Body mass index is weight divided by height squared. It takes ten seconds to work out and nurses record it on almost every admission, because it feeds nutrition screening, dosing-weight decisions and equipment planning. This calculator gives the adult WHO category and the weight range that would put the same person between 18.5 and 24.9. It is a screening number. It does not measure body fat, and it says nothing about where fat sits.
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BMI = weight (kg) ÷ height (m)². In US units, BMI = 703 × weight (lb) ÷ height (in)². A person who weighs 70 kg and is 1.70 m tall has a BMI of 70 ÷ 2.89 = 24.2 kg/m².
The 703 in the US version is just the unit conversion folded into one number, so both versions give the same answer for the same person. Square the height, not the weight, and convert centimetres to metres first. Entering 170 instead of 1.70 is the classic slip, and it gives a BMI of 0.0024, which at least is obviously wrong.
| Category | BMI (kg/m²) |
|---|---|
| Severe underweight (thinness) | below 16.0 |
| Underweight | 16.0–18.4 |
| Normal weight | 18.5–24.9 |
| Overweight (pre-obese) | 25.0–29.9 |
| Obese class I | 30.0–34.9 |
| Obese class II | 35.0–39.9 |
| Obese class III | 40.0 or higher |
WHO subdivides underweight further (16.0–16.9 moderate, 17.0–18.4 mild); most charts, and this calculator, group those together. "Morbid obesity" is an older label for class III that many services have dropped.
A WHO expert consultation in 2004 found that many Asian populations carry a higher risk of type 2 diabetes and cardiovascular disease at BMIs below 25. It kept the international categories but proposed extra public health action points along the scale. Several countries use them as their own cut-offs, and NICE in the UK recommends lower thresholds for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background.
| Action point (kg/m²) | Commonly used to mark |
|---|---|
| 23.0 | Increased risk (overweight in several Asian national guidelines) |
| 27.5 | High risk (obesity in several Asian national guidelines) |
| 32.5 | Further risk step |
| 37.5 | Further risk step |
| Patient | Working | BMI | WHO category |
|---|---|---|---|
| 70 kg, 170 cm | 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 | 24.2 | Normal weight |
| 154 lb, 5 ft 7 in (67 in) | 703 × 154 ÷ (67 × 67) = 108,262 ÷ 4,489 | 24.1 | Normal weight |
| 95 kg, 165 cm | 95 ÷ (1.65 × 1.65) = 95 ÷ 2.7225 | 34.9 | Obese class I |
| 48 kg, 163 cm | 48 ÷ (1.63 × 1.63) = 48 ÷ 2.6569 | 18.1 | Underweight |
| 120 kg, 180 cm | 120 ÷ (1.80 × 1.80) = 120 ÷ 3.24 | 37.0 | Obese class II |
Round at the end, not in the middle. Rounding 1.65² to 2.7 instead of 2.7225 moves the third example from 34.9 to 35.2, which crosses a category line on paper even though nothing about the patient changed.
Nutrition screening. BMI is step one of the Malnutrition Universal Screening Tool (MUST) used in UK hospitals and care homes: above 20 scores 0, 18.5–20 scores 1, and below 18.5 scores 2, before weight loss and acute illness are added. Many US screening tools ask for the same measurement. If the patient cannot stand, MUST allows height from ulna length or a recalled height, and the method used should be documented.
Dosing weight. For most drugs the dose follows actual body weight. For some, including aminoglycosides and several anaesthetic and critical care drugs, a high BMI is the trigger to use ideal body weight or an adjusted body weight instead. The drug reference or pharmacy protocol decides which, not the BMI on its own.
Equipment and safety. A BMI of 40 or more often triggers a bariatric care plan: bed and chair weight limits, larger blood pressure cuffs, moving and handling assessment. Getting the right cuff matters. A cuff that is too small reads high.
BMI cannot tell muscle from fat, so muscular patients score high and frail older patients who have lost muscle can score normal while being undernourished. Fluid counts as weight: oedema, ascites and fluid overload all push BMI up, which is why a daily weight in heart failure is tracked as a trend and not as a BMI. An amputation lowers weight without changing the frame, and pregnancy changes everything, so neither fits the standard table.
For children and teenagers aged 2 to 19, adult categories do not apply. BMI is plotted on a sex-specific BMI-for-age chart instead. The CDC categories are underweight below the 5th percentile, healthy weight from the 5th to below the 85th, overweight from the 85th to below the 95th, obesity at the 95th percentile or above, and severe obesity at 120% of the 95th percentile or above.
The WHO normal range for adults is 18.5 to 24.9 kg/m². Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30.0 or above is obesity, split into class I (30–34.9), class II (35–39.9) and class III (40 or higher).
Multiply weight in pounds by 703, then divide by height in inches squared. For 154 lb and 5 ft 7 in (67 in): 703 × 154 = 108,262, and 67 × 67 = 4,489, so BMI = 108,262 ÷ 4,489 = 24.1.
The WHO international categories are the same, but a 2004 WHO expert consultation found that health risk in many Asian populations rises at lower BMIs and proposed extra action points at 23, 27.5, 32.5 and 37.5. Several Asian countries use 23 as the overweight threshold and 27.5 or similar for obesity.
Not on its own. BMI cannot separate muscle from fat, so muscular people can score as overweight, and older adults who have lost muscle can score normal while being undernourished. Fluid retention also raises BMI. It works as a screening number alongside a proper assessment.
No. For ages 2 to 19, BMI is plotted on a sex-specific BMI-for-age growth chart. The CDC calls the 85th to below the 95th percentile overweight and the 95th percentile or above obesity.