BMI Calculator
This BMI calculator finds your body mass index from your height and weight, using either metric units (kilograms and centimeters) or US units (pounds, feet, and inches). Enter your weight and height, choose the unit system that matches your inputs, and the tool returns your BMI score along with the standard weight category it falls into. Body mass index is a simple ratio of weight to height. Doctors, insurers, and public health agencies use it as a first-pass screening tool because it only requires two measurements to calculate. It does not measure body fat percentage, muscle mass, or where fat is stored on the body. Treat the result as a quick reference point, not as a full health assessment.
Quick answer
BMI compares your weight to the square of your height. It does not measure body fat percentage, muscle mass, or bone density directly.
What this tells you
- •BMI compares your weight to the square of your height. It does not measure body fat percentage, muscle mass, or bone density directly.
- •The four standard adult categories are underweight (below 18.5), normal weight (18.5 to 24.9), overweight (25.0 to 29.9), and obesity (30.0 and above).
- •The metric formula divides weight in kilograms by height in meters squared. The US formula multiplies weight in pounds by 703, then divides by height in inches squared.
- •BMI is a population-level screening estimate, not a diagnosis. Two people with the same BMI can have very different amounts of body fat and muscle.
- •This calculator uses adult BMI categories only. Children, teens, pregnant people, and very muscular individuals need different reference standards.
- •A single BMI reading is a snapshot. Tracking the number over months or years, alongside other health markers, tells you more than one score alone.
How to Use
- 1Select your unit system: Metric (kilograms and centimeters) or US (pounds, feet, and inches).
- 2Enter your current weight in the field for your chosen unit system.
- 3Enter your height. Metric mode uses a single centimeters field, while US mode uses separate feet and inches fields.
- 4Review your BMI score, the category it falls into (underweight, normal weight, overweight, or obesity), and the short interpretation text.
- 5Recalculate any time your weight changes, or switch unit systems if you are comparing a reading from a doctor's office that used different units.
How It Works
Formula
Metric: BMI = weight (kg) ÷ height (m)²
US: BMI = 703 × weight (lb) ÷ height (in)²BMI divides your weight by the square of your height to produce a single number that scales roughly the same way across different heights. In the metric formula, weight is measured in kilograms and height must be converted to meters before squaring. In the US formula, weight is measured in pounds and height is measured in inches. The constant 703 approximates the conversion factor needed to make the pounds-and-inches formula produce the same scale of number as the metric version. Once the calculator has your BMI value, it compares that number against four standard adult weight ranges to assign a category. The formula assumes an adult body and treats weight as a single number, so it does not separate fat mass from lean mass, and it does not adjust for frame size, sex, or age.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Example 1: Metric units, normal weight
Height in meters is 175 ÷ 100 = 1.75. Squaring that gives 1.75 × 1.75 = 3.0625. Dividing weight by that figure gives 70 ÷ 3.0625 = 22.9, which falls inside the normal weight range of 18.5 to 24.9.
Example 2: US units, obesity range
Total height in inches is (5 × 12) + 8 = 68 inches, and 68 × 68 = 4,624. The US formula gives 703 × 200 ÷ 4,624 = 140,600 ÷ 4,624 = 30.4, which falls in the obesity range of 30.0 and above.
Example 3: Metric units, underweight range
Height in meters is 170 ÷ 100 = 1.70, and 1.70 × 1.70 = 2.89. Dividing weight by that figure gives 50 ÷ 2.89 = 17.3, which places the result in the underweight range, below 18.5. A reading in this range is worth discussing with a healthcare provider, since it can reflect insufficient nutrition or another underlying factor.
Example 4: US units, overweight range
Total height in inches is (5 × 12) + 6 = 66 inches, and 66 × 66 = 4,356. The US formula gives 703 × 180 ÷ 4,356 = 126,540 ÷ 4,356 = 29.0, which sits near the top of the overweight range of 25.0 to 29.9, close to the obesity threshold.
Example 5: Metric units, overweight range
Height in meters is 160 ÷ 100 = 1.60, and 1.60 × 1.60 = 2.56. Dividing weight by that figure gives 65 ÷ 2.56 = 25.4, just above the normal weight cutoff of 24.9 and inside the overweight range.
Example 6: US units, normal weight
Total height in inches is (5 × 12) + 10 = 70 inches, and 70 × 70 = 4,900. The US formula gives 703 × 160 ÷ 4,900 = 112,480 ÷ 4,900 = 23.0, which falls comfortably inside the normal weight range of 18.5 to 24.9.
Standard Adult BMI Categories
How the four standard adult BMI ranges used by this calculator are defined.
| Category | BMI range | General interpretation |
|---|---|---|
| Underweight | Below 18.5 | May indicate insufficient nutrition or another factor worth discussing with a provider |
| Normal weight | 18.5 to 24.9 | Generally associated with lower weight-related health risk at a population level |
| Overweight | 25.0 to 29.9 | May benefit from monitoring weight trends and lifestyle factors over time |
| Obesity | 30.0 and above | Associated with higher average health risk, worth discussing with a healthcare provider |
These ranges apply to adults age 20 and over. They do not apply to children, teens, or pregnant people, who need different growth or reference charts.
Common mistakes
- Entering height in centimeters when the calculator is set to US units, or vice versa
- Confusing total height in inches with the inches portion only when using feet and inches, for example entering only 8 instead of the full 5 ft 8 in
- Treating BMI as a precise measure of body fat rather than a general screening number
- Applying adult BMI categories to children or adolescents, who need age- and sex-specific percentile charts instead
- Ignoring that athletes and people with high muscle mass may score high on the BMI scale without carrying excess body fat
- Assuming a normal BMI rules out every weight-related health concern, when waist size, blood pressure, and lab results also matter
Limitations
BMI is a general screening estimate based only on height and weight. It does not measure body fat percentage, muscle mass, bone density, or where fat is stored on the body, and two people with the same BMI can have very different body compositions. The categories used here reflect standard adult ranges and are not adjusted for age, sex, ethnicity, or frame size, all of which can shift what a healthy weight range looks like for a given person. This calculator is not designed for children, teens, pregnant people, or bodybuilders and other highly muscular athletes, who need different reference standards. A single BMI reading is a snapshot, not a trend, and it works best alongside other health information rather than as a standalone measure.
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