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How to Calculate BMI
BMI (Body Mass Index) is a simple calculation that gives you a number representing your weight relative to your height. It's used by doctors, insurance companies, and public health organizations as a quick screening tool for weight categories.
This guide walks through both the metric and imperial BMI formulas, includes worked examples, and explains how to interpret the result. To skip the math, use our BMI calculator.
Important caveat: BMI is a screening tool, not a diagnostic tool. It doesn't distinguish muscle from fat, and can mislead for athletes, the elderly, and certain ethnic groups. Read on for context.
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The BMI formula
The formula differs by measurement system:
BMI = weight (kg) ÷ height² (m²)
BMI = (weight (lbs) ÷ height² (in²)) × 703
The '703' multiplier in the imperial formula converts the result to match the metric scale. Without it, the imperial calculation would produce a much smaller number that wouldn't fit the standard BMI categories.
In both cases, height is SQUARED. Doubling your height doesn't double your expected weight — it quadruples it (roughly). That's why BMI uses height-squared as the denominator.
Worked example — metric
Calculate BMI for a person weighing 70 kg and 1.75 meters (175 cm) tall.
- STEP 1: Square the height: 1.75 × 1.75 = 3.0625
- STEP 2: Divide weight by squared height: 70 ÷ 3.0625 = 22.86
BMI = 22.86 — within the 'Normal' range (18.5-24.9).
Worked example — imperial
Calculate BMI for a person weighing 165 lbs and 5'9" tall (69 inches).
- STEP 1: Square the height: 69 × 69 = 4,761
- STEP 2: Divide weight by squared height: 165 ÷ 4,761 = 0.03466
- STEP 3: Multiply by 703: 0.03466 × 703 = 24.36
BMI = 24.36 — also within the 'Normal' range.
(Note: this person has the same approximate body proportions as the metric example above. The two formulas produce comparable results when measuring the same person.)
BMI categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25.0 - 29.9 | Overweight |
| 30.0 - 34.9 | Obesity (Class I) |
| 35.0 - 39.9 | Obesity (Class II) |
| 40.0+ | Severe obesity (Class III) |
These categories are based on the World Health Organization (WHO) and CDC standards for adults aged 20+. Children and teens use different age-and-sex-specific percentile charts. Different populations may have different optimal ranges — for example, Asian populations have higher cardiovascular risk at lower BMIs, so some countries use 23 as the overweight threshold instead of 25.
BMI limitations
BMI is a useful population-level screening tool, but it has significant individual limitations:
- DOESN'T DISTINGUISH MUSCLE FROM FAT. A 200-lb bodybuilder and a 200-lb sedentary person at the same height have identical BMIs but very different health profiles. Athletes are often categorized as 'overweight' or 'obese' by BMI despite low body fat.
- DOESN'T MEASURE FAT DISTRIBUTION. Visceral fat (around organs) is much more dangerous than subcutaneous fat (under skin), but BMI can't tell them apart. Waist-to-hip ratio and waist circumference are better indicators of visceral fat.
- LESS ACCURATE FOR ELDERLY. Older adults often have less muscle mass and more body fat than their BMI suggests. A 'normal' BMI in an older adult may mask sarcopenia (muscle loss).
- LESS ACCURATE FOR DIFFERENT ETHNICITIES. The standard BMI thresholds were developed from primarily white European populations. Asian, Pacific Islander, and South Asian populations often have higher health risk at lower BMIs. African American populations may have lower health risk at higher BMIs.
- NOT VALID FOR PREGNANCY. Pregnant women shouldn't use standard BMI calculations.
- NOT VALID FOR CHILDREN. Kids use age-and-sex-specific percentile charts, not adult BMI categories.
For a more complete picture, combine BMI with body fat percentage, waist-to-hip ratio, and other health markers like blood pressure and cholesterol.
BMI vs body fat percentage
BMI is a single number from height and weight. Body fat percentage measures the actual ratio of fat to lean tissue. They tell different stories:
For most people, body fat percentage is a more meaningful number than BMI. Use BMI as a starting screen, then measure body fat (via DEXA, Bod Pod, calipers, or the US Navy method) for accuracy.
What to do based on your BMI
- NORMAL (18.5-24.9): Maintain current habits. Continue regular activity, balanced nutrition, and annual checkups.
- UNDERWEIGHT (Below 18.5): Often indicates inadequate nutrition or underlying health issues. Talk to a doctor — could indicate hormonal imbalances, malabsorption, or eating disorders. Don't try to gain weight rapidly with junk food; focus on nutrient-dense calories.
- OVERWEIGHT (25-29.9): Increased risk of cardiovascular disease and type 2 diabetes. Most people in this range can reach healthy BMI through moderate dietary changes and increased activity. Aim for 0.5-1 lb/week loss using our calorie calculator to find your target intake.
- OBESITY (30+): Significantly elevated health risks. Talk to a doctor about a comprehensive plan. May include lifestyle changes, behavioral support, and in some cases medication or surgery. Don't go it alone if BMI is 30+.
- REGARDLESS OF BMI: Health is more than weight. Track blood pressure, cholesterol, blood sugar, fitness level, mental health, and sleep. Many 'normal BMI' people have poor metabolic health, and many 'overweight' people are metabolically healthy.