Body Mass Index Calculator USA: How It Works

The Body Mass Index (BMI) calculator on this page works out your BMI from your height and weight, then places it in the standard underweight, normal, overweight, or obese category used in the USA. You enter your height in feet and inches and your weight in pounds, and the calculator returns your BMI value and the category it falls into. BMI is a screening tool, not a direct measure of body fat, so the result should be read alongside other health indicators.

Formula

The BMI formula is weight in kilograms divided by height in meters squared (kg/m²). For US units, the formula is adjusted with a conversion factor of 703:

BMI = (weight in pounds ÷ (height in inches)²) × 703

Where:

• Weight in pounds (lb) – the number you enter.
• Height in inches (in) – total height = (feet × 12) + inches.
• 703 – the conversion factor that makes the result match the metric formula.

The result is a number with units of kg/m², even though you entered pounds and inches.

Worked Example

Using the default values from the calculator above: height 5 feet 9 inches and weight 150 lb.

1. Convert height to inches: (5 × 12) + 9 = 60 + 9 = 69 inches.
2. Square the height: 69 × 69 = 4,761.
3. Divide weight by height squared: 150 ÷ 4,761 = 0.0315.
4. Multiply by 703: 0.0315 × 703 = 22.1.

The BMI is 22.1 kg/m², which falls in the normal weight category (18.5–24.9).

How to Read Your Result

In the USA, the National Institutes of Health (NIH) and the World Health Organization (WHO) define these adult BMI categories:

• Below 18.5 – Underweight
• 18.5 to 24.9 – Normal weight
• 25.0 to 29.9 – Overweight
• 30.0 and above – Obesity

These cutoffs apply to adults aged 20 and older. For children and teens (ages 2–19), BMI is plotted on age- and sex-specific growth charts, and the categories are percentiles (e.g., 85th–94th percentile is considered overweight). This calculator is for adults only.

Common Mistakes and Misconceptions

A frequent error is mixing units – entering weight in kilograms and height in feet, or using the metric formula with pounds. Always use the same system: either all metric (kg and meters) or all US (lb and inches). Another mistake is rounding intermediate values too early, which can shift the result by a few tenths. Also, BMI does not distinguish muscle from fat – a muscular athlete may have a high BMI without excess fat. It also does not account for age, sex, or fat distribution (e.g., waist circumference). Therefore, BMI is a screening tool, not a diagnostic test. If your BMI is outside the normal range, a healthcare provider should do a fuller assessment.

Why BMI Is Used in the USA

BMI has been used since the 19th century (developed by Adolphe Quetelet) and was adopted by US health agencies in the 1980s as a simple, low-cost way to estimate body fat at the population level. It is used in clinical guidelines, insurance risk assessment, and public health research. Its main strength is that it requires only height and weight, which are easy to measure. Its main weakness is that it does not measure body composition directly. Despite its limitations, it remains a standard first step in many health screenings.

Alternatives to BMI

If you want a more accurate measure of body fat or health risk, consider these alternatives:

• Waist-to-hip ratio – reflects fat distribution; higher ratios indicate more abdominal fat.
• Waist circumference – a simple measure of abdominal obesity; >35 inches for women and >40 inches for men is considered high risk in the USA.
• Body fat percentage – measured via skinfold calipers, bioelectrical impedance, or DEXA scans.
• Waist-to-height ratio – a ratio above 0.5 suggests increased risk.

Each has its own measurement error and cost. For most people, BMI plus waist circumference gives a reasonable picture without special equipment.

Limitations and Accuracy

BMI can misclassify people at the extremes. For example, a very muscular person (e.g., a bodybuilder) may have a BMI of 28 (overweight) but have low body fat. Conversely, an older adult with low muscle mass may have a normal BMI but high body fat. Also, BMI does not reflect where fat is stored – visceral fat (around organs) is more harmful than subcutaneous fat. For individuals of Asian descent, some organizations suggest lower BMI cutoffs for overweight (23) and obesity (27.5) because they have higher risk at lower BMIs. Always interpret BMI in context.

Frequently Asked Questions

Is BMI accurate for athletes?

No, BMI can overestimate body fat in athletes because muscle weighs more than fat. A high BMI in an athlete does not necessarily mean unhealthy body fat. For athletes, body fat percentage measurements are more informative.

What is a healthy BMI for a 70-year-old?

For older adults, some research suggests a slightly higher BMI (around 25–27) may be associated with lower mortality risk, possibly because a little extra weight provides reserves during illness. However, the standard healthy range (18.5–24.9) is still commonly used, but always consult a doctor.

Can BMI be used for children?

Yes, but the interpretation is different. For children and teens, BMI is compared to growth charts for the same age and sex, producing a percentile. A BMI above the 95th percentile is considered obese. This calculator is for adults only.

What is the difference between BMI and body fat percentage?

BMI is a mathematical formula based on height and weight, while body fat percentage directly measures the proportion of fat in your body. BMI does not distinguish fat from muscle or bone. Body fat percentage can be measured with calipers, bioelectrical impedance, or DEXA scans.

Does BMI apply to pregnant women?

No, BMI is not used to assess pregnant women because weight gain during pregnancy is normal and necessary. Healthcare providers use different weight gain guidelines based on pre-pregnancy BMI.

How often should I check my BMI?

There is no fixed rule, but checking once a year or when your weight changes significantly is reasonable. BMI is a long-term indicator, not a daily metric. Focus on trends rather than single readings.

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in height_ft | Height (ft) | ft | 5 | 0
in height_in | Height (in) | in | 9 | 0
in weight_lb | Weight (lb) | lb | 150 | 0
out bmi | Body Mass Index | kg/m² | (weight_lb / (height_ft * 12 + height_in)^2) * 703 | 1
out bmi_category | Category | - | (bmi < 18.5 ? 1 : (bmi < 25 ? 2 : (bmi < 30 ? 3 : 4))) | 0