What is BMI?
BMI, or body mass index, is a single number that expresses your weight relative to your height. It was designed in the 19th century as a way to describe populations, and it is still used today because it needs only two measurements that anyone can take at home: weight and height.
The result is expressed in kg/m² — kilograms per square metre. A BMI of 22 means you carry 22 kilograms of body mass for every square metre of height. Health bodies group those numbers into categories that correlate, on average, with different levels of health risk.
The BMI formula
There are two versions of the same calculation. Pick whichever matches the units you already use — both produce an identical result.
Metric formula (kilograms and metres)
Weight in kilograms, height in metres — not centimetres. Divide centimetres by 100 first.
Imperial formula (pounds and inches)
The 703 factor converts pounds-per-square-inch onto the same kg/m² scale, so the categories still apply.
Worked example — metric
Let's calculate BMI for someone who weighs 70 kilograms and stands 175 centimetres tall.
70 kg, 175 cm
An adult weighing 70 kg with a height of 175 cm (1.75 m).
- 1
Convert height to metres
175 cm ÷ 100 = 1.75 m
- 2
Square the height
1.75 × 1.75 = 3.0625 m²
- 3
Divide weight by height squared
70 ÷ 3.0625 = 22.86
- 4
Round to one decimal place
BMI ≈ 22.9 kg/m²
Result
BMI 22.9 — within the healthy weight range (18.5 – 24.9)
Worked example — imperial
Now the same calculation for someone weighing 154 pounds at 5 feet 9 inches tall. Note that height must be converted entirely into inches before you square it.
154 lb, 5 ft 9 in
An adult weighing 154 lb with a height of 5 ft 9 in.
- 1
Convert height to inches
(5 × 12) + 9 = 69 in
- 2
Square the height
69 × 69 = 4,761 in²
- 3
Divide weight by height squared
154 ÷ 4,761 = 0.032346
- 4
Multiply by the 703 conversion factor
0.032346 × 703 = 22.74
Result
BMI 22.7 — within the healthy weight range (18.5 – 24.9)
BMI chart and categories
Once you have your number, compare it against the standard adult categories used by the World Health Organization and most national health services.
| Category | BMI range (kg/m²) | What it generally indicates |
|---|---|---|
| Underweight | Below 18.5 | May indicate insufficient energy intake |
| Healthy weight | 18.5 – 24.9 | Associated with the lowest average health risk |
| Overweight | 25.0 – 29.9 | Slightly raised risk of metabolic conditions |
| Obesity class I | 30.0 – 34.9 | Moderately raised risk |
| Obesity class II | 35.0 – 39.9 | High risk |
| Obesity class III | 40.0 and above | Very high risk |
These bands are population averages. Some health authorities apply lower thresholds for people of South Asian, Chinese, and other Asian descent — often treating 23.0 as the start of the overweight band — because metabolic risk tends to rise at a lower BMI in these groups.
BMI for children and teenagers
The arithmetic is identical for under-20s, but the interpretation is not. Children's body composition changes constantly as they grow, so a raw BMI number is meaningless on its own.
Instead, a child's BMI is plotted on an age-and-sex-specific growth chart and converted into a percentile — a comparison against other children of the same age and sex.
| Category | Percentile range |
|---|---|
| Underweight | Below the 5th percentile |
| Healthy weight | 5th to below the 85th percentile |
| Overweight | 85th to below the 95th percentile |
| Obesity | 95th percentile and above |
Where BMI falls short
BMI is popular because it's cheap and fast, not because it's precise. Knowing its blind spots stops you from over-reacting to the number.
- It can't tell muscle from fat.Muscle is denser than fat, so athletes and regular lifters frequently land in the “overweight” band while carrying low body fat.
- It ignores fat distribution. Fat carried around the abdomen carries more metabolic risk than fat on the hips and thighs, yet both produce the same BMI.
- It shifts with age. Older adults tend to lose muscle and bone density, so a stable BMI can mask a real change in body composition.
- It doesn't adjust for frame size or ethnicity. Risk thresholds genuinely differ between populations, and a single global cut-off can't reflect that.
- It says nothing about behaviour. Two people with an identical BMI can have completely different diets, fitness levels, blood pressure, and blood sugar.
Pair BMI with a waist measurement, a sense of your body-fat percentage, and — most usefully — a record of what you actually eat. That combination tells a far more complete story than any one figure.
What to do with your BMI
BMI describes where you are now. To change it, you need to know how much energy your body uses — and that's where the other two numbers come in.
- Calculate your BMR to find the calories your body burns at complete rest. See the BMR guide →
- Multiply that by an activity factor to get your TDEE, the total calories you burn in a day. See the TDEE guide →
- Apply a modest deficit or surplus to your TDEE depending on whether you want to lose, maintain, or gain weight.
- Track what you actually eat, because an estimated calorie target only works if you know what you're consuming against it.
Frequently asked questions
How do I calculate my BMI manually?+
Divide your weight in kilograms by your height in metres squared. For example, 70 kg ÷ (1.75 m × 1.75 m) = 70 ÷ 3.0625 = 22.9. If you use pounds and inches, multiply the result of weight ÷ height² by 703 instead.
What is the BMI formula in pounds and inches?+
BMI = (weight in pounds ÷ height in inches²) × 703. The 703 factor converts the imperial units into the metric kg/m² scale, so both formulas give the same number for the same person.
What is a healthy BMI range?+
For most adults, a BMI between 18.5 and 24.9 is classed as a healthy weight. Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30.0 or above falls into the obesity categories.
Is BMI accurate for muscular people?+
Not always. BMI uses only height and weight, so it cannot distinguish muscle from fat. A muscular athlete can register as overweight or obese while carrying very little body fat. Body composition, waist circumference, and how you feel day to day are more informative in that case.
Does BMI differ for men and women?+
The formula and the adult category thresholds are the same for men and women. Women generally carry a higher proportion of body fat at the same BMI, which is one reason BMI is treated as a population screening tool rather than an individual diagnosis.
How is BMI different from BMR and TDEE?+
BMI describes your weight relative to your height. BMR is the number of calories your body burns at complete rest, and TDEE is your total daily calorie burn including activity. BMI tells you where you are; BMR and TDEE tell you how much to eat to change it.
Should I use BMI to set a calorie goal?+
Use BMI as a starting reference point, not as a calorie target. To decide how much to eat, calculate your BMR, multiply it by an activity factor to get your TDEE, then apply a modest deficit or surplus depending on your goal.
What is a normal BMI for my age?+
For adults aged 20 and over, the healthy range is 18.5 to 24.9 regardless of age — the categories do not shift as you get older. Under-20s are different: their BMI is plotted on age-and-sex-specific growth charts and read as a percentile rather than compared to the adult bands.
How do I calculate BMI in kg and cm?+
Convert your height from centimetres to metres by dividing by 100, then divide your weight in kilograms by that figure squared. For 70 kg at 175 cm: 175 ÷ 100 = 1.75 m, then 1.75 × 1.75 = 3.0625, and finally 70 ÷ 3.0625 = 22.9. Using centimetres directly is the most common mistake and gives a nonsensical result.
Why is my BMI high when I am not overweight?+
BMI uses only height and weight, so it cannot separate muscle from fat. If you carry significant muscle from strength training or sport, you can land in the overweight or even obese band while having low body fat. Waist circumference and a body composition measurement give a far more useful picture in that situation.
Is BMI different for Asian populations?+
Several health authorities apply lower thresholds for people of South Asian, Chinese, and other Asian descent, often treating 23.0 rather than 25.0 as the start of the overweight band. Metabolic risk tends to rise at a lower BMI in these groups, so the standard global cut-offs can understate risk.
How often should I check my BMI?+
Every few months is plenty. BMI moves slowly and daily weight fluctuates by a kilogram or more from water, food, and salt, so frequent checks mostly measure noise. Tracking a weekly average weight is more informative than recalculating BMI often.
Keep learning
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