BMI Calculator

WHO BMI Classification

The World Health Organization (WHO) defines the following BMI categories for adults over 20 years old. These ranges apply regardless of age or sex.

Health Risks by BMI Category

How BMI Is Calculated

BMI is calculated by dividing your weight by the square of your height. The formula differs slightly between metric and imperial units:

Worked Example: Two People, Two Numbers

Ella stands 165 cm and weighs 61 kg. Dan stands 183 cm and weighs 98 kg. Here is the division written out for both, with nothing rounded until the last step.

Ella sits in the middle of the normal band. Dan sits 0.7 points under the obesity line, and the calculator above will tell him he is 14.6 kg over a BMI of 24.9. What the arithmetic cannot tell him is which 14.6 kg. At 98 kg and 14% body fat he carries roughly 84 kg of lean tissue; at 98 kg and 32% body fat, roughly 67 kg. The formula returns 29.3 in both cases, because the only instruments it consulted were a scale and a tape measure.

Run the boundaries backwards and the healthy range comes out at 62.0–83.4 kg for Dan and 50.4–67.8 kg for Ella. That is a 21 kg window at Dan's height and a 17 kg window at Ella's — a screening band several clothing sizes wide, which is about as fine as this measure ever claimed to be.

BMI by Height and Weight

Find your height across the top and your weight down the side. Every cell uses the same division the calculator runs, so your result should match the nearest cell within a rounding step. If it doesn't, check which units you typed.

Read the 80 kg row across and the categories drift under your eyes: obese at 155 cm, overweight at 175 cm, normal at 185 cm. Ten kilograms is worth 3.5 BMI points at 170 cm and 2.6 points at 195 cm, so tall people cross fewer boundaries for the same weight change. The squared term in the denominator is doing that, and it is also why BMI is known to run high in tall people and low in short ones.

The same grid read the other way gives the weight at which each boundary falls for your height.

The BMI 23 column is there for a reason. A WHO expert consultation published in The Lancet in 2004 reviewed Asian population data, kept 25 and 30 as the international classification, and identified 23 and 27.5 as extra public-health action points, because body fat percentage and type 2 diabetes risk run higher at a given BMI in many Asian populations. Several national guidelines have since used 23 as their overweight threshold. At 165 cm the two thresholds are 5.5 kg apart.

BMI Prime & Ponderal Index

BMI Prime is the ratio of your BMI to the upper limit of normal (25). A BMI Prime of 1.0 means you are exactly at the boundary. Values below 1.0 are in the normal or underweight range; above 1.0 means overweight or obese.

Ponderal Index (also called Corpulence Index) uses height cubed instead of squared, which makes it more accurate for very tall or very short individuals. Normal range is 11–15 kg/m³. Unlike BMI, the Ponderal Index does not systematically overestimate body fat in tall people or underestimate it in short people.

Where BMI Misleads

BMI reads two numbers off a person and has no term for anything else. Five groups get a systematically wrong answer from it, and knowing which one you belong to is worth more than the result itself.

• Muscular builds. Muscle tissue is denser than fat, roughly 1.06 against 0.9 g/cm³, so a body that trades fat for muscle at a fixed weight keeps its BMI and loses girth. Dan's 29.3 above holds whether his lean mass is 84 kg or 67 kg. A rugby forward and a sedentary man of the same height and weight are identical to the formula, which is why body recomposition can run for months without the scale or the category moving.
• Older adults. Muscle mass falls from the thirties onward, so an unchanged BMI at 70 usually hides more fat than the same BMI did at 30. Winter and colleagues (American Journal of Clinical Nutrition, 2014) pooled 32 studies of adults aged 65 and over and found mortality risk climbing below a BMI of about 23, with no clear excess between 25 and 30. Chasing a 27 down to a 23 at that age can cost the muscle needed to stay steady on the stairs.
• Asian and South Asian populations. The 2004 WHO consultation put its action points at 23 and 27.5. A BMI of 24 reads as normal on the international scale and as overweight on the Asian one, from the same body on the same morning. Which scale your clinician uses depends on where you live and which guideline they follow.
• Pregnancy. BMI has no meaning while pregnant, because the weight on the scale includes a fetus, a placenta, amniotic fluid and expanded blood volume. Antenatal care uses pre-pregnancy BMI instead: the Institute of Medicine's 2009 guidance ties a recommended gestational weight-gain range to the BMI a woman carried before conception, and that is the only role the number plays here.
• Children and teenagers. The adult table does not apply under 20. A BMI of 19 is ordinary for a 17-year-old and high for a 6-year-old, because body composition changes as children grow. Paediatricians read children against BMI-for-age percentiles from the CDC 2000 growth charts or the WHO charts, where the 85th to 94th percentile means overweight and the 95th and above means obesity. Checking a child against the adult categories produces an answer that is wrong more often than right.

What to Measure Beside It

BMI is a screen, and a screen exists to point you at the next measurement. None of the measures below needs equipment beyond a tape, and each one answers a question BMI cannot.

• Waist-to-height ratio. Waist circumference divided by height, with 0.5 as the line. NICE recommended it in its 2022 obesity guideline update as an addition to BMI for adults with a BMI under 35. Dan is 183 cm, so his waist wants to stay under 91.5 cm. One division, and it sees the central fat BMI steps over.
• Waist circumference alone. The NIH/NHLBI clinical guidelines (1998) flag raised risk above 102 cm in men and 88 cm in women, measured at the top of the hip bone after a normal breath out. Pair it with a hip measurement in the waist-to-hip ratio calculator.
• Body fat percentage. This is the thing BMI stands in for. The body fat calculator estimates it from tape measurements using the US Navy method. Consumer bioimpedance scales disagree with DEXA by several percentage points, so read any single figure as a band rather than a value.
• The direction your weight is moving. A weekly average drifting 0.4 kg down says more than one BMI reading ever will, and daily weight swings a kilogram or two on water, sodium and gut contents. Why the scale lies from one morning to the next covers the noise you have to average out.

If what you actually wanted was a goal weight rather than a category, the ideal weight calculator runs the Devine, Hamwi, Robinson and Miller formulas side by side so you can see how far four accepted methods disagree about the same body. And if the plan is to change the number, the BMR calculator is where the energy arithmetic starts.

BMI Questions People Actually Ask

• What counts as a healthy BMI?
  WHO puts the normal band at 18.5 to 24.9 for adults over 20, with no adjustment for age or sex. At 165 cm that band runs from 50.4 kg to 67.8 kg, and at 185 cm from 63.3 kg to 85.6 kg.
• Is BMI accurate if I lift weights?
  Not usually. Muscle is denser than fat, so a trained body reads high. A man at 98 kg and 183 cm scores 29.3 whether he carries 84 kg of lean tissue or 67 kg of it. Check body fat percentage or waist-to-height ratio before treating that 29.3 as a finding.
• Why do Asian BMI thresholds start at 23?
  A WHO expert consultation published in The Lancet in 2004 found body fat percentage and type 2 diabetes risk running higher at a given BMI in many Asian populations. It kept 25 and 30 as the international classification and identified 23 and 27.5 as extra action points. At 165 cm those two thresholds are 5.5 kg apart.
• Can I use BMI while pregnant?
  No. The weight includes a fetus, a placenta, amniotic fluid and expanded blood volume, so the ratio measures nothing useful. Antenatal guidance uses pre-pregnancy BMI, which is what the Institute of Medicine's 2009 gestational weight-gain ranges rest on.
• Does BMI work for children?
  Not with the adult table. Paediatricians use BMI-for-age percentiles from the CDC 2000 or WHO growth charts, where the 85th to 94th percentile means overweight and the 95th and above means obesity. A BMI of 19 is ordinary at 17 and high at 6.
• Does BMI account for age?
  The formula has no age term at all. That matters most after 65, where Winter and colleagues (American Journal of Clinical Nutrition, 2014) pooled 32 studies and found mortality risk rising below a BMI of about 23 rather than above 25.
• How much weight changes my BMI category?
  It depends on height. Ten kilograms is worth 3.5 BMI points at 170 cm and 2.6 points at 195 cm. Moving from a BMI of 30 to a BMI of 25 means 13.6 kg at 165 cm and 18.1 kg at 190 cm.
• Is waist-to-height ratio better than BMI?
  It answers a different question, and it catches the fat BMI cannot see. NICE's 2022 update recommends using it alongside BMI for adults under a BMI of 35, with 0.5 as the boundary, so a 170 cm adult aims to keep the waist under 85 cm.