kg and one inch is 0.0254 m, so the factor is 0.45359237 divided by 0.0254 squared, which is 703.0696. The value is conventionally rounded to 703, which is close enough that the displayed result to one decimal place is unaffected in ordinary cases.

BMI needs only height and weight and estimates nothing about composition. Body fat percentage estimates what proportion of body mass is fat, using measurements such as circumferences or a scan. They answer different questions, and it is entirely possible to sit in one BMI band while a composition measurement tells a different story.

No. This calculator implements the adult scale only. For anyone under 18, BMI is read against age- and sex-specific percentile charts rather than fixed thresholds, because normal body composition changes throughout growth. Entering a child&rsquo;s figures here would produce a number, but the band it lands in would not mean what the adult band means.

Treat it as one screening input rather than a conclusion. It is a weight-for-height ratio computed from two measurements, and it carries no information about your composition, health history, or circumstances. What the number means for a particular person is a question for a clinician or a registered dietitian who can see the rest of the picture.

The thresholds are fixed, but their usefulness varies. The WHO itself describes BMI as a surrogate marker of fatness and notes that additional measurements such as waist circumference add information the index does not carry. Research has also found that the relationship between BMI and body composition differs across populations, which is why some countries apply different cut-offs.

Yes. It is free to use with no account, no sign-up, and no limit on calculations. Everything runs inside this page in your browser.