Body Mass Index (BMI) Calculator

Using this BMI calculator is straightforward. Follow these steps to get an accurate result:

1. Select your unit system. Choose between imperial (pounds and feet/inches) or metric (kilograms and centimeters) using the toggle at the top of the calculator.
2. Enter your weight. For the most accurate reading, weigh yourself in the morning before eating or drinking, wearing minimal clothing. Enter the value in pounds (imperial) or kilograms (metric).
3. Enter your height. Stand against a wall without shoes, with your heels, back, and head touching the surface. Mark the top of your head and measure down to the floor. In imperial mode, enter feet and inches separately. In metric mode, enter your height in centimeters.
4. Review your results. The calculator will instantly display your BMI value, your weight category, and the healthy weight range for your height.

Interpreting Your BMI Result

Your BMI falls into one of four standard categories defined by the WHO:

• Underweight: BMI below 18.5 — may indicate insufficient nutrition or an underlying health condition.
• Normal weight: BMI 18.5 to 24.9 — generally associated with the lowest health risk.
• Overweight: BMI 25.0 to 29.9 — may indicate an elevated risk for chronic conditions.
• Obese: BMI 30.0 or above — associated with significantly increased health risks.

When to Talk to a Doctor

If your BMI falls outside the normal range, or if you have concerns about your weight regardless of your BMI, schedule an appointment with your healthcare provider. This is especially important if you have a family history of heart disease, diabetes, or other weight-related conditions. Your doctor can use additional assessments—including waist circumference, blood pressure, blood glucose, and cholesterol levels—to give you a more complete picture of your health.

Body Mass Index (BMI) is a numerical value derived from a person's weight and height that serves as a widely used screening tool for weight-related health risks. The concept was originally developed by Belgian mathematician and statistician Adolphe Quetelet in the 1830s as part of his work on "social physics," where he sought to define the characteristics of the "average man." Quetelet's Index, as it was initially known, was not originally intended as a measure of individual health but rather as a statistical tool for studying populations. It was not until the mid-20th century that researchers began applying BMI to individual health assessments, and in 1995 the World Health Organization (WHO) formally adopted BMI thresholds to classify underweight, normal weight, overweight, and obesity on a global scale.

BMI measures the ratio of your weight to the square of your height, expressed in units of kg/m². A higher BMI generally indicates greater body mass relative to height. However, it is critical to understand what BMI does not measure. BMI cannot distinguish between lean muscle mass and body fat. It does not account for differences in body composition related to age, sex, or ethnicity. For example, older adults tend to have more body fat than younger adults at the same BMI, women typically carry more fat than men at equivalent BMI values, and certain ethnic groups may face elevated health risks at lower BMI thresholds than others.

Despite these well-documented limitations, BMI remains widely used because it is simple, inexpensive, and non-invasive. It requires only a scale and a tape measure, making it practical for large-scale public health surveillance and routine clinical screenings. For a more comprehensive picture of body composition and health risk, healthcare providers often recommend using BMI alongside alternative metrics such as waist-to-hip ratio, body fat percentage (measured via DEXA scans or bioelectrical impedance), and waist circumference, which is a strong independent predictor of cardiovascular and metabolic disease risk.

Medical Disclaimer: BMI is a screening tool and should not be considered a diagnostic measure of health status. It does not account for muscle mass, bone density, age, sex, or ethnicity. Always consult with a qualified healthcare provider for personalized health assessment and medical advice.

BMI is calculated using one of two formulas depending on your unit system. Both formulas produce the same result; they differ only in the units of measurement and a conversion factor.

Metric Formula

BMI = weight (kg) ÷ [height (m)]²

Imperial Formula

BMI = 703 × weight (lbs) ÷ [height (in)]²

Variable Definitions

Why Does the Imperial Formula Use 703?

The constant 703 is a unit conversion factor that bridges the gap between imperial and metric measurements. It is derived from the relationship between pounds/inches and kilograms/meters. Specifically, 1 kg = 2.20462 lbs and 1 m = 39.3701 inches. When you work through the algebra:

703 ≈ 1 ÷ (0.453592 ÷ 0.0254²) ≈ 1 ÷ (0.453592 ÷ 0.00064516) ≈ 0.00064516 ÷ 0.453592 × 10⁶ ÷ 10⁶

Put more simply: since the metric formula uses kilograms and meters, and the imperial formula uses pounds and inches, multiplying by 703 converts the imperial inputs so the result matches the metric calculation exactly. This means a BMI of 25.0 represents the same body-mass-to-height ratio whether you measure yourself in pounds and inches or kilograms and meters.

Mathematical Relationship

Both formulas are algebraically equivalent. If you convert your weight from pounds to kilograms (divide by 2.20462) and your height from inches to meters (multiply by 0.0254), then plug those values into the metric formula, you will get the same BMI as the imperial formula with the 703 multiplier. This equivalence ensures consistent health classifications worldwide regardless of the measurement system used.

Example 1: Normal Weight (Imperial)

A person who is 5'8" tall and weighs 155 lbs.

First, convert height to inches: 5 × 12 + 8 = 68 inches.

Apply the imperial formula:

BMI = 703 × 155 ÷ 68²

BMI = 108,965 ÷ 4,624

BMI = 23.6

A BMI of 23.6 falls within the Normal weight category (18.5–24.9). This individual is within a healthy range.

Example 2: Overweight (Imperial)

A person who is 5'10" tall and weighs 195 lbs.

Convert height to inches: 5 × 12 + 10 = 70 inches.

Apply the formula:

BMI = 703 × 195 ÷ 70²

BMI = 137,085 ÷ 4,900

BMI = 28.0

A BMI of 28.0 places this person in the Overweight category (25.0–29.9). A healthcare provider might recommend lifestyle adjustments to reduce health risks.

Example 3: Overweight (Metric)

A person who is 175 cm tall and weighs 80 kg.

Convert height to meters: 175 ÷ 100 = 1.75 m.

Apply the metric formula:

BMI = 80 ÷ 1.75²

BMI = 80 ÷ 3.0625

BMI = 26.1

A BMI of 26.1 falls in the Overweight range. This example shows how the metric formula works without any conversion factor.

Example 4: Athletic Person — BMI Limitations

Consider a 6'0" (72 inches), 215 lb athlete with a muscular build and a body fat percentage of only 12%.

BMI = 703 × 215 ÷ 72²

BMI = 151,145 ÷ 5,184

BMI = 29.2

A BMI of 29.2 places this individual in the Overweight category—just one point below the obesity threshold. However, with a body fat percentage of 12%, this person is well within a healthy range by body composition standards. This example clearly illustrates the key limitation of BMI: it cannot distinguish between muscle mass and fat mass. Athletes, bodybuilders, and individuals with dense bone structures frequently register high BMI values despite having excellent body composition. In such cases, metrics like body fat percentage, waist-to-hip ratio, and waist circumference provide a far more accurate assessment of health risk than BMI alone.