BMI Calculator
Free Body Mass Index calculator for adults and children ages 2–20. US and metric units. Includes healthy weight range, ideal weight, BMI history, and personalized guidance.
Optional for adults. Required for children (2–19).
Adds metabolic risk context alongside your BMI score.
<18.5 Healthy
18.5–24.9 Overweight
25–29.9 Obese
30+
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Medical Disclaimer: This BMI calculator is for informational and educational purposes only. BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, body fat distribution, age, sex, or ethnicity. Always consult a licensed healthcare provider before making any health decisions.
What Is BMI?
Body Mass Index (BMI) is a value derived from height and weight used as a screening tool to estimate whether a person has a healthy body weight. The CDC, WHO, and NHLBI all use BMI as an initial indicator of potential weight-related health risks. Our free BMI calculator on Calcxi applies the exact same formulas used by these health authorities.
BMI calculator for adults (age 20+): Categories are the same regardless of age or sex — underweight (<18.5), healthy weight (18.5–24.9), overweight (25–29.9), and obese (30+). This standard is used by every major health organization worldwide. Use the Adult (20+) tab above to get started.
BMI calculator for children and teens (ages 2–19): The same height and weight formula applies, but results are interpreted as a percentile on CDC growth charts compared to peers of the same age and sex. A healthy child BMI falls between the 5th and 84th percentile. Select the Child / Teen (2–19) tab above to calculate BMI for children.
A healthy BMI goes hand in hand with proper hydration. After calculating your BMI, use our Water Intake Calculator to find out exactly how much water your body needs daily based on your weight and activity level.
Adult BMI Categories (CDC/WHO)
| Category | BMI Range | Health Implication |
|---|---|---|
| Underweight | < 18.5 | Possible nutrient deficiency or health condition |
| Healthy Weight | 18.5 – 24.9 | Associated with lowest health risks |
| Overweight | 25.0 – 29.9 | Elevated risk for heart disease, diabetes |
| Obese Class I | 30.0 – 34.9 | High risk — consult a healthcare provider |
| Obese Class II | 35.0 – 39.9 | Very high risk |
| Obese Class III | ≥ 40.0 | Extremely high risk (severe obesity) |
BMI Formula
The same formula is used by the CDC, WHO, and NHLBI. Our BMI calculator applies both US and metric versions with full precision:
BMI = 703 × weight(lbs) ÷ height²(inches²)
// Metric Units
BMI = weight(kg) ÷ height²(m²)
// Example: 5'10", 160 lbs → BMI = 703 × 160 ÷ 70² = 23.0 (Healthy Weight ✓)
Frequently Asked Questions
According to the CDC and WHO, a BMI between 18.5 and 24.9 is considered a healthy BMI for adults age 20 and older. This range is associated with the lowest risk of weight-related diseases. However, BMI is a screening tool your doctor considers waist circumference, blood pressure, and lab values for a complete picture. To support a healthy BMI, also check our Water Intake Calculator to stay properly hydrated.
A BMI calculator for adults uses fixed cutoffs (18.5, 25, 30) that apply to everyone age 20+. A BMI calculator for children uses the same height/weight formula, but the result is plotted on CDC growth charts as a percentile for the child's specific age and sex. A healthy child BMI falls between the 5th and 84th percentile. This calculator handles both adult and child modes automatically, just select the correct tab above.
BMI is a useful screening tool but cannot distinguish between muscle and fat. A muscular athlete may have an "overweight" BMI with low body fat. It also doesn't account for where fat is stored (abdominal fat is riskier). Use BMI as one data point alongside waist circumference and your doctor's assessment.
Waist circumference measures abdominal fat, which is more metabolically dangerous than fat stored elsewhere. Per NHLBI, a waist >40 inches (men) or >35 inches (women) raises risk for diabetes and heart disease even with a normal BMI. This calculator includes waist for additional context.
Evidence-based approaches: (1) Create a moderate calorie deficit of 500–750 calories/day for ~1 lb/week loss; (2) 150 minutes of moderate aerobic activity weekly (CDC guideline); (3) Prioritize whole foods, lean protein, and vegetables; (4) Get 7–9 hours of sleep nightly. Always consult your healthcare provider first.
Related Health Tools
BMI is one piece of your health picture. Explore these free tools on Calcxi to get a more complete view of your wellness.
Medical Disclaimer: This tool is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Results use CDC and WHO formulas. Consult a licensed healthcare provider before making health changes. If you have concerns about your weight or body image, please seek professional support.
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