Everyday Tools
BMI Calculator
Enter height and weight in the units you prefer to get body mass index and the matching CDC category.
Calculation inputs
Choose your units, then fill in only the matching fields.
Results
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Complete the fields and select Calculate to show results here.
About this calculator
A body mass index (BMI) calculator estimates body fat level from height and weight using a simple formula, giving a quick, general screening number rather than a diagnosis. It is commonly used by individuals and clinicians as one starting data point among several.
How the calculation works
- BMI = weight in kilograms ÷ (height in metres)²
- CDC ranges: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 and above obesity
Notes and assumptions
BMI is a screening measure for populations, not a diagnosis for an individual.
How it works in plain English
BMI divides weight in kilograms by height in metres squared. When height is entered in feet and inches, or weight in pounds, the calculator first converts both to metric units, since the formula depends on a consistent metric relationship between mass and area.
The resulting number is then compared against broad category ranges set out by health organizations such as the CDC, sorting the value into underweight, normal weight, overweight, or obese bands. These bands describe population-level statistical patterns between BMI and health risk, not an individual medical assessment.
Because BMI only uses height and weight, it cannot distinguish muscle mass from fat mass, bone density, or how weight is distributed around the body. Athletes with high muscle mass, older adults with reduced muscle, and people of different body frames can all get BMI values that don't reflect their actual health status, which is why it works best as a general screening figure rather than a precise health measurement.
The formula
- BMI = weight (kg) / height (m)^2
- Metric conversions: 1 lb = 0.45359237 kg, 1 inch = 0.0254 m
Worked example
A person who is 5 feet 9 inches tall and weighs 170 pounds first has their height converted: 5 ft 9 in equals 69 inches, or 69 x 0.0254 = 1.7526 m. Their weight converts to 170 x 0.45359237 = 77.11 kg. Dividing weight by height squared gives 77.11 / (1.7526)^2 = 77.11 / 3.072 = 25.1. A BMI of 25.1 falls just inside the overweight category under standard CDC ranges, though this single number does not account for muscle mass or body composition. If that same person lost 10 pounds and dropped to 160 pounds, their new BMI would be (160 x 0.45359237) / 3.072 = 23.6, moving them into the normal weight range under the same CDC bands. This illustrates how BMI shifts with weight change alone, holding height constant, without saying anything about the composition of that weight loss.
BMI categories
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obese |
Ranges follow World Health Organization and CDC classifications for adults.
Frequently asked questions
Is BMI an accurate measure of health?
BMI is a general screening tool that correlates with body fat at a population level, but it does not measure body fat directly and can be misleading for very muscular people, older adults, or those with unusual body composition. It works best alongside other measurements, not on its own.
Does BMI apply the same way to children?
No, children and teens are assessed using age- and sex-specific BMI percentile charts rather than the fixed adult category ranges, because body composition changes significantly during growth. This calculator's categories are intended for adults. Because BMI trends differently at each age and between boys and girls during growth spurts, using flat adult thresholds for a child would misclassify many healthy kids. A pediatrician or growth chart calculator designed for children is the appropriate tool for that age group.
Why do two people with the same BMI look different?
BMI does not separate muscle from fat or account for where fat is stored on the body. Two people of the same height and weight can have very different body compositions, so the same BMI number can reflect different health pictures.
Should I use BMI alone to judge my health?
No. BMI is best treated as one general indicator among several, alongside measures like waist circumference, activity level, and bloodwork. A qualified healthcare professional can interpret it together with your full health picture. Combining BMI with other context, such as changes over time or how clothes fit, generally gives a more complete picture than any single number in isolation. It's a starting point for a conversation, not a final verdict on someone's health status.
What units does the BMI formula use?
The standard formula uses kilograms for weight and metres for height. If you enter pounds or feet and inches, those figures are converted to metric first so the calculation matches the standard international formula. Feet and inches convert to metres using the standard 0.0254 metres-per-inch factor, and pounds convert to kilograms using the standard 0.45359237 kilograms-per-pound factor. Doing this conversion automatically means you can enter measurements in whichever unit system feels most natural.