BMI is weight in kilograms divided by height in metres squared: 70 kg at 1.75 m is 70 ÷ 3.0625 = 22.9. In imperial units, divide weight in pounds by height in inches squared and multiply by 703. For adults, 18.5 to 24.9 is the healthy-weight range under WHO and CDC categories.
BMI is a screening measure, not a diagnosis. It uses only height and weight, so it cannot separate fat from muscle: a 90 kg, 1.80 m rugby player and a 90 kg, 1.80 m desk worker both score 27.8 and land in the overweight band. WHO applies the same adult thresholds worldwide but suggests lower action points of 23 and 27.5 for many Asian populations. The sections below explain where the number is useful and where it misleads, and the TDEE calculator covers the next question most people ask: how many calories that body burns in a day.
BMI Calculator
Checked against: 70 kg at 1.75 m = 22.9 (healthy weight); 154 lb at 5 ft 9 in = 22.7 Last reviewed:
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Sources and method
- Obesity and overweight - World Health Organization
- Adult BMI Calculator - US Centers for Disease Control and Prevention
- Calculate Your Body Mass Index - NIH National Heart, Lung, and Blood Institute
A screening estimate for general information, not a diagnosis or medical advice. Talk to a doctor or registered dietitian before acting on it.
The formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = (weight (lb) ÷ height (in)²) × 703
The 703 in the imperial version is simply the conversion factor that makes pounds and inches produce the same answer as kilograms and metres. Both routes give an identical result, so use whichever units you know your own measurements in.
The one step people get wrong is squaring the height. Height goes in as metres, not centimetres: 175 cm is 1.75 m, and 1.75² is 3.0625. Dividing by 175² instead produces a number ten thousand times too small.
BMI categories for adults
These are the World Health Organization classifications for adults. WHO applies them from age 18; the US CDC uses the same bands from age 20 and percentile charts below that.
| BMI | WHO category | Weight at 1.65 m | Weight at 1.80 m |
|---|---|---|---|
| Below 16.0 | Severe thinness | under 43.6 kg | under 51.8 kg |
| 16.0 to 16.9 | Moderate thinness | 43.6–46.0 kg | 51.8–54.8 kg |
| 17.0 to 18.4 | Mild thinness | 46.3–50.1 kg | 55.1–59.6 kg |
| 18.5 to 24.9 | Healthy weight | 50.4–67.8 kg | 59.9–80.7 kg |
| 25.0 to 29.9 | Overweight | 68.1–81.4 kg | 81.0–96.9 kg |
| 30.0 to 34.9 | Obese, class I | 81.7–95.0 kg | 97.2–113.1 kg |
| 35.0 to 39.9 | Obese, class II | 95.3–108.6 kg | 113.4–129.3 kg |
| 40.0 and above | Obese, class III | 108.9 kg and up | 129.6 kg and up |
The boundaries are conventions drawn from population health data, not biological cliffs. A BMI of 24.9 and a BMI of 25.1 describe nearly identical bodies, even though they sit in different rows of the table.
Worked examples
Metric
Someone weighing 70 kg at 1.75 m: 1.75² = 3.0625, and 70 ÷ 3.0625 = 22.9. That falls in the healthy weight range.
Imperial
Someone weighing 154 lb at 5 ft 9 in. First convert the height to inches: (5 × 12) + 9 = 69 in. Then 69² = 4,761, and (154 ÷ 4,761) × 703 = 22.7. The same person, the same category.
Working backwards to a weight range
To find the healthy weight range for a height, multiply the height in metres squared by 18.5 and by 24.9. At 1.75 m that is 3.0625 × 18.5 = 56.7 kg at the lower end and 3.0625 × 24.9 = 76.3 kg at the upper end.
| Height | Healthy range (kg) | Healthy range (lb) |
|---|---|---|
| 150 cm (4 ft 11 in) | 41.6–56.0 kg | 92–124 lb |
| 155 cm (5 ft 1 in) | 44.4–59.8 kg | 98–132 lb |
| 160 cm (5 ft 3 in) | 47.4–63.7 kg | 104–141 lb |
| 165 cm (5 ft 5 in) | 50.4–67.8 kg | 111–149 lb |
| 170 cm (5 ft 7 in) | 53.5–72.0 kg | 118–159 lb |
| 175 cm (5 ft 9 in) | 56.7–76.3 kg | 125–168 lb |
| 180 cm (5 ft 11 in) | 59.9–80.7 kg | 132–178 lb |
| 185 cm (6 ft 1 in) | 63.3–85.2 kg | 140–188 lb |
| 190 cm (6 ft 3 in) | 66.8–89.9 kg | 147–198 lb |
| 195 cm (6 ft 5 in) | 70.3–94.7 kg | 155–209 lb |
What BMI cannot tell you
BMI knows two things about you: your height and your weight. Everything it appears to say about your health is inferred from those two numbers, and that inference breaks down in several common situations.
It cannot distinguish muscle from fat
Muscle is denser than fat, so a muscular person weighs more for the same size. Rugby players, sprinters, rowers and regular weight trainers routinely record BMI values in the overweight or obese range while carrying very little body fat. The number is arithmetically correct and clinically meaningless.
It ignores where fat is stored
Fat around the abdomen carries considerably more metabolic risk than the same amount stored on the hips and thighs. Two people with identical BMI can have quite different risk profiles depending on distribution, which BMI has no way of seeing.
It shifts with age
Adults tend to lose muscle and bone density with age while gaining fat. An older person can therefore sit inside the healthy BMI band while carrying a higher proportion of body fat than the number implies.
It does not apply to everyone
BMI is not appropriate for children and teenagers, who need age-and-sex percentile charts; for pregnant or breastfeeding people; for people with significant oedema or fluid retention; or for people with limb loss or certain skeletal conditions. Its risk thresholds also fit some ethnic groups better than others.
Measures worth using alongside it
- Waist circumference — measured at the midpoint between the lowest rib and the top of the hip bone. It captures abdominal fat, which BMI misses entirely.
- Waist-to-height ratio — divide waist by height in the same units. Keeping the result below 0.5 is a simple, widely cited target that scales naturally across different heights.
- Body fat percentage — more informative but harder to measure reliably; home scales using bioelectrical impedance are heavily affected by hydration.
- How you actually are — blood pressure, resting heart rate, blood glucose, cholesterol, sleep, energy and fitness say far more about health than any single ratio.
Common mistakes
- Using centimetres instead of metres. The height must be squared in metres. This is by far the most frequent error.
- Forgetting the 703 factor in the imperial formula, which leaves the answer far too small.
- Treating a category boundary as meaningful. Moving from 24.9 to 25.0 changes a label, not your body.
- Applying adult BMI to a child. The categories simply do not transfer.
- Reading BMI as a body fat measurement. It is a ratio of weight to height and nothing more.
A note on how to use this number
BMI is best understood as a prompt rather than an answer. If yours falls outside the healthy range, that is worth raising with a doctor who can look at the rest of the picture — your measurements, your history, your bloods, how you feel. If it falls inside the healthy range, that is reassuring but not a clean bill of health on its own.
This calculator is an educational tool and does not provide medical advice. Speak to a qualified healthcare professional before making changes to your diet, exercise or medication.
Frequently asked questions
What is the formula for BMI?
In metric units, BMI is weight in kilograms divided by height in metres squared. In imperial units, it is weight in pounds divided by height in inches squared, then multiplied by 703. Both give the same number for the same person.
What is a healthy BMI range?
For adults, the World Health Organization classifies 18.5 to 24.9 as healthy weight, below 18.5 as underweight, 25 to 29.9 as overweight, and 30 or above as obese. WHO applies these thresholds from age 18 (the CDC from 20); they are not used for children, who are assessed on percentile charts.
Is BMI accurate for muscular people?
No. BMI uses only height and weight, so it cannot tell muscle from fat. A trained athlete with low body fat can easily record a BMI in the overweight or obese range. For anyone carrying substantial muscle, BMI on its own is misleading.
Does BMI work for children?
Not in the adult form. Children and teenagers are assessed with BMI-for-age percentile charts that compare them against others of the same age and sex, because body composition changes throughout growth. Use a paediatric chart or ask a doctor.
Why is BMI still used if it has so many limitations?
BMI is still used because it needs only scales and a tape measure and has been applied the same way since WHO adopted it in 1995, so results compare across decades and countries. It began as Adolphe Quetelet’s index in the 1830s and was renamed body mass index by Ancel Keys in 1972 as a population statistic, not an individual diagnosis, and that is where it works best.
Is BMI different for men and women?
The formula and the thresholds are the same. Women typically carry a higher proportion of body fat than men at the same BMI, so an identical number can reflect a different body composition, which is one more reason to treat BMI as a starting point rather than a verdict.
Should BMI thresholds differ by ethnicity?
Several health bodies think so. Risk of type 2 diabetes and cardiovascular disease rises at lower BMI values in people of South Asian, Chinese and some other Asian backgrounds, and lower action thresholds are sometimes recommended. Ask a clinician what applies to you.
What is a better measure than BMI?
No single measure replaces it, but waist circumference and waist-to-height ratio add useful information because they reflect where fat is stored. Keeping your waist under half your height is a simple, widely cited target that works across heights.