Calculate your Body Mass Index instantly. Free, accurate, supports metric and imperial units for men and women worldwide.
Enter your height, weight and age then click Calculate
A BMI calculator is a free online tool that uses your height and weight to calculate your Body Mass Index — a widely used screening measure that helps determine whether your weight is in a healthy range for your height. The term “BMI calculator” is searched millions of times every month globally, making it one of the most used health tools on the internet. According to the Centers for Disease Control and Prevention (CDC), BMI is a calculated measure of weight relative to height and is categorized into underweight, healthy weight, overweight, and obesity.
Body Mass Index was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has since become the global standard for population-level weight classification. While it has important limitations (covered later in this guide), the BMI calculator remains the most practical and widely used first-line screening tool for assessing weight-related health risks — used by doctors, healthcare providers, public health organizations, and researchers worldwide.
⚡ Quick fact: According to the World Health Organization (WHO), more than 1 billion people worldwide are living with obesity as of 2024 — including 650 million adults, 340 million adolescents, and 39 million children. The BMI calculator is the primary tool used to identify and track this global health crisis.
Our BMI calculator uses the standard formula endorsed by the WHO and CDC. There are two versions depending on whether you use metric or imperial measurements:
BMI = weight (kg) ÷ height (m)²
Example: A person weighing 70kg who is 1.75m tall: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 — Normal weight.
BMI = (weight in lbs × 703) ÷ height in inches²
Example: A person weighing 154 lbs who is 5ft 9in (69 inches) tall: BMI = (154 × 703) ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7 — Normal weight.
According to the American Cancer Society, both formulas produce equivalent results and our BMI calculator supports both metric and imperial units for users worldwide.
Once your BMI calculator produces a result, it falls into one of the following categories defined by the World Health Organization. These are the globally accepted standards used by healthcare providers worldwide:
| BMI Category | BMI Range (kg/m²) | Health Risk Level |
|---|---|---|
| Severe Thinness | Below 16 | Very High Risk |
| Moderate Thinness | 16.0 – 16.9 | High Risk |
| Mild Thinness (Underweight) | 17.0 – 18.4 | Moderate Risk |
| Normal / Healthy Weight | 18.5 – 24.9 | Low Risk |
| Overweight | 25.0 – 29.9 | Increased Risk |
| Obese Class I | 30.0 – 34.9 | High Risk |
| Obese Class II | 35.0 – 39.9 | Very High Risk |
| Obese Class III (Severe) | 40.0 and above | Extremely High Risk |
It is important to note that BMI categories are the same for both men and women aged 20 and over. However, children and teenagers use different BMI-for-age percentile charts (see the children’s section below).
A healthy BMI calculator result falls between 18.5 and 24.9 kg/m². This range is associated with the lowest risk of weight-related health problems for most adults. However, “healthy” is a nuanced concept — as the National Heart, Lung, and Blood Institute (NHLBI) emphasizes, BMI is just one piece of the puzzle. Your healthcare provider will consider BMI alongside other factors including blood pressure, cholesterol levels, blood glucose, family history, physical activity, and diet.
The CDC recommends that adults aim to maintain a BMI within the healthy weight range through a combination of balanced nutrition and regular physical activity. Research consistently shows that maintaining a BMI in the 18.5–24.9 range significantly reduces the risk of cardiovascular disease, type 2 diabetes, certain cancers, and other chronic conditions.
If your BMI calculator result places you in the overweight or obese category, it is important to understand the associated health risks. According to the CDC’s comprehensive obesity research, being overweight significantly increases the risk of the following conditions:
The American Cancer Society notes that maintaining a healthy weight is one of the most important things you can do to reduce your cancer risk — second only to not smoking.
A BMI calculator result below 18.5 also carries significant health risks that are often underappreciated. Being underweight can indicate malnutrition, an underlying medical condition, or an eating disorder. The associated risks include:
While our BMI calculator is a valuable screening tool, it is important to understand its limitations. In 2023, the American Medical Association (AMA) passed a landmark policy statement acknowledging that BMI has significant limitations as a health metric and should not be used as the sole diagnostic criterion for assessing a patient’s health.
🏥 2023 AMA Policy: The American Medical Association officially recognized that BMI “is an imperfect measure” and called for physicians to use it in combination with other valid measures of risk such as waist circumference, visceral fat measurements, body composition, and metabolic markers. This represents a significant shift from using BMI as a standalone diagnostic tool.
One of the most important developments in BMI calculator science is the recognition that standard BMI cutoffs may not apply equally across different ethnic groups. This is particularly important for people of Asian descent.
The WHO has published specific recommendations for Asian populations, noting that health risks associated with excess body fat begin at lower BMI values. WHO expert consultation recommends considering the following alternative cutoffs for Asian populations:
| BMI Category | Standard WHO Cutoff | Asian Population Cutoff |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Normal weight | 18.5 – 24.9 | 18.5 – 22.9 |
| At risk / Overweight | 25.0 – 29.9 | 23.0 – 27.4 |
| Obese | 30.0 and above | 27.5 and above |
This means a person of South Asian, East Asian, or Southeast Asian descent with a BMI of 24 would be classified as “normal weight” by standard WHO criteria, but may already be at increased risk of type 2 diabetes and cardiovascular disease according to Asian-specific guidelines.
The standard BMI calculator formula is the same for children and adults, but the interpretation is completely different. For children and teens aged 2 through 19, BMI is assessed using age- and sex-specific BMI-for-age percentiles rather than fixed cutoff values. This is because body fat changes substantially with age and differs between boys and girls during growth and development.
According to the CDC’s child and teen BMI calculator guidelines, the following percentile ranges are used:
Parents and caregivers concerned about a child’s BMI should always consult a pediatrician rather than using adult BMI categories to assess children’s health.
Our BMI calculator measures the ratio of weight to height, but it does not directly measure body fat. Body fat percentage is a more precise measure of health — it tells you what proportion of your body weight consists of fat tissue versus lean tissue (muscle, bone, organs, water).
Healthy body fat percentage ranges differ significantly by age and sex:
| Age Group | Healthy Body Fat % (Men) | Healthy Body Fat % (Women) |
|---|---|---|
| 20–39 years | 8% – 19% | 21% – 32% |
| 40–59 years | 11% – 21% | 23% – 33% |
| 60–79 years | 13% – 24% | 24% – 35% |
While body fat percentage provides more detailed information, it requires specialized equipment (DEXA scan, hydrostatic weighing, or bioelectrical impedance) to measure accurately. The BMI calculator remains the most accessible and practical screening tool for most people.
If your BMI calculator result falls outside the healthy range, here are evidence-based strategies recommended by Harvard Health Publishing:
The science around the BMI calculator and its use continues to evolve. Here are the most significant recent developments:
In June 2023, the American Medical Association adopted a landmark new policy on BMI, officially recognizing that “BMI is an imperfect measure due to its historical exclusion of diverse populations and sexes.” The AMA now recommends using BMI as one of many tools — alongside waist circumference, visceral fat measurements, body composition assessments, and metabolic health markers — to assess weight-related health risks.
In 2024, a major Lancet commission proposed new diagnostic criteria for obesity that go beyond BMI. The commission recommended that obesity should be diagnosed based on both excess body fat AND evidence of health impairment — not BMI alone. This represents a potential paradigm shift in how obesity is defined and treated medically.
The WHO continues to use BMI as the primary population-level screening tool while acknowledging its limitations. Updated 2025 WHO guidance emphasizes that waist-to-height ratio (WHtR) may be a more sensitive predictor of cardiometabolic risk than BMI alone, particularly for identifying central obesity which carries the highest health risks.
A normal or healthy BMI calculator result for adults aged 20 and over is between 18.5 and 24.9 kg/m². This range is defined by the WHO and CDC as being associated with the lowest risk of weight-related health problems. A BMI below 18.5 is considered underweight, 25–29.9 is overweight, and 30 or above is classified as obese.
The BMI calculator uses the same formula and the same healthy range for both men and women. However, it has known limitations — women naturally carry more body fat than men at the same BMI, and muscular individuals of either sex may have a high BMI despite being metabolically healthy. For this reason, BMI is best used as a screening tool rather than a definitive health assessment.
Using our free BMI calculator is simple. Select your preferred unit system (metric or imperial), enter your height and weight, add your age and gender, then click Calculate. Your BMI result, category, healthy weight range, and BMI prime will appear instantly. No sign-up or registration is required.
BMI Prime is the ratio of your actual BMI to the upper limit of the normal BMI range (25 kg/m²). A BMI Prime of 1.0 means your BMI is exactly at the healthy weight upper limit. Below 1.0 is healthy, above 1.0 means overweight or obese. It allows for a quick intuitive comparison — a BMI Prime of 1.2, for example, means your BMI is 20% above the healthy upper limit.
The BMI formula is the same for children, but the interpretation is different. Children and teenagers aged 2–19 should use a BMI-for-age percentile chart rather than adult BMI categories. The CDC provides separate BMI calculators for children and teens. Our calculator is designed for adults aged 18 and over.
The healthy weight range depends on your height. For example, for a person who is 5ft 7in (170cm) tall, the healthy weight range is approximately 57–72kg (126–159 lbs), corresponding to a BMI of 18.5–24.9. Our BMI calculator automatically shows your specific healthy weight range based on your entered height.
Yes — according to WHO and CDC classification, a BMI of exactly 25 falls into the overweight category (25.0–29.9). However, being at 25 is only marginally above the healthy range and may not represent a significant health risk on its own, particularly for muscular individuals. A BMI of 24.9 is classified as healthy weight. Your doctor can help you interpret your specific result in the context of your overall health.
Most health professionals recommend checking your BMI every 3–6 months if you are actively trying to change your weight, or annually as part of a general health check. BMI changes slowly — it is not a daily tracking metric. Focus on consistent lifestyle habits (nutrition, exercise, sleep) rather than frequent BMI measurements, which can create unnecessary anxiety.
No — the standard BMI calculator does not distinguish between weight from fat and weight from muscle. This is one of the most cited limitations of BMI. A bodybuilder or strength athlete may have a BMI in the overweight or obese range despite having very low body fat and excellent metabolic health. In such cases, body fat percentage measurement or waist circumference provides more meaningful health information.