⚖️ BMI Calculator

Find out where you stand with our free BMI calculator. Enter your weight and height to instantly see your Body Mass Index, your weight category according to CDC and WHO standards, and the healthy weight range for your height. BMI is a quick screening tool used by healthcare professionals worldwide to flag potential weight-related health risks.

While BMI is not a diagnostic measure on its own, it provides a useful starting point for conversations with your doctor about nutrition, exercise, and long-term health goals.

What Is BMI?

Body Mass Index (BMI) is a numeric value derived from a person’s weight and height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it was originally called the “Quetelet Index” and later adopted by the World Health Organization as a simple population-level screening tool for weight categories.

BMI does not measure body fat directly. Instead, it uses the statistical relationship between weight and height to estimate whether a person is likely underweight, at a healthy weight, overweight, or obese. Because it requires only two inputs and no special equipment, it remains the most widely used initial assessment in clinical settings, public health studies, and insurance underwriting.

How to Calculate BMI

There are two common formulas depending on the units you use:

Metric Formula

BMI = Weight (kg) ÷ Height (m)²

Example: 180 lbs = 81.6 kg, 5′10″ = 1.78 m → BMI = 81.6 ÷ (1.78)² = 25.8

Imperial Formula

BMI = (Weight (lbs) × 703) ÷ Height (in)²

Example: 180 lbs, 70 in → BMI = (180 × 703) ÷ (70)² = 126,540 ÷ 4,900 = 25.8

The factor of 703 converts the metric formula to work with pounds and inches.

BMI Categories

The CDC and WHO use the same thresholds for adults aged 20 and older:

Below 18.5 — Underweight: May indicate malnutrition, an eating disorder, or other health issues. Associated with weakened immune function and bone loss.
18.5 – 24.9 — Normal weight: Generally associated with the lowest risk of weight-related disease.
25.0 – 29.9 — Overweight: Increased risk of heart disease, type 2 diabetes, and high blood pressure.
30.0 – 34.9 — Obesity Class I: Moderate health risk. Lifestyle changes are strongly recommended.
35.0 – 39.9 — Obesity Class II: High risk. Medical intervention may be advised.
40.0 and above — Obesity Class III: Very high risk, sometimes called “severe” or “morbid” obesity.

Practical Examples

Example 1: A 5′4″ (64 in) woman weighing 135 lbs. BMI = (135 × 703) ÷ 64² = 94,905 ÷ 4,096 = 23.2 (Normal weight).

Example 2: A 6′0″ (72 in) man weighing 220 lbs. BMI = (220 × 703) ÷ 72² = 154,660 ÷ 5,184 = 29.8 (Overweight, borderline obese).

Example 3: A 5′9″ (69 in) person weighing 125 lbs. BMI = (125 × 703) ÷ 69² = 87,875 ÷ 4,761 = 18.5 (Low end of normal).

Limitations of BMI

BMI is a screening tool, not a definitive diagnosis. It has several well-documented limitations:

Muscle vs. fat: Athletes and people who do strength training may have a high BMI due to muscle mass, not excess fat. A bodybuilder with 10% body fat could technically fall in the “obese” BMI range.
Age: Older adults tend to lose muscle and gain fat. A “normal” BMI in a 75-year-old may mask a concerning body composition.
Sex: Women naturally carry more body fat than men. BMI uses the same thresholds for both.
Ethnicity: Research shows that health risks may emerge at different BMI levels for different populations. For example, Asian populations face elevated risks at lower BMI thresholds.
Fat distribution: BMI says nothing about where fat is stored. Visceral fat (around the organs) poses greater health risks than subcutaneous fat (under the skin), regardless of total BMI.

For a more complete picture, consider supplementing BMI with waist circumference, waist-to-hip ratio, or a body composition analysis (DEXA scan, bioelectrical impedance).

Tips for a Healthy Weight

Focus on trends, not a single number. Track your weight over weeks and months rather than obsessing over daily fluctuations, which can swing 2–4 lbs from water alone.

Small changes add up. Cutting 250 calories per day and adding a 30-minute daily walk can lead to roughly 1 lb of fat loss per week—a sustainable, evidence-backed pace.

Talk to your doctor. If your BMI is outside the normal range, a healthcare provider can run additional tests (blood pressure, cholesterol, blood sugar) to assess your actual health risk.

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Frequently Asked Questions

What is a healthy BMI range?

According to the CDC and WHO, a BMI between 18.5 and 24.9 is considered normal weight for adults. Below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is classified as obese.

How is BMI calculated using pounds and inches?

The imperial formula is BMI = (weight in pounds × 703) / (height in inches)². For example, a person weighing 150 lbs at 5'6" (66 inches) has a BMI of (150 × 703) / 66² = 24.2.

Is BMI the same for men and women?

The BMI formula and category thresholds are the same for both sexes. However, women naturally carry a higher percentage of body fat than men, which means a woman and man with identical BMI scores may have different body fat levels.

Why is BMI not always accurate?

BMI does not distinguish between muscle and fat. A muscular athlete may have a high BMI but low body fat. BMI also does not account for fat distribution, bone density, age, or ethnicity—all of which affect health risk.

What should I do if my BMI is too high or too low?

Consult your doctor. BMI is a screening tool, not a diagnosis. Your doctor can assess additional factors like blood pressure, cholesterol, blood sugar, and family history to determine your actual health risk and recommend a plan.

Are there better alternatives to BMI?

Waist circumference (above 40 inches for men, 35 for women signals elevated risk), waist-to-hip ratio, and body composition tests (DEXA scan, bioelectrical impedance) provide more nuanced assessments. BMI remains popular because it is free, fast, and requires no equipment.