BMI Calculator
Calculate your Body Mass Index (BMI) using the CDC-standard weight(kg) / height(m)² formula. Enter height and weight in metric or US units to get your BMI and, for adults 20+, the CDC weight category.
Calculator Inputs
Enter your height as a single number matching the unit selected below. In US units, enter TOTAL inches (e.g. 5'9" = 69 in), not separate feet and inches.
Enter your weight matching the unit selected below.
Collected for context only — sex does not change the BMI formula or thresholds.
Used only to determine whether the adult CDC weight category applies (age 20+). Age never changes the BMI number itself.
- BMI
- 22.86
- Category
- Healthy weight
- Healthy Adult BMI Range
- 18.5–24.9
About these results
CalcSpectrum calculators produce automated estimates using established formulas and the specific values you enter. Because real-world outcomes depend on input accuracy, mathematical assumptions, and rounding conventions, please verify critical figures with an appropriately qualified financial, tax, medical, legal, or other professional, as relevant.
Health & Fitness Notice: Wellness and body metric calculations provide informational estimates based on generalized population formulas. They do not provide medical diagnosis, clinical advice, or treatment plans. Consult an appropriately qualified medical or healthcare professional for personal guidance.
How It's Calculated
Formula
Body Mass Index (BMI) is a simple screening ratio of weight to height, calculated as weight in kilograms divided by height in meters squared. It's the metric the CDC and most public health bodies use to sort adults into four broad weight categories: Underweight, Healthy weight, Overweight, and Obesity. The metric formula (kg / m²) is the authoritative one; the commonly cited "703 × lb / in²" US-customary version is an approximation using a rounded conversion constant, not a separate formula — both forms describe the same physical ratio, and this calculator always computes from the metric form internally regardless of which units you enter. Sex and age are collected here but do not change the BMI number itself: BMI is a pure function of weight and height. Age matters only for one thing — whether the adult CDC category label applies. The flat adult thresholds below are validated only for ages 20 and up; for children and teens (2-19), CDC instead uses age- and sex-specific BMI-for-age percentile growth charts, which are a different system this calculator does not attempt to replicate. If you enter an age under 20, you'll still see your raw BMI number, just without an adult category attached to it. This is an estimate for general informational and educational purposes, based on a published formula — it is not a medical diagnosis, and it is not a substitute for individualized advice from a qualified healthcare provider. Specifically for BMI: it cannot distinguish muscle mass from fat mass, does not account for body composition, frame size, or sex-specific fat distribution, and is a population screening tool — not a diagnosis of individual body fatness or health.
Worked Examples
Metric example: 70 kg, 175 cm (1.75 m)
- Convert height to meters: 175 cm = 1.75 m
- Apply the formula: BMI = weight / height² = 70 / (1.75 × 1.75) = 70 / 3.0625
- BMI ≈ 22.86
- For an adult (age 20+): 18.5 ≤ 22.86 < 25.0, so the category is Healthy weight
US units example: 154.324 lb, 68.898 in (equivalent body)
- Convert to metric internally: 154.324 lb ≈ 70 kg, 68.898 in ≈ 1.75 m
- Apply the same metric formula: BMI = 70 / (1.75 × 1.75) ≈ 22.86
- The result matches the metric example above — both unit paths describe the same body
- For an adult (age 20+): category is Healthy weight
Frequently Asked Questions
Why do sex and age not change my BMI number?
BMI is defined purely as weight divided by height squared — there is no sex or age term anywhere in the formula, and the CDC's adult classification thresholds (Underweight/Healthy/Overweight/Obesity) are the same regardless of sex or exact adult age. Sex and age are collected here for context and because age gates whether an adult category is shown at all, but neither one adjusts the arithmetic.
Why doesn't my BMI get a category if I'm under 20?
The CDC's flat adult thresholds only apply to adults (age 20 and up). For children and teens (ages 2-19), the CDC instead uses BMI-for-age percentile growth charts that account for the fact that healthy body composition changes substantially with age and sex during growth — a flat number line doesn't work the same way it does for adults. This calculator still computes and shows your raw BMI number at any age, but intentionally does not guess at a percentile system it doesn't implement; that keeps the tool from giving a falsely precise-looking category for children and teens.
What are the known limitations of BMI?
BMI cannot tell the difference between muscle mass and fat mass — a very muscular person can have a "high" BMI without excess body fat, while someone with low muscle mass and higher fat percentage can have a "normal" BMI. It also doesn't account for frame size, sex-specific fat distribution, or age-related muscle loss. BMI is a fast, free population-screening tool, not an individualized diagnosis of body fatness or health — it's best used as one data point, not the whole picture.
Which unit should I use for height if I'm in the US?
Enter your height as a single number in total inches, not separate feet and inches. For example, 5 feet 9 inches is 5×12+9 = 69 inches.
What does the healthy BMI range shown actually mean?
18.5–24.9 is the CDC's Healthy weight range for adults. It's shown as a constant reference alongside your own result so you can see at a glance how far your number is from that range, regardless of which category you land in.