Body Mass Index (BMI) is a screening metric that relates body weight to height. It is commonly used to categorize weight status in adults, but it does not directly measure body fat or body composition.

Is BMI an accurate measure of health?

BMI can be useful at a population level, but it has limitations for individuals. It does not distinguish between muscle and fat, and it does not account for fat distribution, fitness level, or individual variation.

Why might BMI be misleading for muscular people?

BMI is based only on height and weight. Individuals with higher muscle mass may have a higher BMI despite having low body fat and good overall fitness.

BMI Calculator

Q: Do you store my data?

No. This BMI calculator runs entirely in your browser. No account is required and no personal data is collected or stored.

Estimate Body Mass Index (BMI) using a standard adult equation. BMI compares weight to height and is commonly used as a broad screening reference at the population level.

For educational purposes only. These are general estimates, not medical, nutritional, or training advice.

Calculate Your BMI

Choose units, enter height and weight, then view BMI and the standard adult category label.

How to Use This BMI Calculator

Choose imperial or metric units, enter your height and weight, then calculate your BMI. BMI is best interpreted as a broad reference — not a direct measurement of body fat, fitness, or individual health.

Use consistent units so results are comparable over time.
Focus on trends instead of overreacting to one result.
Interpret in context with other consistent indicators when possible.

What BMI Measures

Body Mass Index (BMI) is a height-adjusted weight metric. It converts weight and height into one number so large groups can be compared using a standardized scale. Because it’s simple and consistent, BMI is widely used in public health research and basic screening contexts.

How BMI Is Calculated

BMI uses the same underlying concept in both systems. Imperial calculations include a conversion factor to match metric units.

Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lb) ÷ height (in)²

Standard Adult Category Ranges

These labels are commonly used to describe BMI ranges. They are best understood as broad screening categories — not a direct measurement of body fat, fitness level, or individual health status.

Underweight: < 18.5
Normal: 18.5–24.9
Overweight: 25.0–29.9
Obesity: 30.0+

Why BMI Can Be Misleading

BMI cannot separate lean mass from fat mass. Two people can share the same BMI while having very different body compositions. That’s one reason BMI is generally more reliable at the group level than it is for interpreting one individual in isolation.

Muscular builds: higher lean mass can raise BMI even when body fat is relatively low.
Body proportions: frame size and limb length can change how weight “reads” at the same height.
Fat distribution: BMI does not describe where weight is carried.
Population differences: typical body composition patterns vary, but BMI uses one set of cutoffs.

Interpreting BMI in Context

A practical way to view BMI is as a rough signal that may or may not match what’s happening in the body. When people talk about interpreting BMI responsibly, they typically mean combining it with other repeatable indicators (for example: long-term trend data, waist measurements, strength/performance trends, or body composition estimates).

Common Misinterpretations (And What BMI Can’t Tell You)

BMI is easy to calculate, which is why it’s used so often — but that simplicity also leads to common misunderstandings. Treat BMI as a rough screening reference, not a complete picture of health, fitness, or body composition.

BMI is not body fat percentage: it does not measure fat mass directly and can’t separate fat from lean tissue.
A category label is not a diagnosis: “overweight” or “obesity” are broad screening terms, not a medical conclusion on their own.
Muscle can inflate BMI: lifters and athletes can land in higher categories with relatively low body fat.
Short-term weight changes can mislead: hydration, sodium, carbs, and digestion can shift scale weight without real fat change.
Where weight is carried matters: BMI does not capture fat distribution (for example, waist changes) which can affect how risk is discussed.
Two people can share the same BMI and look very different: frame size, limb length, and muscle mass change what “the same BMI” means in practice.

When Another Metric Is More Useful

If your goal is understanding body composition or tracking change over time, BMI is often most helpful when paired with other repeatable indicators. For example, waist trends and body composition estimates can add context, and performance trends can show whether training is moving in the right direction. If you want a related tool, compare trends with the Body Fat Calculator.

Related Calculators

BMI is one simple screening metric. These tools can add context using different models.

Body Fat Calculator — compare BMI with a body composition estimate.
Ideal Weight Calculator — view common reference ranges by height.
TDEE Calculator — estimate maintenance calories based on activity.
Macro Calculator — translate calories into protein/carbs/fats targets.

BMI Calculator FAQ

Is BMI accurate for athletes?

BMI can be misleading for very muscular people because it cannot separate lean mass from fat mass. Two people can share the same BMI while having very different body compositions.

Why can BMI categories feel “off” compared to how I look?

Category labels are broad. Water retention, muscle mass, and body proportions can all change how weight relates to appearance and health markers.

Do you store my data?

No. This calculator runs locally in your browser. No account is required and no personal data is collected.

Reviewed & Updated

Calculator logic and on-page content reviewed for clarity and educational accuracy. Last review: December 2025.