BMI Calculator
Enter your height and weight in metric or imperial units to get your body mass index, the category it falls in, and the weight range that would put you in the healthy band.
- UnderweightBelow 18.5
- Healthy weight18.5 – 24.9
- Overweight25 – 29.9
- Obesity class I30 – 34.9
- Obesity class II35 – 39.9
- Obesity class III40 and above
This is not medical advice. Talk to a doctor before acting on any number here.
BMI is a rough screening ratio for adults. It takes no account of muscle mass, body composition, age, sex, ethnicity or pregnancy.
How to use this calculator
- Pick your units. Metric takes kilograms and centimetres; imperial takes pounds with feet and inches.
- Enter your weight and height. The result updates as you type — there is nothing to submit.
- Read the number and its band. The band is highlighted in the list of adult categories so you can see where you sit relative to the thresholds.
- Check the healthy weight range for your height, which is often more useful than the index itself.
The formula
BMI is weight divided by height squared, in metric units:
BMI = weight (kg) ÷ height (m)²
The imperial version folds the unit conversion into a single constant:
BMI = 703 × weight (lb) ÷ height (in)²
Rearranging the first formula gives the healthy weight range for any height:
lowest healthy weight = 18.5 × height (m)²
highest healthy weight = 24.9 × height (m)²
Because height is squared, BMI rises steeply as height falls. That is the main reason the index is criticised: it systematically reads high for tall people and low for short people compared with actual body fat.
Worked examples
Metric: 70 kg at 175 cm
- Height in metres: 175 ÷ 100 = 1.75, and 1.75² = 3.0625
- BMI: 70 ÷ 3.0625 = 22.9
- That sits in the healthy band
- Healthy range for this height: 56.7 kg to 76.3 kg
Imperial: 154 lb at 5 ft 9 in
- Height in inches: 5 × 12 + 9 = 69, and 69² = 4,761
- BMI: 703 × 154 ÷ 4,761 = 22.7
- Also healthy, and close to the metric example because it is nearly the same person
Why the same BMI can mean two things
Two people both measure 180 cm and 95 kg, giving a BMI of29.3 — the top of the overweight band.
- One is a rugby forward carrying a lot of muscle and about 12% body fat
- The other is sedentary with roughly twice that proportion of body fat
- The index cannot tell them apart, which is exactly why it is a starting point rather than a conclusion
Frequently asked questions
For adults, the World Health Organization and the CDC both use the same bands: below 18.5 is underweight, 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight, and 30 and above is obesity, split into three classes.
These cut-offs are population screening thresholds. They describe risk across large groups, not the health of any individual person.
No, and it was never meant to be. BMI is a ratio of weight to height, so it cannot tell muscle from fat. A muscular athlete often scores as overweight while carrying very little body fat, and someone with low muscle mass can score as healthy while carrying a lot.
It is also less reliable for older adults, for people who are very tall or very short, and it varies in meaning across ethnic groups — some health bodies use lower thresholds for people of South Asian descent.
Not this version. Children and teenagers are assessed with age-and-sex-specific percentile charts, because body composition changes a lot during growth. A raw BMI number means something quite different at 8 than at 38.
This calculator uses the adult bands, so treat it as applying to people aged 20 and over.
No. BMI is not a meaningful measure during pregnancy or while breastfeeding, because the expected weight change is part of a healthy pregnancy rather than a sign of anything.
Your midwife or doctor will track weight against pregnancy-specific guidance instead.
Waist circumference and waist-to-height ratio say more about where fat is carried, which matters more for health risk than total weight does. A common rule of thumb is keeping your waist under half your height.
Blood pressure, resting heart rate, blood sugar, cholesterol, activity levels and sleep all tell you far more than a single ratio can.
One kilogram is about 2.2046 pounds, and one inch is exactly 2.54 centimetres. So 154 pounds is roughly 70 kg, and 5 feet 9 inches is 69 inches, which is about 175 cm.
The imperial BMI formula folds the conversion into a constant: 703 × pounds ÷ inches². This calculator converts to metric first and uses the standard kg/m² formula, which gives the same answer.