BMI Calculator
Body mass index for adults, in metric or US units — with the healthy weight range for your height, and an honest account of what BMI can and can't tell you.
Your measurements
These categories apply to adults aged 20 and over. BMI for children and teenagers is read from age- and sex-specific percentile charts instead, so this calculator’s labels do not apply to them.
Your BMI
Healthy weight for your height
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What this tool does
Body mass index is weight divided by height squared. It is a single number that puts your weight in the context of your height, and it is the measure most health services use as a first, rough screening step. This calculator works it out in either metric or US units, places it against the standard adult categories, and shows what weight range would put you in the healthy band at your height.
How BMI is calculated
The metric formula is straightforward: BMI = kg ÷ m². Someone who is 1.75 m and 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
In US units the same relationship needs a conversion factor: BMI = (lb ÷ in²) × 703. The 703 exists purely to reconcile pounds and inches with the metric definition — it has no meaning of its own. This calculator converts to metric internally, so both routes give an identical answer.
The adult categories
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity, class I |
| 35.0 – 39.9 | Obesity, class II |
| 40.0 and above | Obesity, class III |
These are the World Health Organization cut-offs, also used by the US National Institutes of Health and the CDC for adults aged 20 and over.
What BMI does not measure
This is the part usually left off calculator pages, and it matters. BMI was devised in the 1830s as a way of describing populations, not diagnosing individuals, and it has real limits:
- It cannot tell muscle from fat. Muscle is denser than fat, so athletes and physically heavy-working people routinely land in the “overweight” band while carrying very little fat.
- It ignores where fat is carried. Fat around the abdomen carries substantially more metabolic risk than the same weight on hips and thighs, and BMI cannot see the difference. Waist circumference and waist-to-height ratio capture this better.
- The cut-offs are not equally valid across populations. Health risks associated with a given BMI appear at lower thresholds in people of South Asian, Chinese and some other Asian backgrounds; several health bodies use a lower overweight threshold of 23 for these groups.
- It shifts with age. Older adults tend to lose muscle and gain fat at an unchanged BMI, so the same number describes a different body at 70 than at 30.
- It does not apply during pregnancy, nor to children and teenagers, whose BMI is interpreted against age- and sex-specific percentile charts.
BMI is best treated as one data point that suggests whether a conversation is worth having — not as a verdict on your health.
Useful measurements alongside it
Waist circumference is measured at the midpoint between the lowest rib and the top of the hip bone. A common threshold of raised risk is above 94 cm (37 in) for men and 80 cm (31.5 in) for women, with thresholds again varying by population.
Waist-to-height ratio is simply waist divided by height in the same units, with a rough target of staying under 0.5. It needs no chart, adjusts naturally for body size, and predicts metabolic risk at least as well as BMI in most studies.
This is not medical advice
This calculator provides a general-information figure and nothing more. It cannot account for your medical history, body composition, family history or bloodwork, and it is not a diagnosis of anything. If your result concerns you, or if you are planning a significant change to your weight, diet or exercise, speak to a doctor or a registered dietitian who can look at the whole picture.
Nothing you enter is stored
Your height and weight are calculated in your own browser and never transmitted. Health data is exactly the sort of thing that should not be sitting in somebody’s server logs, and here it isn’t — there is no server involved. If you use the share button, the figures go into the page’s URL, so treat that link as private.
Frequently asked questions
How is BMI calculated?
BMI is your weight in kilograms divided by your height in metres squared. In US units the equivalent is weight in pounds divided by height in inches squared, multiplied by 703. Someone 1.75 m tall weighing 70 kg has a BMI of 22.9.
What is a healthy BMI for an adult?
The World Health Organization defines 18.5 to 24.9 as the healthy weight range for adults aged 20 and over. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above falls into the obesity categories.
Is BMI accurate for athletes?
Often not. BMI cannot distinguish muscle from fat, and muscle is denser, so people carrying a lot of muscle frequently register as overweight while having very low body fat. For them, body composition or waist measurements are far more informative.
Does BMI work the same for everyone?
No. Health risks appear at lower BMI values in people of South Asian, Chinese and some other Asian backgrounds, and several health bodies use a lower overweight threshold of 23 for these groups. BMI also shifts in meaning with age, and does not apply during pregnancy.
Can I use this calculator for my child?
No. Children and teenagers are assessed with age- and sex-specific BMI percentile charts, not the adult cut-offs, because normal body composition changes throughout growth. Ask a paediatrician or use a dedicated child growth chart.
Should I measure my waist as well?
It is worth doing. Waist circumference and waist-to-height ratio capture where fat is carried, which BMI cannot see, and abdominal fat carries more metabolic risk. A common rule of thumb is keeping your waist under half your height.
Is my height and weight data sent anywhere?
No. Everything is calculated in your browser and nothing is transmitted or stored. If you use the share link, the numbers are placed in the page URL, which only you control.
More calculators
The rest are in Calculators — including the Age Calculator and the Percentage Calculator.