BMI Calculator — Calculate Your Body Mass Index Online Free
Knowing your Body Mass Index takes seconds — enter your height and weight, pick metric or imperial units, and get your BMI value, category, and ideal weight range instantly. No account, no download, no ads blocking your results.
BMI is one of the most widely used screening tools in clinical and public health settings worldwide. While it has limits (more on that below), it gives you a fast, evidence-based starting point for understanding where your body weight sits relative to your height.
How to Use the BMI Calculator
- Select your unit system — choose Metric (kg and cm) or Imperial (lbs and ft/in) at the top.
- Enter your data — select gender, then fill in weight, height, and optionally your age.
- Click "Calculate BMI" — your BMI value, WHO category, and ideal weight range appear instantly on the right.
Body Mass Index — What It Is and Why It Matters
BMI is calculated by dividing your weight in kilograms by the square of your height in meters (BMI = kg/m²). For imperial units, the formula adds a correction factor: BMI = 703 × lbs ÷ in².
The result places you in one of six categories defined by the World Health Organization (WHO): underweight (< 18.5), normal weight (18.5–24.9), overweight (25.0–29.9), and three obesity classes (30.0–34.9, 35.0–39.9, and ≥ 40.0). These thresholds are used globally in clinical screening to flag people who may benefit from further health evaluation.
Common Use Cases
- Pre-appointment check: Many doctors ask for your BMI at annual check-ups. Calculating it in advance lets you arrive informed and ask better questions.
- Fitness baseline: Tracking BMI over time alongside body measurements gives a simple, repeatable number to monitor long-term weight management progress.
- Insurance and clinical forms: Some life insurance applications and health risk assessments request BMI. This calculator gives you the exact value instantly.
- Parenting and teen health: BMI for children and teens uses age- and sex-specific percentile charts — not the same thresholds as adults. Use this calculator as a reference starting point, then consult a pediatrician for accurate percentile interpretation.
- Meal planning and nutrition goals: Dietitians often use BMI alongside TDEE to set calorie targets. Knowing your BMI helps frame realistic, evidence-based nutrition goals.
Frequently Asked Questions
What is a healthy BMI for women?
The healthy BMI range for women (and men) according to WHO is 18.5 to 24.9. Research does show that women typically carry slightly more body fat at any given BMI compared to men, so context matters — but the target range is the same. Values between 18.5 and 24.9 are consistently associated with the lowest all-cause mortality risk in large population studies.
Is BMI accurate for athletes and muscular people?
BMI is a less reliable screening tool for highly muscular individuals. Because it only considers weight and height — not body composition — a bodybuilder or elite athlete can register as "overweight" or "obese" even with very low body fat. In these cases, body fat percentage measurement or DEXA scanning provides a more accurate picture. BMI is designed as a population-level screening tool, not an individual diagnostic measure.
How do I calculate BMI with height in feet and inches?
Use the imperial formula: BMI = (weight in lbs × 703) ÷ (height in inches)². For example, a person who is 5'9" (69 inches) and weighs 165 lbs: BMI = (165 × 703) ÷ (69²) = 116,000 ÷ 4,761 ≈ 24.4. Our calculator handles this automatically — just select "Imperial" and enter your feet and inches separately.
What BMI categories does the WHO define for adults?
The World Health Organization defines six categories for adults aged 20 and over. Underweight: BMI below 18.5. Normal weight: 18.5 to 24.9. Overweight: 25.0 to 29.9. Obesity Class I: 30.0 to 34.9. Obesity Class II: 35.0 to 39.9. Obesity Class III (severe): 40.0 and above. Children and adolescents use age- and sex-specific percentile charts instead.
Can BMI change with age?
For adults (20+), the same WHO thresholds apply regardless of age or sex. However, body composition changes with age — muscle mass typically decreases and body fat increases even if weight stays the same. This means two people with identical BMIs at different ages can have very different health profiles. Older adults with BMIs in the low normal range may actually have insufficient muscle mass, which is why clinicians consider BMI alongside other metrics.
Resources
- WHO — Body mass index (BMI) — Official WHO guidance on healthy weight and BMI thresholds used worldwide.
- CDC — About Adult BMI — The US Centers for Disease Control explanation of how BMI is interpreted for adults.
- NIH — Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity — The clinical reference for BMI categories used by healthcare providers in the United States.
