BMI & Weight Category Calculator
Calculates your Body Mass Index (BMI) from weight and height in US units, then classifies it into a weight category. Use it to quickly screen for underweight, healthy, overweight, or obese ranges.
About this calculator
Body Mass Index is a widely used screening metric that estimates body fatness from height and weight. The imperial formula is: BMI = (weight_lbs / height_in²) × 703, where 703 is a unit-conversion constant that makes the result equivalent to the metric formula (kg/m²). Standard WHO categories are: Underweight < 18.5, Normal weight 18.5–24.9, Overweight 25–29.9, and Obese ≥ 30. Age and ethnicity are included because research shows BMI thresholds carry different health risks across populations — for example, Asian adults face elevated metabolic risk at BMIs above 23. While BMI does not measure body fat directly, it remains a practical first-pass clinical tool used globally.
How to use
Example: a person weighs 170 lbs and stands 68 inches tall (5 ft 8 in). Step 1 — Square the height: 68² = 4,624. Step 2 — Divide weight by that value: 170 / 4,624 = 0.03677. Step 3 — Multiply by 703: 0.03677 × 703 ≈ 25.8. A BMI of 25.8 falls in the Overweight category (25–29.9). For a 40-year-old of Asian ethnicity, this would warrant closer attention given lower risk thresholds for that demographic group.
Frequently asked questions
What are the BMI categories and what does each one mean for my health?
The four standard BMI categories are Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above). Being outside the normal range is associated with increased risk of conditions like type 2 diabetes, cardiovascular disease, and joint problems. However, BMI is a population-level screening tool, not a definitive health diagnosis — a muscular athlete can have a high BMI without excess body fat. Always pair your BMI result with a conversation with a healthcare provider for context.
Why does ethnicity affect BMI health risk thresholds?
Large epidemiological studies have found that people of Asian descent develop metabolic complications such as insulin resistance and hypertension at lower BMI values than individuals of European descent. As a result, several health organizations recommend a lower 'overweight' cut-off of 23 and 'obese' cut-off of 27.5 for Asian populations. Conversely, some studies suggest that Black adults may have higher lean mass at the same BMI, meaning standard thresholds may overestimate health risk in that group. Ethnicity-adjusted thresholds make the calculator more clinically meaningful across diverse users.
How accurate is BMI as a measure of body fat percentage?
BMI correlates moderately with measured body fat but is far from precise at the individual level, with studies showing the same BMI can correspond to a 10–15% range in actual body fat percentage. It cannot distinguish between fat mass and muscle mass, so bodybuilders and athletes are routinely misclassified as overweight or obese. For a more accurate body composition assessment, methods like DEXA scanning, hydrostatic weighing, or skinfold calipers are recommended. Despite its limitations, BMI remains useful as a quick, cost-free initial screen for large populations.