What Is BMI and Why It Matters
Our free BMI Calculator helps you quickly work out your Body Mass Index and see your healthy weight range using the standard WHO BMI categories. Just enter your age, height, and weight—then switch easily between metric (cm, kg) and US units (feet & inches, pounds)—to get an instant BMI score with a clear label (Underweight, Normal, Overweight, or Obesity), plus your healthy target range in kg and lb. Built to be fast, mobile-friendly, and privacy-focused, the calculator runs in your browser (we don’t store your inputs) and also shows helpful metrics like BMI Prime and Ponderal Index for extra context.
Body Mass Index (BMI) is a simple screening measure that compares your weight to your height to estimate whether you’re in a healthy weight range. While not perfect, it is widely used by the World Health Organization and the CDC as a quick health indicator.
How the BMI Calculator Works (Metric & US Units)
The BMI formula divides weight by height squared. Depending on the units you use:
- Metric: BMI = weight (kg) ÷ height² (m²)
- US: BMI = 703 × weight (lb) ÷ height² (in²)
Our calculator lets you switch instantly between the two systems and also calculates BMI Prime and the Ponderal Index for more context.
BMI Chart for Adults (WHO Categories)
Adults (age 20+) are typically classified using the following World Health Organization (WHO) BMI ranges:
| Classification |
BMI (kg/m²) |
| Severe Thinness |
< 16 |
| Moderate Thinness |
16 – 17 |
| Mild Thinness |
17 – 18.5 |
| Normal |
18.5 – 24.9 |
| Overweight |
25 – 29.9 |
| Obesity Class I |
30 – 34.9 |
| Obesity Class II |
35 – 39.9 |
| Obesity Class III |
≥ 40 |
Note: Some regions and health authorities may define slightly different cutoffs.
Why BMI Matters
Maintaining a healthy BMI is associated with lower risks of chronic diseases and better overall well-being. A BMI that is too high or too low can indicate potential health issues. Although it is not perfect, BMI is a practical way for most people to assess whether they may need to adjust their diet, exercise routine, or lifestyle.
Health Risks of High BMI
People with overweight or obesity are more likely to experience a range of health problems, including:
- High blood pressure and heart disease
- Type 2 diabetes
- Stroke
- Sleep apnea and breathing problems
- Joint issues like osteoarthritis
- Increased risk of certain cancers (colon, breast, kidney, liver)
Even modest weight reduction can improve health outcomes and reduce these risks.
Health Risks of Low BMI
Being underweight also carries potential dangers, such as:
- Weakened immune system and frequent illness
- Brittle bones and higher fracture risk (osteoporosis)
- Nutritional deficiencies and anemia
- Fertility problems in women
- Growth and development issues in children and teens
Risks associated with being overweight
Being overweight increases the risk of a number of serious diseases and health conditions. Below is a list of said risks, according to the Centers for Disease Control and Prevention (CDC):
- High blood pressure
- Higher levels of LDL cholesterol, which is widely considered "bad cholesterol," lower levels of HDL cholesterol, considered to be good cholesterol in moderation, and high levels of triglycerides
- Type II diabetes
- Coronary heart disease
- Gallbladder disease
- Osteoarthritis, a type of joint disease caused by breakdown of joint cartilage
- Sleep apnea and breathing problems
- Certain cancers (endometrial, breast, colon, kidney, gallbladder, liver)
- Low quality of life
- Mental illnesses such as clinical depression, anxiety, and others
- Body pains and difficulty with certain physical functions
- Generally, an increased risk of mortality compared to those with a healthy BMI
As can be seen from the list above, there are numerous negative, in some cases fatal, outcomes that may result from being overweight. Generally, a person should try to maintain a BMI below 25 kg/m2, but ideally should consult their doctor to determine whether or not they need to make any changes to their lifestyle in order to be healthier.
Limitations of BMI
BMI should not be used as the only measure of health. It does not distinguish between fat, muscle, or bone mass. For example:
- Athletes with high muscle mass may have a “high BMI” but low body fat.
- Older adults may have a “normal BMI” but higher body fat percentage due to loss of muscle.
- Different ethnic groups may face higher health risks at lower BMI levels.
Therefore, BMI is best considered alongside other factors such as waist circumference, diet, physical activity, and overall health assessments.
What is BMI Prime?
BMI Prime is a ratio of your BMI compared to the upper limit of the “normal” range (25 kg/m²). A value below 1.0 means you are within or below the healthy range, while values above 1.0 indicate overweight or obesity. This provides an easy way to see how far you are from the upper limit of the recommended range.
What is the Ponderal Index?
The Ponderal Index (PI) is similar to BMI but takes height into account differently by cubing the height instead of squaring it. PI can be more accurate than BMI for people who are very tall or very short, since BMI sometimes overestimates or underestimates body fat at height extremes.
Key Takeaway
BMI is a helpful starting point for evaluating weight and potential health risks, but it should not be viewed in isolation. Always consult a healthcare professional for a complete health assessment, especially if your BMI is outside the normal range or you have other health concerns.
Important Reminder
The FlowCuboStudio BMI Calculator is for educational purposes only and should not replace professional medical advice. Always speak with a qualified health provider about your results.
