Body Mass Index, or BMI, is one of the simplest health measures there is. Doctors use it as a first check of whether your weight is in a healthy range for your height. This guide explains how to calculate BMI by hand in metric and imperial units, how to read the chart, and what the number can and cannot tell you. For an instant answer, use our free BMI Calculator.
The BMI formula (metric)
BMI = weight in kg ÷ (height in metres)²
Example: weight 72 kg, height 170 cm (1.70 m).
- Square the height: 1.70 × 1.70 = 2.89
- Divide: 72 ÷ 2.89 = 24.9
The BMI formula (imperial)
BMI = 703 × weight in pounds ÷ (height in inches)²
Example: 160 lb, 5 ft 7 in (67 inches).
- 67 × 67 = 4,489
- 703 × 160 ÷ 4,489 = 25.1
If you know your weight in kilograms but your height in feet, convert first with Feet to CM Converter and KG to LBS Converter, or simply let the calculator handle mixed units.
Reading the BMI chart
| BMI | WHO category | Common cut-offs for South Asians |
|---|---|---|
| Below 18.5 | Underweight | Underweight |
| 18.5 to 24.9 | Normal | Normal up to 22.9 |
| 25 to 29.9 | Overweight | Overweight from 23 |
| 30 and above | Obese | Obese from 27.5 (some guidelines use 25) |
Research shows that people of South Asian origin tend to develop diabetes and heart disease at lower BMI levels, which is why many doctors in Pakistan and India use lower thresholds. Our BMI for South Asians article explains this in detail.
Your healthy weight range
You can also turn BMI around to find a healthy weight for your height:
Weight = BMI × height²
At 1.70 m, a BMI of 18.5 to 24.9 corresponds to about 53.5 kg to 72 kg. The calculator shows this range automatically.
What BMI does not measure
BMI does not distinguish fat from muscle, does not show where fat is stored and is not designed for children, pregnant women or very muscular athletes. Waist size is a useful second check: our BMI vs waist-to-height ratio guide explains how.
When to talk to a doctor
BMI is a screening tool, not a diagnosis. If your BMI is outside the healthy range, or if you have concerns about weight, blood pressure or blood sugar, a doctor can give proper advice. Keep a record of your weight with the date; the Date Difference Calculator helps you see how long changes took.
Frequently asked questions
What is the formula for BMI?
BMI = weight in kg divided by height in metres squared.
What is a normal BMI?
18.5 to 24.9 by WHO standards; many South Asian guidelines use 18.5 to 22.9.
Is BMI accurate for athletes?
Not always. Muscle weighs more than fat, so muscular people can have a high BMI without excess fat.
Can I use BMI for children?
Children need age- and sex-specific BMI charts, so ask a paediatrician.