# BMI calculator

> Body mass index (BMI) from height and weight in metric or imperial units, with the WHO adult category and the healthy weight range for your height.

Interactive version: https://www.calcopenly.com/health/bmi-calculator
Subject: Health and fitness calculators

Body mass index is weight in kilograms divided by the square of height in metres. Pounds, stones, feet and inches are converted first. The result is placed in the WHO adult bands: under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, and obesity classes I, II and III from 30, 35 and 40. The healthy weight range is the span of weights that keeps BMI between 18.5 and 25 at your height.

BMI is used as a first screen for weight-related health risk and in population surveys. At the default 70 kg and 175 cm, BMI is 70 ÷ 1.75² = 22.9, and the healthy range for that height runs from 56.7 to 76.6 kg.

The bands are population cut-offs linked to average disease risk, not a diagnosis. BMI cannot tell muscle from fat, so muscular people can read high. It does not apply to pregnancy or to children, whose BMI is read against age- and sex-specific percentiles.

## How to calculate BMI by hand

In metric units, divide weight in kilograms by height in metres, squared. Someone who weighs 82 kg and is 1.80 m tall has a BMI of 82 ÷ (1.80 × 1.80) = 82 ÷ 3.24 = 25.3.

In pounds and inches, multiply weight by 703 and divide by height in inches, squared. The 703 is a unit conversion: one pound per square inch of height equals 0.45359237 ÷ 0.0254² = 703.07 kg/m². For 140 lb at 5 ft 6 in (66 in), BMI is 703 × 140 ÷ 4,356 = 22.6.

Three habits keep hand calculations honest:

- Square the height, not the weight. Forgetting the square gives a number around 45 for most adults.
- Convert feet and inches to inches before squaring: 5 ft 6 in is 66 in, not 5.6 ft.
- Round only at the end. The calculator keeps 50 digits and shows one decimal, rounded half-up.

## BMI categories for adults

WHO uses the same cut-offs for men and women aged 20 and over. The CDC adult categories match them.

| BMI (kg/m²) | WHO classification |
|---|---|
| Below 16.0 | Underweight: severe thinness |
| 16.0 to 16.9 | Underweight: moderate thinness |
| 17.0 to 18.4 | Underweight: mild thinness |
| 18.5 to 24.9 | Normal (healthy) weight |
| 25.0 to 29.9 | Pre-obesity (overweight) |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |

A result that displays as 25.0 can come from a BMI of 24.96, which is still in the healthy band. The calculator places you in a band using the unrounded value, so the category shown is the one the full-precision number belongs to.

## Healthy weight for your height

Multiplying 18.5 and 25 by your height in metres, squared, gives the band of weights that keeps BMI in the normal range. The table below is computed that way. Heights in feet and inches are rounded to the nearest inch.

| Height | Healthy weight (kg) | Healthy weight (lb) |
|---|---|---|
| 150 cm (4 ft 11 in) | 41.6 to 56.3 | 92 to 124 |
| 155 cm (5 ft 1 in) | 44.4 to 60.1 | 98 to 132 |
| 160 cm (5 ft 3 in) | 47.4 to 64.0 | 104 to 141 |
| 165 cm (5 ft 5 in) | 50.4 to 68.1 | 111 to 150 |
| 170 cm (5 ft 7 in) | 53.5 to 72.3 | 118 to 159 |
| 175 cm (5 ft 9 in) | 56.7 to 76.6 | 125 to 169 |
| 180 cm (5 ft 11 in) | 59.9 to 81.0 | 132 to 179 |
| 185 cm (6 ft 1 in) | 63.3 to 85.6 | 140 to 189 |
| 190 cm (6 ft 3 in) | 66.8 to 90.3 | 147 to 199 |
| 195 cm (6 ft 5 in) | 70.3 to 95.1 | 155 to 210 |

The band is wide on purpose: at 175 cm it spans 20 kg. BMI says nothing about where in that band a given person is healthiest. For a single target figure, the [ideal weight calculator](/health/ideal-weight-calculator) compares the Devine, Robinson, Miller and Hamwi formulas.

## What BMI does and does not tell you

BMI tracks body fat reasonably well across large groups, which is why it is used in national health surveys and clinical screening. Across 239 prospective studies covering 10.6 million adults, the Global BMI Mortality Collaboration ([Lancet, 2016](https://doi.org/10.1016/S0140-6736(16)30175-1)) found all-cause mortality was lowest at a BMI of 20 to 25 and rose steadily above it.

For one person, BMI misses three things:

- **Composition.** Muscle is denser than fat, so a rugby player at 100 kg and 1.85 m has a BMI of 29.2 with little excess fat. A [body fat calculator](/health/body-fat-calculator) using the US Navy tape method separates the two.
- **Distribution.** Fat around the waist carries more metabolic risk than fat on the hips. NICE advises adults to keep their waist measurement below half their height; a waist-to-height ratio of 0.5 or more signals increased risk even at a normal BMI.
- **Age and sex.** Women carry more body fat than men at the same BMI, and older adults lose muscle, so the same number can mean different things.

BMI is not used for children and teenagers against these fixed cut-offs, and it is not a measure of health during pregnancy.

## BMI prime and the ponderal index

The calculator also shows two related indices.

**BMI prime** is your BMI divided by 25, the upper limit of the normal range. A value of 1.00 sits exactly on that limit; 0.92 is 8% below it and 1.20 is 20% above it. It turns the result into a fraction of the threshold, which is easier to track over time than the raw number.

**The ponderal index** divides weight by height cubed instead of squared (kg/m³). Because body volume grows roughly with the cube of height, it reads less high for very tall people and less low for very short ones than BMI does. It is mostly used for newborns and in research, and has no widely adopted adult cut-offs.

## Where BMI comes from

The Belgian statistician Adolphe Quetelet noticed in the 1830s that adult weight rises roughly with the square of height, and proposed weight ÷ height² as a way to describe the "average man". Ancel Keys and colleagues compared several weight-for-height indices in 1972 and found this one correlated best with measured body fat. They named it the body mass index. WHO adopted the current cut-offs in its 1995 and 2000 technical reports on obesity.

## Next steps after checking your BMI

- To estimate how many calories your body uses each day, use the [BMR and TDEE calculator](/health/bmr-tdee-calculator).
- To set a daily calorie and protein target for weight change, use the [calorie and macro planner](/health/calorie-macro-calculator).
- To check the result against a measurement of fat rather than weight, use the [body fat calculator](/health/body-fat-calculator).

A BMI outside the normal range is a reason to talk to a doctor, who can weigh it against blood pressure, blood tests, waist size and family history.

## Inputs

- **Weight**
- **Height**: Accepts 5'11" or 5 ft 11 in

## Results

- BMI (kg/m²) — main result
- Category (WHO adult)
- Healthy weight from (kg)
- Healthy weight to (kg)
- BMI prime
- Ponderal index (kg/m³)

## Formula

$$
\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}
$$

## Worked examples

### 70 kg, 175 cm

- Weight: 70 kg
- Height: 175 cm
- **BMI: 22.9 kg/m²**
- **Category (WHO adult): Healthy**
- Checked against: 70 / 1.75² = 22.857 (hand calculation; CDC calculator)

### 200 lb, 6 ft

- Weight: 200 lb
- Height: 6 ft
- **BMI: 27.1 kg/m²**
- **Category (WHO adult): Overweight**
- Checked against: 703 × 200 / 72² = 27.12 (CDC imperial formula)

### 50 kg, 1.7 m

- Weight: 50 kg
- Height: 1.7 m
- **BMI: 17.3 kg/m²**
- **Category (WHO adult): Underweight**
- Checked against: 50 / 2.89 = 17.30

## Questions

### What is a healthy BMI for an adult?

WHO and CDC class an adult BMI of 18.5 to 24.9 as healthy (normal), 25 to 29.9 as overweight and 30 or more as obesity, with class III obesity from 40. These are population cut-offs tied to average risk of conditions such as type 2 diabetes and heart disease, not a verdict on one person's health. For someone 175 cm tall, the healthy band means 56.7 to 76.6 kg.

### How do you calculate BMI in pounds and inches?

Multiply weight in pounds by 703 and divide by height in inches squared. The factor converts lb/in² to kg/m²: 0.45359237 ÷ 0.0254² is 703.07. Someone weighing 200 lb at 6 ft (72 in) has 703 × 200 ÷ 5,184 = 27.1, which falls in the overweight band. CDC gives the same formula for its adult BMI calculator.

### Is BMI accurate for athletes and older adults?

Not always, because BMI measures weight for height and cannot separate fat from muscle and bone. A muscular athlete can exceed 25 with little body fat, and an older adult who has lost muscle can carry excess fat below 25. CDC describes BMI as a screening measure, not a diagnosis. Waist size adds information: WHO links a waist above 94 cm in men and 80 cm in women to increased risk of metabolic complications.

### Are BMI cut-offs different for Asian populations?

The international WHO bands apply to all adults, but a 2004 WHO expert consultation (Lancet 363:157–163) found that many Asian populations have higher risk of type 2 diabetes and cardiovascular disease at lower BMI. It kept the standard cut-offs and added public-health action points at 23, 27.5, 32.5 and 37.5. Some national guidelines use 23 as the start of raised risk. This calculator reports the international bands.

### How is BMI assessed in children and teenagers?

For ages 2 to 19, CDC compares BMI with growth charts for the same age and sex instead of fixed cut-offs. Below the 5th percentile is underweight, the 85th to below the 95th percentile is overweight, and the 95th percentile or above is obesity. The adult bands in this calculator should not be applied to children, because normal body fat changes with age and differs between boys and girls.

### How accurate is the BMI calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 3 worked examples whose answers come from independent sources; for example, “70 kg, 175 cm” is checked against 70 / 1.75² = 22.857 (hand calculation; CDC calculator).

### Where does the method come from?

WHO — A healthy lifestyle: body mass index; CDC — Adult BMI categories.

## Sources

- [WHO — A healthy lifestyle: body mass index](https://www.who.int/europe/news-room/fact-sheets/item/a-healthy-lifestyle---who-recommendations)
- [CDC — Adult BMI categories](https://www.cdc.gov/bmi/adult-calculator/bmi-categories.html)

_Note: medical information, not professional advice._