BMI calculator — body mass index in metric or imperial
Work out your body mass index in kg/cm or lb/ft, see the category gauge, your healthy weight range, and what BMI does and does not measure.
- Runs in your browser
- Nothing you enter is sent
- Free, no account
5 ft 8.9 in
158.7 lb · 11 st 4.7 lb
BMI is a screening number for adults aged 20 and over, not a diagnosis. It is not valid during pregnancy or for children, who are assessed on percentile charts.
The formula, with your numbers
- Metric
- BMI = weight (kg) ÷ height (m)²
- BMI = 72 ÷ (1.75 × 1.75) = 72 ÷ 3.0625 = 23.5
- Imperial
- BMI = 703 × weight (lb) ÷ height (in)²
- BMI = 703 × 158.7 ÷ 4,746.9 = 23.5
- Healthy range
- 18.5 × 3.0625 to 24.9 × 3.0625 = 56.7 kg – 76.3 kg
Both forms give the same answer — the 703 factor just converts pounds and inches into kilograms and metres.
What this means
A BMI of 23.5 falls in the “Healthy weight” band. Your weight sits inside the range associated with the lowest all-cause mortality risk for your height. Nothing here needs changing on BMI grounds alone.
For your height, a BMI of 18.5 to 24.9 corresponds to 56.7 kg – 76.3 kg. You are inside that range.
BMI knows nothing about muscle, bone density or where fat is stored. A 95 kg rower and a 95 kg sedentary adult of the same height get an identical score. If you train seriously with weights, check body fat percentage or waist-to-height ratio — keeping your waist under half your height is a better single indicator of metabolic risk than BMI is.
| Category | BMI |
|---|---|
| Underweight | under 18.5 |
| Healthy weightYou | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obesity class I | 30 – 35 |
| Obesity class II | 35 – 40 |
| Obesity class III | 40 and above |
How to calculate your BMI
- Pick metric or imperial with the unit toggle — your choice is remembered on this device.
- Enter your height. In imperial mode you get separate feet and inches boxes.
- Enter your weight. Every conversion is shown underneath the box, so you can sanity-check it.
- Read the gauge. The needle sits on your score, the highlighted arc is your category, and the healthy weight range for your exact height appears beside it.
Nothing is submitted anywhere. The arithmetic runs in JavaScript inside this page, so the numbers you type never leave your browser.
The BMI formula
Body mass index is weight divided by height squared. In metric units that is kilograms divided by metres squared:
BMI = kg ÷ m²
In imperial units the same relationship needs a conversion factor of 703, because pounds and inches are not SI units:
BMI = 703 × lb ÷ in²
A 1.75 m, 72 kg adult has a BMI of 72 ÷ (1.75 × 1.75) = 72 ÷ 3.0625 = 23.5. The same person in imperial is 5 ft 9 in (69 in) and 158.7 lb: 703 × 158.7 ÷ 4,761 = 23.4. The tiny difference is rounding, not disagreement.
BMI categories for adults
| Category | BMI range | What it generally indicates |
|---|---|---|
| Underweight | Below 18.5 | Higher risk of nutrient deficiency, reduced bone density and impaired immunity |
| Healthy weight | 18.5 – 24.9 | The band with the lowest all-cause mortality in large population studies |
| Overweight | 25.0 – 29.9 | Slightly raised cardiometabolic risk on average, but heavily confounded by muscle mass |
| Obesity class I | 30.0 – 34.9 | Clearly raised risk of type 2 diabetes, hypertension and joint problems |
| Obesity class II | 35.0 – 39.9 | Substantially raised risk; medical support usually improves outcomes |
| Obesity class III | 40.0 and above | Highest statistical risk band, sometimes called severe obesity |
These cut-offs come from the World Health Organization and apply to adults of any sex aged 20 and over. Several Asian countries use lower thresholds — 23 for overweight and 27.5 for obesity — because cardiometabolic risk rises at a lower BMI in those populations.
Healthy weight ranges by height
The range below is simply BMI 18.5 to 24.9 converted back into weight. Notice how wide it is: at 1.75 m there is more than 20 kg between the bottom and the top of "healthy".
| Height | Healthy weight (metric) | Healthy weight (imperial) |
|---|---|---|
| 1.55 m · 5 ft 1 in | 44.4 – 59.8 kg | 98 – 132 lb |
| 1.60 m · 5 ft 3 in | 47.4 – 63.7 kg | 104 – 140 lb |
| 1.65 m · 5 ft 5 in | 50.4 – 67.8 kg | 111 – 149 lb |
| 1.70 m · 5 ft 7 in | 53.5 – 72.0 kg | 118 – 159 lb |
| 1.75 m · 5 ft 9 in | 56.7 – 76.3 kg | 125 – 168 lb |
| 1.80 m · 5 ft 11 in | 59.9 – 80.7 kg | 132 – 178 lb |
| 1.85 m · 6 ft 1 in | 63.3 – 85.2 kg | 140 – 188 lb |
| 1.90 m · 6 ft 3 in | 66.8 – 89.9 kg | 147 – 198 lb |
Where BMI breaks down
BMI was devised in the 1830s by Adolphe Quetelet as a way of describing populations, not individuals, and it has exactly two inputs. That makes it cheap, reproducible and easy to compare across studies — and blind to almost everything that matters clinically.
- Muscle reads as fat. Muscle is denser than adipose tissue, so trained athletes routinely land in the "overweight" band with single-digit body fat. Most professional rugby players and sprinters would be classed as obese on BMI alone.
- Fat distribution is invisible. Visceral fat around the organs carries far more metabolic risk than subcutaneous fat on the hips and thighs, but BMI cannot tell them apart. A waist measurement can.
- It is height-biased. Because weight scales closer to height cubed than squared for similar body shapes, BMI slightly over-reads for tall people and under-reads for short ones. The Ponderal index shown in the results uses height cubed and is more stable at the extremes.
- Age and ethnicity shift the meaning. Older adults lose muscle and gain fat at the same BMI, and health risk appears at different thresholds across populations.
- It is not valid for everyone. Children, pregnant people, and anyone with significant oedema or amputation need different tools entirely.
Better checks to run alongside BMI
If your BMI is anywhere near a boundary, two extra numbers tell you far more than a second decimal place would.
- Waist-to-height ratio. Measure your waist at the navel and divide by your height in the same units. Under 0.5 is the low-risk zone, and it works across heights, sexes and ethnic groups better than BMI does.
- Body fat percentage. A tape measure and the US Navy equation give a usable estimate in two minutes, and it separates the muscular from the merely heavy.
Both are on this site, and both run entirely on your own device — no account, no upload, no tracking.
Questions
What is a healthy BMI?
For adults, the WHO puts the healthy range at 18.5 to 24.9. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese. The cut-offs are the same for men and women.
Why does BMI say I am overweight when I am not?
BMI only knows your height and weight, so it cannot tell muscle from fat. Muscular people, sprinters, rowers and rugby players routinely score 26 to 30 with low body fat. If that sounds like you, use a waist measurement or the body fat calculator instead.
Does BMI work for children?
No. Children and teenagers are assessed against age- and sex-specific percentile charts, not fixed adult cut-offs, so this calculator is for adults aged 20 and over.
Should Asian populations use different cut-offs?
The WHO notes that health risk rises at a lower BMI in many Asian populations, and several countries use 23 as the overweight threshold and 27.5 for obesity. The calculator shows both the standard bands and where you sit numerically so you can apply either.
Is my BMI stored or uploaded anywhere?
No. Every number is calculated by JavaScript in your own browser. Nothing is sent to a server and nothing is logged.