Instantly check your Body Mass Index, weight category, and healthy weight range — in metric or imperial units, based on WHO standards.
Results update live as you type.
Your Body Mass Index
Category
Normal
healthy zone
Healthy range
125 – 169
lbs at 5'9"
To healthy weight
0 lbs
you are on target
Ponderal index
13.4
kg/m³
| Category | BMI range | Health risk |
|---|---|---|
| Underweight | Below 18.5 | Malnutrition risk |
| Normal weight | 18.5 – 24.9 | Low risk |
| Overweight | 25.0 – 29.9 | Moderate risk |
| Obese (Class I) | 30.0 – 34.9 | High risk |
| Obese (Class II) | 35.0 – 39.9 | Very high risk |
| Obese (Class III) | 40.0 + | Extremely high risk |
The Formula
Body Mass Index is a simple ratio of your weight to your height squared. Devised by Belgian statistician Adolphe Quetelet in the 1830s and adopted by the World Health Organization as the standard adult screening tool, it gives a quick way to classify whether a person's weight is in a healthy range for their height.
Formula
About This Tool
A BMI calculator — also called a body mass index calculator or simply a weight calculator — is a free health screening tool that estimates how appropriate your weight is for your height. Enter your height, weight, age, and sex and the calculator returns your BMI to one decimal place, your WHO category, and the healthy weight range associated with your height.
Our online BMI calculator for adults uses the World Health Organization classification thresholds: underweight under 18.5, healthy 18.5–24.9, overweight 25–29.9, and obese 30 and above. It supports both metric (kg, cm) and US imperial (lbs, ft/in) units, with instant live results as you change any value.
BMI is not a perfect measurement — it does not directly measure body fat, distinguish muscle from fat, or account for athletes, pregnant women, the elderly, or different ethnic body types. But for the general population, it is a fast, evidence-based starting point used by doctors, nutritionists, and public-health bodies worldwide.
Use this free BMI calculator to track your number over time, see how losing or gaining a few pounds changes your category, and identify a sensible target weight range. All calculations run entirely in your browser — no sign-up and no data collected.
Instant Live Results
BMI updates in real time as you type or drag a slider — no submit button needed.
Metric & Imperial
Switch between US (lbs, ft/in) and metric (kg, cm) with one click.
WHO Categories
Underweight, normal, overweight, and three obese sub-classes — exactly as the WHO defines them.
Healthy Range
See the exact weight range that places you in the healthy BMI band for your height.
100% Free & Private
No account, no tracking — every calculation runs locally in your browser.
Visual Gauge
A colour-coded gauge instantly shows where you sit on the BMI spectrum.
Four simple inputs give you a complete BMI assessment in seconds.
Toggle between US / Imperial (feet, inches, pounds) and Metric (centimetres, kilograms). The calculator converts automatically — pick whichever unit you have handy.
Type your height directly in the field. For imperial, fill in feet and inches separately. For metric, just enter your height in centimetres. Be as accurate as possible — half an inch matters.
Add your current weight (pounds or kilograms). For best accuracy, weigh yourself first thing in the morning, after using the bathroom and before breakfast. Tip: drag the slider to see how your BMI changes with weight.
Provide your age (must be 20+ for adult BMI categories) and biological sex. These don't change the BMI number itself, but help us flag if BMI may be a less reliable indicator for your situation.
Your BMI number, category, and healthy weight range update instantly. The colour gauge shows where you sit on the spectrum from underweight to severely obese.
Compare your number against the category table to see your health-risk band. Use the highlighted healthy range as a sensible weight target, and pair BMI with waist measurement or body fat percentage for a fuller picture.
Everything you need to know about BMI, the body mass index formula, and how to interpret your result.
BMI (Body Mass Index) is a screening number that estimates whether a person's body weight is appropriate for their height. It is calculated by dividing weight in kilograms by the square of height in metres (kg/m²). BMI does not directly measure body fat percentage, but for the general population it correlates well with body fat and is associated with health risks like heart disease, type 2 diabetes, and high blood pressure. The World Health Organization, CDC, and NHS all use BMI as a first-pass screening tool for adults aged 20 and older.
The BMI formula in metric units is: BMI = weight (kg) ÷ height (m)². In imperial units it becomes: BMI = (weight (lbs) × 703) ÷ height (in)². So a person who is 175 cm tall and weighs 72 kg has a BMI of 72 ÷ (1.75)² = 72 ÷ 3.0625 = 23.5, which falls in the healthy range. Our BMI calculator handles both unit systems and the conversion for you instantly.
The World Health Organization classifies adult BMI as follows: Below 18.5 — underweight; 18.5 to 24.9 — healthy / normal weight; 25.0 to 29.9 — overweight; 30.0 to 34.9 — Obese Class I; 35.0 to 39.9 — Obese Class II; 40.0 or higher — Obese Class III (severe / morbid obesity). These bands apply to adults of all ethnicities, though some health bodies use slightly lower cutoffs (23+ overweight, 27.5+ obese) for people of South Asian, East Asian, and Polynesian heritage who develop metabolic risks at lower BMI levels.
BMI is a useful screening tool but it has real limitations. Because it uses only height and weight, it cannot distinguish muscle from fat. A muscular athlete or bodybuilder may have a BMI of 28+ and be classified as overweight despite having very low body fat. Conversely, an inactive person of "normal" BMI may carry a high proportion of body fat (sometimes called "skinny fat"). BMI is also less reliable for the elderly (who lose muscle), pregnant women, growing children, and people of certain ethnic backgrounds. For a complete picture pair your BMI result with a waist measurement, body fat percentage, and a conversation with your doctor.
The healthy BMI range is identical for men and women: 18.5 to 24.9. However, at any given BMI women naturally carry a slightly higher percentage of body fat than men because of essential reproductive fat stores. Women's healthy body fat percentage is around 21-32% and men's is around 8-19%. This is why two people with the same BMI can look quite different, and why body fat percentage or waist-to-hip ratio can complement the BMI number — especially for very lean or very muscular individuals.
This BMI calculator is designed for adults aged 20 and older. For children and teenagers (aged 2-19), BMI is interpreted using age- and sex-specific percentile charts published by the CDC and WHO rather than fixed adult cutoffs. A child's BMI changes naturally with growth, so a number that looks "high" for an adult may be perfectly normal for a 10-year-old. For paediatric BMI assessment please use a dedicated children's BMI percentile calculator or consult your child's paediatrician.
According to the WHO and CDC: a BMI of 25.0 to 29.9 is classified as overweight, and a BMI of 30.0 or higher is classified as obese. Obesity is further divided into three classes: Class I (30-34.9), Class II (35-39.9), and Class III, sometimes called "severe" or "morbid" obesity (40+). The higher your obesity class, the greater the statistical risk of type 2 diabetes, heart disease, hypertension, sleep apnoea, fatty liver disease, joint problems, and several cancers. Even modest weight loss of 5-10% of body weight produces measurable health improvements.
Healthy, sustainable weight loss requires a moderate calorie deficit (around 500 kcal/day below maintenance) achieved through a combination of nutrition and activity. Aim for: 0.5-1 kg (1-2 lbs) loss per week; protein at 1.6-2.2 g/kg of body weight to preserve muscle; resistance training 2-4 times per week; 7-9 hours of sleep; and at least 150 minutes of moderate cardio per week. Crash diets typically rebound — slow and consistent change wins. Use our calorie calculator and TDEE calculator to find your ideal daily intake, and re-check your BMI every 2-4 weeks rather than daily.
BMI is a ratio of mass to height — quick, free, and based only on a scale and a measuring tape. Body fat percentage measures the actual proportion of your body that is fat tissue, which requires more sophisticated tools: skinfold callipers, bioelectrical impedance scales, hydrostatic weighing, BodPod, or DEXA scanning. Body fat percentage is more accurate for athletes and people with significant muscle mass, but it is harder to measure consistently at home. Most people benefit from using both — track BMI weekly for the trend, and check body fat percentage every few months.
Large epidemiological studies consistently show that very low and very high BMI are both associated with higher all-cause mortality and increased risk of chronic disease. The lowest mortality risk in most adult populations sits between BMI 20 and 25. That said, BMI alone is not a diagnosis. A doctor evaluating your overall health risk will also look at waist circumference, blood pressure, blood lipids, blood glucose, family history, smoking status, and fitness level. A high BMI in a physically active, metabolically healthy adult carries different risk than the same BMI in a sedentary adult with high blood pressure.
The BMI formula itself does not change, but the interpretation does. Athletes, bodybuilders, and weight-trained individuals often carry significantly more muscle than the average adult, which weighs more than fat. Because BMI only sees total weight, a 95 kg, 180 cm rugby player can register a BMI of 29 (overweight) despite having 10% body fat. For athletes and very muscular people, body fat percentage and waist-to-height ratio are more meaningful than BMI alone.
The math is exact — our online BMI calculator applies the standard WHO formula and produces a result accurate to one decimal place for the height and weight you provide. Real-world accuracy depends entirely on the quality of your measurements: weigh yourself at the same time of day (ideally first thing in the morning), wearing minimal clothing, on a calibrated scale, and measure your height with shoes off against a wall. With good measurements your calculated BMI will match what a clinician produces in person. This calculator is intended for educational and screening use and is not a substitute for medical advice.