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BMI Calculator
Use this BMI calculator to check your body mass index, see the healthy weight range for your height and, if you add your waist, your waist-to-height ratio. Results update as you type.
This tool is for informational purposes only and is not a substitute for professional medical advice. BMI is a screening measure for adults aged 18 and over; it doesn't measure body fat directly.
How BMI is calculated
The formula
- MetricBMI = weight (kg) ÷ height (m)²
- ImperialBMI = 703 × weight (lb) ÷ height (in)²
- Healthy weight range18.5 × height (m)² to 24.9 × height (m)²
Example: 70 kg at 1.75 m tall is 70 ÷ 3.0625 = 22.9. The healthy range at that height is 56.7–76.3 kg.
Adult BMI categories (WHO)
| Severely underweight | Below 16.0 |
|---|---|
| Underweight | 16.0 – 18.4 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese (class I) | 30.0 – 34.9 |
| Obese (class II) | 35.0 – 39.9 |
| Obese (class III) | 40.0 and above |
Categories use your BMI rounded to one decimal, as it is shown.
BMI cut-offs for Asian populations
Research has found that many people of Asian heritage — including South Asian, Chinese and other East Asian backgrounds — have a higher risk of type 2 diabetes and heart disease at a lower BMI. A WHO expert consultation suggested lower action points, and NICE in the UK uses them too. Turn on Use Asian BMI cut-offs in the calculator to apply them.
| Underweight | Below 18.5 |
|---|---|
| Healthy weight | 18.5 – 22.9 |
| Overweight | 23.0 – 27.4 |
| Obese | 27.5 and above |
Waist-to-height ratio
Divide your waist by your height, in the same units. It shows how much weight you carry around your middle, which BMI can't. The simple rule: keep your waist to less than half your height.
| Healthy | 0.40 – 0.49 |
|---|---|
| Increased risk | 0.50 – 0.59 |
| High risk | 0.60 and above |
How to measure your waist
- Find the bottom of your ribs and the top of your hip bone.
- Wrap a tape measure around your middle at the point halfway between them — for many people this is just above the belly button.
- Breathe out normally and measure without pulling the tape tight.
What BMI doesn't tell you
BMI is a quick screening measure, not a diagnosis. It doesn't distinguish muscle from fat or show where fat is stored, so very muscular people can score as overweight and others can have a "healthy" BMI with a lot of fat around the middle. It is less reliable for older adults, isn't used during pregnancy, and children and teenagers need age- and sex-specific BMI percentiles instead. This tool is for informational purposes only and is not a substitute for professional medical advice — speak to a doctor about your weight and health.
Frequently asked questions
How is BMI calculated?+
BMI is your weight in kilograms divided by your height in meters squared. In imperial units it is 703 × weight in pounds ÷ height in inches squared. For example, 70 kg at 1.75 m gives 70 ÷ 3.0625 = 22.9.
What is a healthy BMI?+
For most adults, 18.5 to 24.9. For people of Asian heritage, the WHO suggests a lower upper limit of 22.9, because health risks tend to start at a lower BMI.
How is my ideal weight range worked out?+
It is the weight that would put you between BMI 18.5 and 24.9 at your height (18.5 to 22.9 with Asian cut-offs). It is a range, not a single target, and the right weight for you also depends on your build, muscle and health.
Why does the waist-to-height ratio matter?+
Fat around the middle is more closely linked with type 2 diabetes and heart disease than weight alone. A ratio under 0.5 — a waist less than half your height — is a simple, widely used target, recommended by NICE in the UK.
Is BMI accurate for athletes, older adults or during pregnancy?+
Not always. BMI can't tell muscle from fat, so very muscular people may score as overweight. It is less reliable in older adults, isn't used during pregnancy, and children and teens need age- and sex-specific BMI percentiles instead.
Is my information stored or sent anywhere?+
No. Everything is calculated in your browser and nothing you enter is uploaded or saved. If you tick “Remember my units”, only your choice of metric or imperial is kept on this device — never your measurements.
What BMI is considered obese?+
For adults, a BMI of 30 or more. The WHO splits it into class I (30–34.9), class II (35–39.9) and class III (40 and above). With Asian cut-offs, obesity starts at 27.5.
Is BMI different for men and women?+
No. The formula and the adult categories are the same for everyone. In this calculator, sex is only used for the optional formula-based ideal weights, never for your BMI or healthy weight range.
How much weight do I need to lose to reach a healthy BMI?+
The calculator shows how much you'd need to lose or gain to reach the nearest edge of the healthy range. At 175 cm and 85 kg, the BMI is 27.8 and the top of the healthy range is 76.3 kg, so the gap is 8.7 kg. It is a guide, not a target to hit at any cost — talk to a doctor before a big change.
What are the Devine, Robinson, Miller and Hamwi ideal weight formulas?+
Older formulas, developed mainly for medication dosing, that give one ideal weight from your height and sex by adding a fixed amount per inch over 5 feet. For a man 5 ft 10 in tall they give Devine 73.0 kg, Robinson 71.0 kg, Miller 70.3 kg, Hamwi 75.0 kg. Choose male or female in the calculator to see them next to your BMI-based range; they aren't shown for heights under 5 ft.
What heights and weights can I enter?+
Heights from 100 to 250 cm (3 ft 4 in to 8 ft 2 in) and weights from 20 to 400 kg (about 44 to 882 lb). These cover adults; BMI categories aren't meant for children, who need age-based percentiles.
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