What is BMI (Body Mass Index)?

BMI stands for Body Mass Index. It’s a simple number that compares your weight to your height to give a fast “screening” estimate of whether your body weight is generally low, typical, or high for your height. BMI is popular because it’s quick, uses only two inputs (height and weight), and can be used across large populations to study weight-related health trends.

That said, BMI is not a perfect measure of health. It does not directly measure body fat percentage, where fat is stored, muscle mass, bone density, or overall fitness. Two people can have the same BMI and very different bodies. For example, a muscular athlete can have a high BMI while still being very lean. Likewise, older adults can have a “normal” BMI but lower muscle mass, which can matter for strength and long-term health. Think of BMI as a starting point—a helpful signal, not a diagnosis.

How this BMI calculator works

This calculator supports metric and imperial units so you can use what you’re comfortable with. Under the hood, the math is straightforward:

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

In metric, we convert centimeters to meters by dividing by 100. In imperial, we convert your height in feet and inches into total inches (feet × 12 + inches). The “703” is simply a conversion factor that makes the imperial version produce the same BMI scale as the metric formula.

BMI categories (what your number means)

Once you calculate BMI, we place it into common categories. These cutoffs are widely used for general guidance:

• Underweight: BMI < 18.5
• Normal weight: BMI 18.5–24.9
• Overweight: BMI 25.0–29.9
• Obesity: BMI ≥ 30.0

Categories are useful because they simplify interpretation. But your personal context matters. If your BMI is a bit above or below “normal,” it doesn’t automatically mean you’re unhealthy—especially if you have high muscle mass or other factors. If you’re unsure, consider pairing BMI with measurements like waist circumference, body fat estimate, energy levels, sleep, and activity habits.

Examples

Example 1 (Metric): Height = 175 cm, Weight = 72 kg. Convert height to meters: 1.75 m. BMI = 72 ÷ (1.75²) = 72 ÷ 3.0625 ≈ 23.5. That’s in the normal range.

Example 2 (Imperial): Height = 5 ft 9 in = 69 in, Weight = 160 lb. BMI = 703 × 160 ÷ (69²) = 112,480 ÷ 4,761 ≈ 23.6. Also normal.

Healthy weight range (based on your height)

Many people don’t just want a BMI number—they want a practical target. That’s why this calculator also estimates a healthy weight range for your height using the “normal” BMI range (18.5–24.9). We reverse the BMI formula: given height, we compute what weight would produce BMI 18.5 (low end) and BMI 24.9 (high end).

This range is a helpful planning tool. If your current weight is outside the range, it doesn’t mean you must “hit” that window immediately. Think of it as a map: it shows where “typical BMI” sits for your height, and you can decide what’s realistic and healthy for your lifestyle.

When BMI can be misleading

BMI works best as a broad screening metric, but it can be less accurate for certain groups:

• Very muscular people: muscle adds weight, raising BMI without necessarily increasing health risk.
• Older adults: lower muscle mass can hide risk even at “normal” BMI.
• Pregnancy: BMI isn’t designed for pregnancy weight changes.
• Kids/teens: BMI-for-age percentiles are typically used instead of adult cutoffs.
• Different body frames: body composition varies; BMI doesn’t capture distribution of fat.

If you want a more complete picture, consider pairing BMI with other tools like body fat estimate, waist–hip ratio, calorie needs (TDEE), and activity tracking.

FAQs

• Is BMI the same as body fat percentage? No. BMI uses only height and weight. Body fat percentage estimates how much of your body is fat versus muscle, bone, and water.
• What’s a “good” BMI? Many guides call 18.5–24.9 “normal.” But “good” depends on your health, muscle mass, and goals.
• Why does the imperial formula use 703? It’s a unit conversion constant so pounds/inches produce the same BMI scale as kg/meters.
• Should I worry if I’m slightly over 25? Not automatically. Small differences can be normal. Use BMI alongside habits, waist size, labs, and how you feel.
• Does BMI apply to athletes? It can be misleading for athletes because high muscle mass increases weight and BMI.
• How often should I check BMI? If you’re monitoring change, monthly is usually enough. Daily weight swings can mislead.
• Can BMI help with weight-loss planning? Yes as a rough baseline. Pair it with calorie needs (TDEE) and sustainable habit changes.
• Is BMI a medical diagnosis? No. It’s a screening tool. For medical advice, consult a clinician—especially if you have symptoms or conditions.