Free BMI Calculator
Body Mass Index with WHO health categories, ideal weight range, and metric or imperial units — using CDC and NHLBI standards.
Under18.5–24.9
Healthy25–29.9
Over30–34.9
Obese I≥35
Obese II+
Your healthy weight range
What is BMI?
Body Mass Index (BMI) is a screening tool that estimates whether your body weight is proportional to your height. It’s calculated with a simple formula — weight in kilograms divided by height in meters squared — and the resulting number is placed on a scale defined by the World Health Organization (WHO), endorsed by the US CDC, the National Heart, Lung and Blood Institute (NHLBI), and the American Heart Association. This free BMI calculator supports both metric (kg + cm) and imperial (lb + ft/in) units and shows your result on a color-coded scale with your ideal healthy weight range for your height.
BMI categories (adult, WHO / CDC standard)
| Category | BMI range (kg/m²) | Health risk |
|---|---|---|
| Underweight | Below 18.5 | Increased risk of nutritional deficiency and low bone density |
| Healthy weight | 18.5 – 24.9 | Lowest average risk |
| Overweight | 25.0 – 29.9 | Elevated risk of hypertension, type 2 diabetes, heart disease |
| Obesity class I | 30.0 – 34.9 | High |
| Obesity class II | 35.0 – 39.9 | Very high |
| Obesity class III | 40.0 and above | Extremely high (severe / morbid obesity) |
How to use the BMI calculator
- Pick your unit system — metric (kg + cm) or imperial (lb + ft/in) using the toggle at the top.
- Enter your weight and height. Age and gender are optional but useful context for interpreting the result.
- Click Calculate BMI. See your value on a color-coded 5-band scale, your health category, and the ideal healthy weight range for your specific height.
The BMI formula in detail
The formula in metric units: BMI = weight (kg) ÷ [height (m)]². So a 70 kg person who is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9, which is in the healthy range. In imperial units, the formula is: BMI = 703 × weight (lb) ÷ [height (in)]². The 703 conversion factor accounts for the difference between metric and imperial units.
Important limitations of BMI
BMI is a fast, cheap screening test that works well at the population level. It has known limitations for individuals:
It doesn’t distinguish muscle from fat
An NFL linebacker with 8% body fat can register a BMI of 32 (obese) because muscle is denser than fat. Bodybuilders, competitive athletes, and people who lift heavy weights routinely score “overweight” on BMI while carrying little actual fat.
It doesn’t account for body composition changes with age
Older adults tend to lose muscle and gain fat even at stable weight. A “healthy” BMI in a 75-year-old may hide sarcopenia (muscle loss) and elevated visceral fat.
It doesn’t apply to pregnant or breastfeeding women
Adult BMI categories were developed for non-pregnant adults. During pregnancy, weight gain is expected and healthy; different guidelines apply.
It doesn’t apply to children the same way
Children under 20 use BMI-for-age percentile charts, not the adult categories. A child’s BMI is compared against the CDC growth chart for their exact age and sex. The 5th to 85th percentile is considered healthy.
Asian populations may need adjusted cutoffs
Research on Asian body composition has led some countries (India, China, Japan, Singapore) to adopt lower cutoffs — often 23 for “overweight” and 27.5 for “obese.” If you have Asian heritage, discuss adjusted cutoffs with your physician.
What to combine BMI with for a fuller picture
- Waist circumference — measures abdominal fat, which is a stronger predictor of metabolic disease than BMI. Adult men: aim for under 94 cm (37 in); women: under 80 cm (31.5 in).
- Waist-to-height ratio — divide waist by height. Aim for under 0.5. This single number outperforms BMI in predicting cardiovascular risk.
- Body fat percentage — measured via DEXA scan, bioelectrical impedance, or skin-fold calipers. Healthy ranges: men 10–20%, women 18–28%.
- Metabolic markers — blood pressure, fasting glucose, HbA1c, HDL and LDL cholesterol, triglycerides. Someone with “overweight” BMI but perfect metabolic markers is at lower risk than someone with “healthy” BMI and poor markers.
Real-world use cases
Personal health tracking — Monthly BMI checks show weight trends over time. Weekly or daily is too noisy — fluid, salt, and food weight can move BMI by 0.5 in a day.
Doctor’s appointments — BMI is often the first metric your doctor calculates. Knowing yours in advance saves time and lets you ask more informed questions.
Insurance quotes — Life insurance premiums are partly BMI-based. Under-25 typically qualifies for best rates; 25–30 is standard; 30+ triggers higher premiums.
Fitness planning — Coaches use BMI as a rough starting point for setting calorie and macro targets, but always in combination with body composition measurements.
BMI calculator comparison
| Feature | This tool | NIH calculator | Doctor’s chart |
|---|---|---|---|
| Metric + Imperial | ✅ | ✅ | Sometimes |
| Visual color-coded scale | ✅ | Text only | Basic |
| Ideal weight range shown | ✅ | ❌ | Sometimes |
| Sign-up required | Never | Never | Appointment |
| Data stored/uploaded | Never | Government site | Medical record |
| Cost | Free | Free | Copay/insurance |
Frequently asked questions (FAQs)
Often no. Muscle is denser than fat, so muscular athletes can register “overweight” while having low body fat. For athletes, use waist circumference or a DEXA body composition scan for a truer picture.
The WHO adult standard: below 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 or above obese. Some Asian countries use adjusted lower cutoffs (23 overweight, 27.5 obese).
The healthy range (18.5–24.9) is identical for both sexes. Body-fat percentage targets differ by sex (women have higher essential fat), but BMI itself does not distinguish.
Not with adult categories. Children use BMI-for-age percentiles (5th to 85th is healthy) that vary by age and sex. Consult a pediatrician or use the CDC’s child-specific BMI calculator.
Monthly is plenty. Daily fluctuations of 1–2 kg from hydration, salt, and meals move BMI trivially. Track the trend over 4–12 weeks, not day to day.
No. Every calculation runs in JavaScript on your device. Nothing is sent to any server or stored anywhere except your own browser’s memory for the current session.