How to Calculate BMI (& What It Means)

BMI stands for Body Mass Index. It is a single number calculated from two measurements — your height and your weight — that tells you how your body mass compares to what is considered a healthy range for your height.

BMI is widely used because it is fast, free, and consistent. Doctors, insurers, and public health researchers rely on it as an initial screening tool. A BMI reading in the overweight or obese range does not mean you have a health problem, but it does signal that further assessment is worthwhile.

What BMI measures: weight-to-height ratio, calibrated to correlate with body fat at the population level.

What BMI does not measure: body fat percentage, muscle mass, where fat is stored, or any metabolic health markers. Those limitations are important to understand before you interpret your result — more on that below.

If you want your result instantly, use the BMI calculator. The rest of this guide explains how the calculation works and what the number means.

There are two versions of the formula — metric and imperial — and they produce identical results.

Metric formula

BMI = weight (kg) ÷ height (m)²

Height must be in metres, not centimetres. If you know your height in cm, divide by 100 first: 175 cm ÷ 100 = 1.75 m.

Imperial formula

BMI = (weight in lbs ÷ height in inches²) × 703

Height must be in total inches, not feet and inches separately. Convert first: 5 ft 8 in = (5 × 12) + 8 = 68 inches.

The multiplier 703 is a unit-conversion constant that maps pounds per square inch to the same scale as kilograms per square metre. Both formulas give the same BMI value.

Using metric units (kg and cm)

1. Measure your weight in kilograms (kg).
2. Measure your height in centimetres (cm), then divide by 100 to get metres.
3. Square your height in metres (multiply it by itself).
4. Divide your weight by the squared height.
5. The result is your BMI — no rounding needed beyond one decimal place.

Using imperial units (lbs and inches)

1. Measure your weight in pounds (lbs).
2. Convert your height to total inches: multiply feet by 12 and add remaining inches.
3. Square your height in inches (multiply it by itself).
4. Divide your weight in lbs by the squared height in inches.
5. Multiply that result by 703.
6. The result is your BMI.

Example 1 — Metric

Person: 68 kg, 170 cm tall.

• Height in metres: 170 ÷ 100 = 1.70 m
• Height squared: 1.70 × 1.70 = 2.89 m²
• BMI: 68 ÷ 2.89 ≈ 23.5
• Category: Normal weight (18.5–24.9)

Example 2 — Imperial

Person: 190 lbs, 5 ft 10 in tall.

• Height in inches: (5 × 12) + 10 = 70 in
• Height squared: 70 × 70 = 4,900 in²
• Step 1: 190 ÷ 4,900 = 0.03878
• Step 2: 0.03878 × 703 ≈ 27.3
• Category: Overweight (25–29.9)

Example 3 — Imperial (shorter person)

Person: 130 lbs, 5 ft 3 in tall.

• Height in inches: (5 × 12) + 3 = 63 in
• Height squared: 63 × 63 = 3,969 in²
• Step 1: 130 ÷ 3,969 = 0.03275
• Step 2: 0.03275 × 703 ≈ 23.0
• Category: Normal weight (18.5–24.9)

Skip the arithmetic with the BMI calculator — it handles both unit systems and outputs your category instantly.

Once you have your BMI value, find it in the standard WHO/CDC adult categories below.

• Below 18.5 — Underweight. May indicate insufficient caloric intake, nutrient deficiency, or an underlying health condition. Underweight is associated with weakened immunity, bone loss, and anemia risk.
• 18.5 to 24.9 — Normal weight. Associated with the lowest average statistical risk for most weight-related conditions across large populations. This does not mean perfect health — a person in this range can still have metabolic problems.
• 25.0 to 29.9 — Overweight. Elevated risk of type 2 diabetes, hypertension, and cardiovascular disease compared to the normal-weight range, especially when combined with other risk factors like high waist circumference.
• 30.0 to 34.9 — Obesity (Class I). Substantially elevated risk of obesity-related conditions including sleep apnea, joint problems, and metabolic syndrome.
• 35.0 to 39.9 — Obesity (Class II). High medical risk with increasing likelihood of comorbidities requiring clinical management.
• 40.0 and above — Obesity (Class III / Severe). Associated with significantly reduced life expectancy and complex medical complications. Clinical intervention is typically indicated.

These categories are thresholds, not cliffs. A BMI of 25.0 does not suddenly make a person unhealthy; 24.9 does not guarantee health. Think of the categories as bands on a spectrum that help guide clinical attention, not labels that define your health.

Children and teenagers

Adult BMI categories do not apply to people under 20. For children and adolescents, BMI is plotted on age- and sex-specific growth charts and interpreted as a percentile, not a fixed cutoff. The CDC provides separate BMI-for-age charts for this purpose.

Asian populations

Research shows that people of South, East, and Southeast Asian descent tend to carry more body fat and face higher metabolic risk at lower BMI values than the standard cutoffs assume. Many Asian health authorities — and the WHO's Asia-Pacific guidelines — recommend lower action points: overweight at BMI 23, and obesity at BMI 27.5.

Older adults

As people age, muscle mass naturally decreases (a process called sarcopenia). An older adult with a "normal" BMI may carry a higher proportion of body fat than the category implies. Some researchers suggest that a BMI of 23–27 may be associated with better outcomes for adults over 65 than the 18.5–24.9 range derived from younger populations.

Athletes and strength trainers

Muscle is denser than fat. A highly muscular person can have a BMI that falls in the overweight or obese range while carrying very low body fat. For athletes, body fat percentage — measured via DEXA, skinfold calipers, or the Navy circumference method — is a more meaningful indicator than BMI.

BMI uses only two variables — height and weight. That makes it fast and consistent, but it also means it cannot capture anything about body composition, fat distribution, or metabolic health.

• Fat location. Visceral fat stored around the abdomen carries higher cardiovascular and metabolic risk than subcutaneous fat elsewhere. Two people can have the same BMI with very different fat distributions and health risks.
• Muscle vs. fat. A muscular person and a sedentary person can have identical BMIs despite very different body compositions.
• Blood markers. Blood pressure, blood sugar, cholesterol, and triglyceride levels are not captured by BMI. Someone with a healthy BMI can have dangerous metabolic readings.
• Fitness level. Cardiorespiratory fitness is one of the strongest predictors of mortality, and it does not correlate with BMI.
• Height bias. Because height is squared in the denominator, taller people tend to have lower BMI values than shorter people at the same actual degree of body fatness — a known systematic bias in the formula.

For a fuller picture of body composition, pair your BMI reading with waist circumference and use the body fat calculator (Navy circumference method — just a tape measure, no special equipment). Together, these give more information than either alone.

BMI is a starting point, not a destination. Once you have your number and category, here is where to go next depending on your goal:

• Understand your body composition. Use the body fat calculator to estimate your body fat percentage using measurements alone.
• Set calorie targets. Start with the BMR calculator to find your basal metabolic rate, then use the calorie calculator for activity-adjusted daily targets.
• Plan protein intake. Higher protein supports muscle retention during fat loss. The protein calculator gives targets based on weight, activity, and goal.
• Talk to a clinician. If your BMI is outside the normal range or you have related health concerns, a healthcare provider can put your number in the full clinical context that calculators cannot replicate.

For a deeper dive into the science — including how BMI compares to body fat percentage and what research says about its limitations — read the full BMI guide.