Body Mass Index (BMI) is the most widely used screening tool for weight categories. It is a single number that puts your height and weight into a clinical context — underweight, healthy, overweight, or obese. The math behind it is simple, but knowing what the number actually means (and what it doesn't) takes a bit more.
This guide covers the exact formula in metric and imperial, two worked examples, the official WHO adult chart, and the places where BMI breaks down. Use the calculator above to get your number — or read on to do it by hand.
The metric BMI formula
The original BMI formula was developed in the 19th century by the Belgian mathematician Adolphe Quetelet and is sometimes called the Quetelet index. In metric units, it is:
BMI = weight (kg) ÷ height (m)²
The two steps are:
- Square your height in meters (multiply it by itself).
- Divide your weight in kilograms by that number.
Worked example — metric
Imagine someone who is 1.75 m tall and weighs 70 kg.
- Height squared: 1.75 × 1.75 = 3.0625
- BMI: 70 ÷ 3.0625 = 22.9
22.9 falls into the WHO "healthy range" (18.5–24.9).
The imperial BMI formula
In the United States, height is measured in inches and weight in pounds. The imperial formula uses a conversion constant (703) so the resulting number matches the metric version:
BMI = (weight (lb) × 703) ÷ height (in)²
The two steps are:
- Square your height in inches.
- Multiply your weight in pounds by 703, then divide by that number.
Worked example — imperial
Imagine someone who is 5'7" (67 in) tall and weighs 154 lb.
- Height in inches squared: 67 × 67 = 4,489
- Weight × 703: 154 × 703 = 108,262
- BMI: 108,262 ÷ 4,489 = 24.1
24.1 is at the high end of the WHO healthy range.
The official WHO adult BMI chart
The World Health Organization defines four adult BMI categories using these cutoffs:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy range |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
"Obese" is often broken into three sub-classes: Class I (30–34.9), Class II (35–39.9), and Class III (≥40), sometimes called "severe" or "morbid" obesity. Higher classes carry higher health risks. See our [categories guide](/articles/bmi-categories-explained) for the full breakdown.
Asian-adjusted thresholds
Research shows that Asian populations tend to develop weight-related health complications at lower BMIs than European-origin populations. The WHO has issued separate cutoffs that lower the "healthy" band and lower the "overweight" and "obese" cutoffs:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 22.9 | Healthy range |
| 23.0 – 27.4 | Overweight |
| 27.5 and above | Obese |
If you have South, East, or Southeast Asian heritage, consider using our [Asian BMI calculator](/calculators/bmi-asian) — it has a built-in toggle so you can see both.
How to find your healthy weight range from BMI
A useful next step after calculating your BMI is to find the weight range that puts your BMI in the healthy band (18.5–24.9). The math is:
- Lower bound: 18.5 × (your height in meters)²
- Upper bound: 24.9 × (your height in meters)²
For someone who is 1.75 m tall:
- Lower: 18.5 × 3.0625 = 56.7 kg (about 125 lb)
- Upper: 24.9 × 3.0625 = 76.3 kg (about 168 lb)
That gives a healthy weight range of roughly 56.7–76.3 kg (125–168 lb) for someone 1.75 m / 5'9" tall. The calculator above does this for you automatically and shows it in your preferred units.
Where BMI breaks down
BMI is useful as a quick screening tool, but it has real limitations. It does not distinguish muscle from fat, and it does not account for bone density, body shape, age, ethnicity, or sex-based differences in body composition.
For these groups, body-fat percentage, waist-to-height ratio, or a clinical assessment by a healthcare provider gives a more accurate picture. See our [accuracy guide](/articles/is-bmi-accurate-in-2026) for the full breakdown.
BMI vs. body-fat percentage
Body-fat percentage measures what BMI cannot: how much of your weight is actually fat versus muscle, bone, and water. A "healthy" body-fat percentage depends on age and sex:
| Group | Essential fat | Athletes | Healthy | Acceptable |
|---|---|---|---|---|
| Men | 2–5% | 6–13% | 14–17% | 18–24% |
| Women | 10–13% | 14–20% | 21–24% | 25–31% |
You can measure body-fat percentage with calipers, bioelectrical impedance scales (BIA), DEXA scans, or hydrostatic weighing. DEXA is the clinical gold standard. Our [BMI vs. body-fat guide](/articles/bmi-vs-body-fat-percentage) compares the two in detail.
Common mistakes to avoid
- Measuring yourself at the wrong time of day. Weight fluctuates 1–2 kg from morning to evening due to food, water, and digestion.
- Using the wrong height. Self-reported height is often off by 1–2 cm; use a stadiometer if possible.
- Treating BMI as a verdict. It's a screening signal — not a diagnosis. Talk to a clinician for personalized guidance.
- Ignoring waist circumference. For waist-to-height ratio, a number above 0.5 signals elevated risk regardless of BMI.
What to do with your number
Once you have your BMI, the right next step depends on which band you fall into. See our [categories guide](/articles/bmi-categories-explained) for action steps per band, or our [action guide](/articles/from-bmi-to-action) for a quick decision tree.