BMI Calculator Guide: Calculate Body Mass Index & Understand Weight Categories
The BMI calculator estimates body mass index using weight and height to categorize weight status. BMI = weight (kg) / height (m)^2. While convenient for population screening, BMI does not distinguish between fat and lean mass, so athletes and some ethnic groups may require additional measures.
What is BMI Calculator?
The BMI calculator converts your inputs into a BMI value and classifies it (e.g., underweight, normal, overweight, obesity classes). It’s used in health screening, insurance, and research because it’s quick and consistent across units.
How to Use the BMI Calculator
- Enter height and weight. Use metric or US units; the tool converts internally.
- Calculate to see BMI and the corresponding category.
- Optionally add waist circumference for cardiometabolic context.
- Track changes over time to monitor trends, not just single values.
Formulas & Methods
- BMI (metric):
BMI = mass_kg / (height_m)^2 - BMI (US):
BMI = 703 × (weight_lb / height_in^2) - Adult categories (typical): Underweight
<18.5, Normal18.5–24.9, Overweight25–29.9, Obesity≥30(Class I30–34.9, Class II35–39.9, Class III≥40).
Assumptions & limitations
- BMI is a screening indicator; it does not directly measure body fat.
- Muscular builds, older adults, and pregnancy can shift interpretation.
- Use alongside body fat %, waist-to-height, and clinical context.
Examples
Example A — Metric
Height 1.75 m, Weight 78 kg → BMI = 78 / 1.75^2 = 25.5 (Overweight).
Example B — US units
Height 5 ft 6 in (66 in), Weight 140 lb → BMI = 703 × 140 / 66^2 ≈ 22.6 (Normal).
| Input | Value | Output | |---|---:|---:| | 1.80 m, 95 kg | — | BMI ≈ 29.3 (Overweight) | | 72 in, 220 lb | — | BMI ≈ 29.8 (Overweight) |
Pro Tips & Best Practices
- Pair BMI with waist-to-height ratio (<0.5 target is often cited) for better risk context.
- Track trends—a slow move of 1–2 BMI points over months is meaningful.
- Keep units consistent; double-check feet/inches conversions.
- For athletes, get a body fat % estimate to avoid misclassification.
Related Calculators
FAQ
Q: What is BMI and how is it calculated?
A: Body Mass Index (BMI) = weight (kg) / height (m)^2. It approximates body fatness from height and weight.
Q: What are the adult BMI categories?
A: Underweight <18.5; Normal 18.5–24.9; Overweight 25–29.9; Obesity ≥30 (Class I 30–34.9, Class II 35–39.9, Class III ≥40).
Q: Is BMI accurate for athletes?
A: BMI can misclassify very muscular people as overweight/obese; consider body fat % and waist measures too.
Q: Does age or sex change BMI thresholds?
A: Adult thresholds are the same by sex; for children/teens, BMI-for-age percentiles are used.
Q: Which units does the calculator accept?
A: Use metric (kg, m) or US units (lb, ft/in); the tool converts to metric for the formula.
Compliance note: Informational only; not medical advice.
Call to Action
Enter your height and weight to see your BMI and category—then explore body fat and calorie tools to round out the picture.