BMI — Body Mass Index — is a simple number that compares your weight to your height. It's calculated the same way everywhere, takes about 10 seconds, and gives doctors a fast first screening for weight-related health risks.
Here's how to calculate it yourself, what the ranges mean, and where the number falls short.
How to calculate BMI
There are two versions of the formula depending on which units you're using.
Metric (kilograms and meters)
Example: You weigh 75 kg and are 1.75 m tall.
BMI = 75 ÷ (1.75 × 1.75) = 75 ÷ 3.0625 = 24.5
Imperial (pounds and inches)
Example: You weigh 160 lbs and are 5'7" (67 inches tall).
BMI = (160 × 703) ÷ (67 × 67) = 112,480 ÷ 4,489 = 25.1
The multiplier 703 is just the conversion factor between the two unit systems — the formula is mathematically identical.
BMI ranges for adults
The World Health Organization and CDC use the same standard categories for adults 20 and older:
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity (Class 1) |
| 35.0 – 39.9 | Obesity (Class 2) |
| 40.0 and above | Obesity (Class 3) |
These categories are the same regardless of age or sex in the standard adult calculation. Some research suggests the thresholds should differ by population group — Asian health organizations, for example, sometimes use lower cutoffs — but the WHO ranges are the international standard.
BMI for children and teenagers
The adult ranges don't apply to children under 20. For kids and teens, BMI is calculated the same way mathematically, but the result is interpreted as a percentile compared to other children of the same age and sex — not as a fixed category.
- Below the 5th percentile: Underweight
- 5th to below the 85th percentile: Healthy weight
- 85th to below the 95th percentile: Overweight
- 95th percentile and above: Obese
A child's BMI number alone means nothing without comparing it to the percentile charts for their age group.
Worked examples at different heights and weights
Example 1: 5'4" (64 inches), 130 lbs
(130 × 703) ÷ (64²) = 91,390 ÷ 4,096 = 22.3 → Normal weight
Example 2: 5'10" (70 inches), 200 lbs
(200 × 703) ÷ (70²) = 140,600 ÷ 4,900 = 28.7 → Overweight
Example 3: 6'0" (72 inches), 220 lbs
(220 × 703) ÷ (72²) = 154,660 ÷ 5,184 = 29.8 → Overweight (just below obese)
Example 4: 5'6" (66 inches), 250 lbs
(250 × 703) ÷ (66²) = 175,750 ÷ 4,356 = 40.3 → Obesity (Class 3)
What BMI doesn't tell you
BMI is a screening tool, not a diagnosis. It has well-documented limitations:
- Muscle mass: Muscle is denser than fat. Athletes with high muscle mass often land in the overweight or obese category despite low body fat percentages. A 200-pound bodybuilder and a 200-pound sedentary person at the same height have the same BMI but very different health profiles.
- Fat distribution: Where you carry fat matters more than how much you carry. Visceral fat (around the abdomen) is more dangerous than subcutaneous fat, and BMI doesn't distinguish between them.
- Age: Older adults tend to have more body fat at the same BMI than younger adults. A BMI of 24 means something different at 25 than at 65.
- Sex: Women naturally carry more body fat than men at the same BMI level, but the same cutoffs are applied to both.
- Ethnicity: Studies consistently show that people of Asian descent have higher health risks at lower BMI values than people of European descent, leading some clinicians to use adjusted thresholds.
Despite these limitations, BMI remains the most widely used initial screening metric because it's fast, free, and requires only a scale and a measuring tape. It's a starting point for a conversation, not a final verdict on health.
Better measures to use alongside BMI
If you want a more complete picture, consider:
- Waist circumference: Greater than 40 inches (men) or 35 inches (women) indicates elevated metabolic risk regardless of BMI.
- Waist-to-height ratio: Ideally below 0.5 for adults under 40; below 0.6 for adults over 50.
- Body fat percentage: Measured directly via DEXA scan, hydrostatic weighing, or estimated via bioelectrical impedance. More accurate than BMI but harder to measure.
Calculate your BMI instantly — no signup required
Open BMI Calculator →Your BMI number is one data point. It's useful to know, but a single number can't capture your health any more than a single blood pressure reading can. Use it as a starting point, not a conclusion.
Related: How Many Calories Should I Eat Per Day? — once you know your BMI category, your daily calorie target is the natural next number to calculate.