How to calculate BMI correctly: formula, charts, and what your result means

The Body Mass Index (BMI) is a measurement that relates weight to height to estimate whether a person has a healthy weight. It was developed by Belgian mathematician Adolphe Quetelet in 1832 and adopted by the WHO as a standard population health indicator.

BMI is the most widely used screening tool in the world by doctors, nutritionists, and public health organizations. With a single number, you get a quick reference of your nutritional status, although it has important limitations we will cover later.

Calculate your BMI instantly with the NexTools BMI calculator, which accepts both metric (kg/cm) and imperial (lb/ft) systems and shows your WHO classification.

The BMI formula is one of the simplest in medicine:

BMI = Weight (kg) / Height (m)²

Example 1 (metric): A person weighing 75 kg who is 1.72 m tall:

BMI = 75 / (1.72 x 1.72) = 75 / 2.9584 = 25.35

Example 2 (imperial): A person weighing 165 pounds who is 5'8" (68 inches):

BMI = (165 / (68 x 68)) x 703 = (165 / 4624) x 703 = 25.09

The 703 factor in the imperial formula converts the result to match the metric system value.

Quick reference table (height 5'7" / 1.70m):

Weight (lb/kg)	BMI	Classification
110 lb / 50 kg	17.3	Underweight
121 lb / 55 kg	19.0	Normal
143 lb / 65 kg	22.5	Normal
154 lb / 70 kg	24.2	Normal
165 lb / 75 kg	26.0	Overweight
176 lb / 80 kg	27.7	Overweight
198 lb / 90 kg	31.1	Obese I

The World Health Organization (WHO) establishes these categories for adults over 18:

BMI	Classification	Health risk
Below 16.0	Severe thinness	Very high
16.0 - 16.9	Moderate thinness	High
17.0 - 18.4	Mild thinness	Moderate
18.5 - 24.9	Normal weight	Low
25.0 - 29.9	Overweight	Moderate
30.0 - 34.9	Obese class I	High
35.0 - 39.9	Obese class II	Very high
40.0 or above	Obese class III (morbid)	Extremely high

Important: These ranges are for the general population. For Asian populations, the WHO recommends different cutoffs: overweight starting at BMI 23 (instead of 25) and obesity at BMI 27.5 (instead of 30), because metabolic complications appear at lower values.

Global data 2025: According to the WHO, 39% of adults worldwide are overweight (BMI >= 25) and 13% are obese (BMI >= 30). In the United States, over 42% of adults are classified as obese.

For children and adolescents (ages 2-19), BMI is not interpreted with the same ranges as adults. Instead, percentiles are used to compare a child's BMI with other children of the same age and sex:

Percentile	Classification
Below 5th	Underweight
5th to 84th	Healthy weight
85th to 94th	Overweight
95th or above	Obese

This means a child with a BMI at the 75th percentile has a BMI higher than 75% of children of the same age and sex, but is still in the healthy range.

Percentiles are updated through population studies. The most commonly used reference charts are from the WHO (ages 0-5) and the CDC (Centers for Disease Control, ages 2-19).

Why age matters: A BMI of 22 may be completely normal for an adult but indicate overweight in an 8-year-old. The body changes dramatically during growth.

BMI is useful as a population screening tool, but has significant limitations at the individual level:

• Does not distinguish muscle from fat: A muscular athlete like a rugby player may have a BMI of 30+ (classified as obese) while having low body fat and excellent health. Muscle weighs more than fat by volume.
• Does not consider fat distribution: Abdominal (visceral) fat is far more dangerous than subcutaneous fat in the hips and legs. Two people with the same BMI can have very different health risks depending on where fat accumulates.
• Varies by ethnicity: Asian populations develop metabolic problems at lower BMIs, while Polynesian populations may be healthy at higher BMIs.
• Does not apply to pregnant women: Special weight gain charts are used during pregnancy.
• Loses accuracy in older adults: Loss of muscle mass and height in old age distorts the calculation.

More precise complementary measurements:

• Waist circumference: Elevated risk if > 40 inches (102 cm) in men or > 35 inches (88 cm) in women.
• Waist-to-hip ratio: Risk if > 0.90 in men or > 0.85 in women.
• Body fat percentage: Measured with bioimpedance, DEXA scan, or skinfold calipers.

If your BMI is outside the normal range (18.5-24.9), here are evidence-based recommendations:

To lower BMI (overweight/obesity):

• Moderate caloric deficit: Reduce by 500 calories daily to lose approximately 1 pound per week. An extreme deficit slows metabolism and causes rebound weight gain.
• Regular physical activity: 150 minutes of moderate or 75 minutes of vigorous activity per week (WHO recommendation). Combining cardio with strength training preserves muscle mass.
• Prioritize protein: 1.2-1.6g of protein per kg of body weight to maintain muscle while losing fat.
• Sleep: Sleeping less than 7 hours increases the hunger hormone (ghrelin) and makes weight loss harder.

To increase BMI (underweight):

• Controlled caloric surplus: Add 300-500 calories daily, prioritizing nutrient-dense foods (nuts, olive oil, avocado, protein).
• Strength training: To gain weight as muscle, not just fat.
• Medical consultation: Low weight can indicate absorption issues, thyroid problems, or other conditions requiring evaluation.

Calculate how many calories you need with the TDEE calorie calculator and track your progress with the BMI calculator.

Multiple epidemiological studies confirm the relationship between out-of-range BMI and increased disease risk:

Overweight and obesity risks (BMI >= 25):

• Type 2 diabetes (3-7x higher risk with BMI >= 30)
• Cardiovascular disease (hypertension, heart attack, stroke)
• Certain cancers (breast, colon, kidney)
• Sleep apnea
• Osteoarthritis (from joint overload)
• Non-alcoholic fatty liver disease

Underweight risks (BMI < 18.5):

• Weakened immune system
• Osteoporosis (fragile bones)
• Anemia and nutritional deficiencies
• Fertility problems
• Higher mortality risk (paradoxically, mortality with underweight is comparable to or higher than with class I obesity)

The "obesity paradox": Recent studies show that people with mild overweight (BMI 25-27) have similar or even slightly lower mortality than normal-weight individuals. This does not mean overweight is healthy, but rather that BMI is an imperfect indicator and other factors (physical activity, diet, muscle mass) matter as much or more than the number on the scale.

BMI was created in 1832 by Adolphe Quetelet, a Belgian astronomer and mathematician, not a doctor. His goal was to describe the "average man" for population statistical studies, not to diagnose individuals. It was not until 1972 that physiologist Ancel Keys renamed it "Body Mass Index" and it became popular in medicine.

Alternatives to BMI gaining traction:

• BRI (Body Roundness Index): Uses waist circumference and height. Better predictor of visceral fat than BMI.
• BAI (Body Adiposity Index): Uses hip circumference and height. Does not require weight.
• FFMI (Fat-Free Mass Index): Adjusts BMI for body composition. Useful for athletes.
• Waist-to-height ratio: Simple and effective. Goal: your waist should measure less than half your height.

Despite its limitations, BMI remains the most widely used measure due to its simplicity and low cost (you only need weight and height). The BMI calculator gives you your result in seconds for a quick health reference.