Body mass index is a quick screening number that relates your weight to your height. It is cheap, needs no equipment and is used by the CDC, the World Health Organization and most clinics as a first look at weight status. This calculator works in US units or metric, places your result on the adult BMI scale and shows the range of weights that would put you in the healthy band at your height.
How to use the BMI calculator
- Choose US (feet, inches and pounds) or Metric (centimeters and kilograms).
- Enter your height and current weight.
- Optionally enter your age. If you are under 20, the result reminds you that percentile charts apply instead of adult categories.
- Read your BMI and category on the tape, then check the scale bar and the table of weight ranges for your height.
BMI formula
BMI is weight in kilograms divided by the square of height in meters:
With pounds and inches you multiply by a conversion factor:
Many charts round the factor to 703, which changes the result by less than 0.01. To turn the formula around and find the weight that matches a target BMI, multiply the target by your height in meters squared. That is how the healthy weight range is built: 18.5 × h² at the low end and 24.9 × h² at the high end.
Worked example
Someone who is 5 ft 10 in tall (70 inches, or 1.778 m) and weighs 170 lb:
BMI = 703.07 × 170 ÷ 70² = 703.07 × 170 ÷ 4,900 = 24.4
Healthy range at 1.778 m: 18.5 × 1.778² = 128.9 lb up to 24.9 × 1.778² = 173.5 lb.
A BMI of 24.4 falls in the healthy band, about 3.5 lb below the top of that range.
The metric version works the same way. At 165 cm and 72 kg, BMI = 72 ÷ 1.65² = 26.4, which is in the overweight range; the healthy range for that height runs from about 50.4 to 67.8 kg.
Adult BMI categories
| BMI | CDC category | WHO term |
|---|---|---|
| Below 18.5 | Underweight | Underweight |
| 18.5 – 24.9 | Healthy weight | Normal weight |
| 25.0 – 29.9 | Overweight | Pre-obesity |
| 30.0 – 34.9 | Obesity, class 1 | Obesity class I |
| 35.0 – 39.9 | Obesity, class 2 | Obesity class II |
| 40 and above | Obesity, class 3 (severe) | Obesity class III |
The calculator rounds BMI to one decimal before choosing the category, so the label always matches the number you see.
What BMI can and cannot tell you
BMI was designed to compare large groups, and at the population level higher BMI tracks with higher rates of type 2 diabetes, high blood pressure and heart disease. For one person it is a starting point, not a diagnosis. Keep these limits in mind:
- Muscle counts as weight. Strength athletes and people with heavy manual jobs can land in the overweight range with little body fat.
- Fat distribution is invisible to BMI. Fat stored around the abdomen carries more risk than the same amount on the hips; a waist measurement captures this.
- Ethnicity shifts the risk curve. A WHO expert panel noted that many Asian populations show higher health risk at lower BMIs and suggested extra action points at 23 and 27.5.
- Age matters. Children use percentile charts, and in older adults muscle loss can hide excess fat behind a normal BMI.
Two extra numbers on the result
BMI Prime divides your BMI by 25, so it reads as a fraction of the healthy upper limit. The ponderal index divides weight by height cubed instead of squared; because it scales better at the extremes, it is sometimes preferred for very tall or very short people. Values of roughly 11 to 15 kg/m³ correspond to the healthy BMI band for average heights.
Related tools
The BMI calculator for women adds a waist-to-height check and notes on pregnancy and menopause. To estimate body composition directly, try the body fat calculator, and compare common formulas with the ideal weight calculator.
This calculator is informational only and is not medical advice. A BMI outside the healthy range is a reason to talk with a healthcare professional, who can consider your full health history.
Frequently asked questions
What is a healthy BMI for adults?
The CDC and the World Health Organization use 18.5 to 24.9 as the healthy (normal) range for adults age 20 and older. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or higher is obesity, which is split into classes 1, 2 and 3 at 35 and 40.
Why does the US BMI formula multiply by 703?
BMI is defined in kilograms per square meter. When you enter pounds and inches, the factor 703.07 converts the units: one pound is 0.45359237 kg and one inch is 0.0254 m, and 0.45359237 ÷ 0.0254² ≈ 703.07. The answer is the same number you would get in metric.
Is BMI accurate for athletes and older adults?
BMI only uses height and weight, so it cannot tell muscle from fat. Muscular athletes are often labeled overweight even when their body fat is low, while older adults who have lost muscle can have a normal BMI with a high fat percentage. A waist measurement or a body fat estimate adds useful context.
How is BMI interpreted for children and teens?
For ages 2 through 19, the same BMI number is compared with CDC growth charts for the child's age and sex and reported as a percentile. A BMI that is healthy for an adult can be high or low for a child, so the adult categories on this page do not apply.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value of 1.00 sits exactly at the limit, 0.74 to 0.99 is in the healthy range, and 1.20 means your BMI is 20% above the healthy upper limit.