BMI Calculator
Enter your height and weight to get your Body Mass Index and the standard World Health Organization weight category. You also get BMI Prime, the Ponderal Index, your healthy weight range for that height, and how much you would need to lose or gain to reach it. Switch between metric and imperial units, your result updates as you type.
Formula
Worked example
An adult weighing 70 kg at 175 cm: 70 ÷ (1.75)² = 70 ÷ 3.0625 ≈ 22.9, a healthy-weight BMI. BMI Prime is 22.9 ÷ 25 ≈ 0.92, and the Ponderal Index is 70 ÷ 1.75³ ≈ 13.1 kg/m³. The healthy weight range for 1.75 m is about 56.7 to 76.6 kg.
What is BMI?
Body Mass Index (BMI) is a single number that relates your weight to your height. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet and is calculated as weight in kilograms divided by the square of height in metres. Because it needs only two easily measured values, BMI is used worldwide as a fast, low-cost screen for whether an adult is likely to be underweight, a healthy weight, overweight, or living with obesity.
How to use this calculator
Choose metric (kilograms and centimetres) or imperial (pounds and inches), then enter your height and weight. Your BMI and its weight category update instantly, along with BMI Prime, the Ponderal Index, the healthy weight range for your height, and how much you would need to lose or gain to reach it. The optional age field flags when adult cut-offs do not apply, and the cut-off selector lets you switch from the standard WHO band to the lower Asian-Pacific band. These results apply to adults aged 20 and over, children and teenagers are assessed against age- and sex-specific percentile charts instead of the fixed adult thresholds.
BMI Prime, Ponderal Index and your healthy weight range
BMI Prime is simply your BMI divided by the upper limit of the healthy band (25 by default). A value below 1.0 means you are within the healthy range, 1.2 means you are 20 percent above the top of it, which makes comparisons easy at a glance. The Ponderal Index divides weight by height cubed rather than squared, so it stays more consistent across very short and very tall people, where ordinary BMI can mislead. The healthy weight range is the span of weights that would put your BMI between the lower and upper cut-offs for your exact height, and the weight-to-lose-or-gain figure is the distance from your current weight to the nearest edge of that range (zero if you are already inside it).
How BMI is calculated
In metric units, BMI = weight (kg) ÷ height (m)². In imperial units the same ratio is scaled by 703, so BMI = 703 × weight (lb) ÷ height (in)². For example, an adult who weighs 70 kg and stands 175 cm tall has a BMI of 70 ÷ 1.75² ≈ 22.9, which falls in the healthy-weight range.
BMI categories and what they mean
The World Health Organization defines the standard adult cut-offs: below 18.5 is underweight, 18.5-24.9 is healthy weight, 25.0-29.9 is overweight, and 30.0 or above is obesity (further divided into classes I, II and III). These thresholds are identical for men and women. Some health authorities apply lower cut-offs for people of South and East Asian descent, where cardiometabolic risk tends to rise at a lower BMI.
Limitations of BMI
BMI measures weight relative to height, not body composition or health. It cannot distinguish muscle from fat, so very muscular people such as athletes and manual workers can be classified as overweight despite low body fat. It can understate risk in older adults who have lost muscle, and it says nothing about where fat is stored, even though abdominal fat carries more risk. Pregnancy, ethnicity and frame size all affect how the number should be read. Treat BMI as a starting point, and pair it with waist circumference, waist-to-height ratio, or a body-fat estimate.
WHO adult BMI categories
| BMI range (kg/m²) | Category | Health risk |
|---|---|---|
| Below 16.0 | Severe thinness | High |
| 16.0-16.9 | Moderate thinness | Increased |
| 17.0-18.4 | Mild thinness | Increased |
| 18.5-24.9 | Healthy weight | Lowest |
| 25.0-29.9 | Overweight | Increased |
| 30.0-34.9 | Obesity (class I) | High |
| 35.0-39.9 | Obesity (class II) | Very high |
| 40.0 and above | Obesity (class III) | Extremely high |
Standard World Health Organization cut-offs for adults aged 20 and over.
Frequently asked questions
Is BMI accurate for athletes?
Not always. BMI counts all body mass the same way, so the extra muscle athletes carry can push their BMI into the overweight range even when their body-fat percentage is low. Very muscular people are better assessed with a body-fat measurement or a waist-based metric.
What is a healthy BMI range?
For most adults aged 20 and over, the World Health Organization considers a BMI between 18.5 and 24.9 to be the healthy-weight range. Some guidelines use a lower upper limit (around 23) for people of Asian descent, because health risks can begin at a lower BMI.
Does BMI work for children?
Not with these cut-offs. Because children and teenagers are still growing, their BMI is compared against age- and sex-specific percentile charts rather than the fixed adult thresholds. Use a dedicated child or teen BMI tool for anyone under 20.
Should I use a waist measurement instead?
Use both. Waist circumference and waist-to-height ratio capture abdominal fat, which BMI ignores. Together with BMI they give a fuller picture of cardiometabolic risk than any one number alone.
How often should I check my BMI?
BMI is most useful as a trend rather than a one-off reading. Checking every few weeks or months is plenty to track gradual change; a single measurement is just a snapshot, and ordinary weight fluctuations can shift it slightly.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy band. A result below 1.0 means you are within the healthy range, and a result of, say, 1.10 means you are 10 percent above the top of it. It is a quick way to see how far you are from the healthy boundary without memorising the raw cut-offs.
What is the Ponderal Index and how is it different from BMI?
The Ponderal Index divides weight by height cubed (kg/m³) rather than height squared. Because BMI tends to overstate the build of tall people and understate that of short people, the cubic version stays steadier across heights, so it can be a useful second reading for people at the extremes of the height range.
How much weight do I need to lose or gain to reach a healthy BMI?
Multiply 18.5 and 25 by your height in metres squared to get the lightest and heaviest healthy weights for your height. If you are above that range, the weight to lose is your current weight minus the upper figure; if you are below it, the weight to gain is the lower figure minus your current weight. This calculator works both out for you in your chosen units.