BMI Calculator
Work out your Body Mass Index in seconds using metric or imperial units, see exactly where it falls on the standard scale, and understand what the result actually means — and doesn’t mean.
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Your BMI, category, and a plain-language explanation will appear here once you calculate.
Your BMI result
- Underweight<18.5
- Healthy18.5–24.9
- Overweight25–29.9
- Obesity≥30
What this means
BMI is a screening tool, not a diagnosis. It does not directly measure body fat and can be misleading for athletes, pregnant people, children and teens, older adults, and people with high muscle mass. Speak with a doctor or qualified health professional to interpret your result and discuss any concerns.
What Is BMI?
Body Mass Index (BMI) is a simple screening number calculated from a person’s height and weight. It was developed in the 19th century by the Belgian mathematician Adolphe Quetelet as a quick way to describe body size across large populations, and it’s still used today by doctors, researchers, and public health agencies as a fast, low-cost starting point for thinking about weight.
BMI does not measure body fat, muscle, or overall health directly. It is best understood as a first-pass screening tool: a number that can flag when a closer conversation with a healthcare professional might be worthwhile, rather than a verdict on anyone’s health.
How Is BMI Calculated?
BMI uses one formula worldwide. In metric units:
BMI = weight (kg) ÷ height (m)²
For example, someone who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.06 ≈ 22.9.
In imperial units, the formula is adjusted with a conversion factor:
BMI = (weight in pounds ÷ height in inches²) × 703
Both formulas produce the same result — the calculator above simply converts feet/inches and pounds into meters and kilograms behind the scenes before applying the metric formula, so you never need to do the conversion by hand.
BMI Categories
For adults aged 20 and over, the World Health Organization and most national health bodies use the same four standard categories:
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity | 30.0 and above |
Obesity is sometimes further divided into class I (30.0–34.9), class II (35.0–39.9), and class III (40.0 and above), which clinicians may use to guide the intensity of a treatment conversation. These sub-classes are informational only and are not shown as a separate result above.
What Does My BMI Mean?
Your BMI category is a general indicator, not a personal health assessment. Here’s how each range is typically interpreted at a population level:
Underweight (below 18.5)
May be associated with nutritional deficiencies or an underlying condition, but can also simply reflect a naturally lean body type. Worth mentioning at a routine check-up, especially alongside symptoms like fatigue or unintended weight loss.
Healthy weight (18.5–24.9)
Associated, on average, with lower risk of weight-related conditions. It does not guarantee good health, and someone in this range can still have risk factors such as high blood pressure or low fitness.
Overweight (25.0–29.9)
May be worth discussing with a healthcare provider, particularly alongside other factors like waist circumference, blood pressure, and family history.
Obesity (30.0 and above)
Associated with higher average risk of conditions such as type 2 diabetes and cardiovascular disease, though individual risk varies widely. A healthcare professional can help interpret what this means for you specifically.
Limitations of BMI
BMI is popular because it’s quick and requires no special equipment, but it has real, well-documented blind spots:
- It can’t tell fat from muscle. Athletes and people with high muscle mass are often classified as “overweight” or “obese” despite low body fat.
- It ignores where fat is stored. Visceral (abdominal) fat carries different health risks than fat stored elsewhere, and BMI can’t distinguish between them.
- It’s not designed for pregnancy. Healthy weight gain during pregnancy follows its own guidelines, not adult BMI categories.
- It’s not designed for children and teens. Growing bodies need age- and sex-specific percentile charts instead.
- It can be less reliable for older adults. Natural muscle loss with age can make BMI look “healthier” than a person’s actual body composition suggests.
- Population differences matter. Some research indicates health risks appear at lower BMI thresholds in certain populations, including many people of Asian descent, which is why some health authorities recommend adjusted cutoffs for these groups.
For these reasons, doctors typically use BMI as one input among several — alongside waist circumference, blood pressure, blood sugar, cholesterol, and personal and family medical history — rather than relying on it alone.
BMI for Different Age Groups
| Age group | How BMI is typically used |
|---|---|
| Children & teens (2–19) | Assessed with BMI-for-age percentile charts specific to sex and age, not the fixed adult categories. A pediatrician should interpret these results. |
| Adults (20–64) | The standard adult categories on this page generally apply, though individual factors such as muscle mass and body frame still matter. |
| Older adults (65+) | Some research suggests a slightly higher BMI may be linked to better outcomes in this group, partly because of natural muscle loss (sarcopenia). Clinicians often weigh BMI alongside strength, mobility, and nutrition. |
If you entered an age under 18 or 65 and over above, keep this context in mind when reading your result, and treat it as a conversation starter with a healthcare provider rather than a final answer.
Frequently Asked Questions
What is a healthy BMI range?
For most adults, a BMI between 18.5 and 24.9 falls into the healthy weight range. This range is associated, on average, with lower risk of weight-related health conditions, though individual health depends on many other factors besides BMI.
Is BMI the same as body fat percentage?
No. BMI only uses height and weight, so it cannot distinguish between fat, muscle, and bone. Two people with the same BMI can have very different body compositions. Body fat percentage requires a different type of measurement, such as skinfold calipers, bioelectrical impedance, or a DEXA scan.
Does BMI apply the same way to everyone?
Not exactly. BMI tends to be less accurate for athletes and people with high muscle mass, pregnant people, growing children and teens, and older adults who have lost muscle mass. Some research also suggests health risks appear at lower BMI values in certain populations, including many people of Asian descent.
What should I do if my BMI is outside the healthy range?
A BMI outside the 18.5 to 24.9 range is not a diagnosis. It simply means it may be worth discussing your weight, lifestyle, and overall health with a doctor or registered dietitian, who can consider your full health picture rather than one number alone.
Is BMI calculated differently for men and women?
No. The BMI formula and the standard adult category cutoffs are the same for men and women. However, men and women naturally tend to carry body fat differently, which is one reason BMI alone does not fully describe an individual’s health.
How is BMI different for children and teens?
Children and teens are not assessed using the adult BMI categories on this page. Instead, clinicians use BMI-for-age percentiles that account for a child’s age and sex, since growth patterns change constantly. A pediatrician is the right person to interpret a child’s BMI.
How often should I calculate my BMI?
There is no fixed schedule. Many people find it useful to check BMI occasionally, such as during a routine health check-up, rather than tracking it frequently. Your doctor can advise how often it makes sense to monitor for your situation.
Can I use this calculator during pregnancy?
This calculator can perform the arithmetic, but standard BMI categories are not designed for pregnancy, since healthy weight gain varies by trimester and pre-pregnancy weight. Prenatal weight guidance should come from your obstetrician or midwife.