BMI Calculator
Calculate your Body Mass Index (BMI) - a screening tool that measures weight relative to height. Learn what your BMI means and why it should be just one part of understanding your health.
BMI Categories (CDC)
- Underweight:
- Below 18.5
- Normal weight:
- 18.5 - 24.9
- Overweight:
- 25.0 - 29.9
- Obese:
- 30.0 - 39.9
- Severely obese:
- 40.0+
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Fill out the form to calculate your BMI
What is BMI?
Body Mass Index (BMI) is a screening tool that uses your weight and height to estimate whether you fall into a healthy weight range. According to the CDC, BMI is "a measure of weight relative to height" used for screening potential weight-related health risks at both individual and population levels.
While BMI has been widely used since the 1970s as a simple, quick assessment tool, it's important to understand it was originally developed for population-level statistics, not individual health diagnosis.
How is BMI Calculated?
BMI uses a simple mathematical formula based on weight and height. The calculation is the same for adults of all ages and both sexes.
Metric: BMI = weight (kg) / height (m)²
Example: 70 kg / (1.75 m)² = 22.9
Imperial: BMI = (weight (lb) / height (in)²) × 703
Example: (154 lb / 69 in²) × 703 = 22.7
BMI Categories
The CDC and WHO define standard BMI categories for adults 20 years and older:
| Category | BMI Range |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 - 24.9 |
| Overweight | 25.0 - 29.9 |
| Obese | 30.0 - 39.9 |
| Severely obese | 40.0 and above |
Serious Limitations of BMI
While BMI is widely used, it has significant scientific limitations that you should understand:
❌ Doesn't Distinguish Muscle from Fat
BMI can't tell the difference between muscle mass and fat mass. As noted by Stanford Medicine researchers in 2024, a powerlifter with substantial muscle mass might be classified as obese despite having very low body fat. Many athletes and people who strength train are incorrectly categorized as overweight or obese.
❌ Ignores Body Composition and Fat Distribution
BMI doesn't account for where fat is stored in your body. Visceral fat (around organs) poses greater health risks than subcutaneous fat (under skin), but BMI treats all weight equally. It also fails to consider bone density, muscle distribution, and overall body composition.
❌ Not Accurate Across Different Demographics
According to a 2025 study in the International Journal of Behavioral Nutrition and Physical Activity, BMI thresholds for health risks vary significantly by ethnicity, age, and gender. Asian populations may face health risks at lower BMIs than the standard categories suggest, while the relationship between BMI and mortality risk differs for Hispanic populations. Early BMI research primarily studied non-Hispanic white men, making it less applicable to other groups.
❌ Gender Differences Not Accounted For
Women and men naturally store fat differently and have different muscle mass distributions, but BMI uses the same formula for everyone. This means BMI may be even less accurate for women than men when assessing health risks.
In 2023, the American Medical Association officially called BMI "an imperfect way to measure body fat" and recommended using it alongside other measures.
Motion's View on BMI
At Motion, we believe BMI is an outdated metric that shouldn't be used as the primary measure of health or fitness progress. Here's why we recommend a different approach:
- 🎯Focus on Habits, Not Numbers: Rather than obsessing over BMI or weight, Motion helps you build consistent movement habits. When you move regularly, eat well, and prioritize your health, your body naturally finds its healthy state - whatever that looks like for you.
- 💪Celebrate Strength and Function: We encourage tracking metrics that matter for quality of life: getting stronger, having more energy, sleeping better, feeling more confident. If you're building muscle through strength training, your BMI might actually increase while your health improves dramatically.
- 📊Use Multiple Health Markers: If you want to track body composition, consider using BMI alongside more accurate measures: waist circumference, body fat percentage, progress photos, how clothes fit, strength improvements, energy levels, and metabolic health markers (blood pressure, cholesterol, blood sugar).
- 🌈Every Body is Different: Motion's adaptive AI creates personalized goals based on YOUR activity patterns, not generic standards. We celebrate progress at every fitness level and body type, because health looks different for everyone.
- 🤝Community Over Comparison: Instead of comparing yourself to population averages, Motion connects you with supportive friends who celebrate your personal progress. We compete on effort and consistency, not weight or BMI.
Bottom line: Use this BMI calculator if you need it for medical forms or insurance purposes, but don't let it define your health journey. Your worth isn't measured by a number on a scale or a BMI category. Focus on building habits that make you feel strong, energized, and confident.
Better Alternatives to BMI
For a more complete picture of your health, consider these alternatives alongside or instead of BMI:
Waist Circumference
Measures abdominal fat, which is more closely linked to health risks than overall weight.
Body Fat Percentage
Distinguishes between fat mass and lean mass, providing a more accurate health picture.
Waist-to-Hip Ratio
Assesses body fat distribution and associated health risks.
Metabolic Health Markers
Blood pressure, cholesterol, blood sugar - often better indicators of health than weight.
Important Considerations
- BMI is a screening tool, not a diagnostic measure of health or body fatness.
- It should be used alongside other health assessments, not in isolation.
- BMI may incorrectly classify athletes, elderly people, and certain ethnic groups.
- Two people with the same BMI can have vastly different body compositions and health statuses.
- Focus on overall health behaviors - consistent movement, nutritious eating, adequate sleep, stress management - rather than hitting a specific BMI target.
- Always consult with a healthcare provider for personalized health guidance.