BMI (Body Mass Index) is one of the simplest health metrics: a single number that compares your weight to your height. The math is just one division — but the imperial and metric versions look slightly different. Here's how to compute it by hand.
The metric formula
BMI = weight (kg) / [height (m)]²
Worked example: 70 kg person, 1.75 m tall.
- Square the height: 1.75 × 1.75 = 3.0625
- Divide weight by squared height: 70 / 3.0625 = 22.86
- BMI ≈ 22.9
The imperial formula
BMI = 703 × weight (lb) / [height (in)]²
The 703 is a unit conversion constant — it converts pounds and inches into the metric kg/m² result. Without it the imperial number would be tiny.
Worked example: 154 lb person, 5'9" (= 69 inches) tall.
- Square the height: 69 × 69 = 4,761
- Multiply weight by 703: 154 × 703 = 108,262
- Divide: 108,262 / 4,761 = 22.74
- BMI ≈ 22.7
(The small difference from the metric example is because 154 lb ≠ exactly 70 kg and 69 in ≠ exactly 1.75 m.)
Why 703?
Curious about where the 703 comes from? It's the unit conversion baked in:
- 1 inch = 0.0254 m, so 1 in² = 0.000645 m²
- 1 lb = 0.4536 kg
- To convert imperial weight/height² to metric: multiply by 0.4536 / 0.000645 = 703.07
Rounded to 703 for hand-calculation, the imperial formula gives you the same answer as the metric formula. So you don't need to convert your height and weight to metric first — the 703 handles it for you.
BMI categories (adult, both genders)
The World Health Organization classification:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Severely obese (Class III) |
Asian-population thresholds (different)
Major health organizations now recommend lower BMI thresholds for people of South Asian and East Asian descent because cardiovascular risk rises at lower BMI in these populations:
- Underweight: below 18.5
- Normal: 18.5 – 22.9
- Overweight: 23.0 – 27.4
- Obese: 27.5 and above
Some health systems use these thresholds for screening; others stick with the WHO numbers. Discuss with your doctor.
Children and teens — BMI percentile
For kids under 20, raw BMI isn't compared to adult thresholds. Instead, BMI is compared to age-and-sex-matched percentile charts published by the CDC. A child at the 85-95th percentile is "overweight"; above 95th is "obese". The same BMI of 22 might be overweight at age 10 and underweight at age 17.
Pediatric BMI requires the CDC growth charts — don't try to compute it from the adult formulas alone.
What BMI doesn't measure
BMI is a useful screening tool but has well-known limitations:
- Doesn't distinguish muscle from fat. Athletes and bodybuilders are routinely classified as "overweight" or "obese" because muscle is denser than fat.
- Doesn't account for body composition or fat distribution. Two people with the same BMI can have very different cardiometabolic risk.
- Was developed for populations, not individuals. Originally designed by Adolphe Quetelet in 1832 to compare populations — not as a health diagnosis.
- Less accurate at extreme heights. Very tall or very short people may get misleading numbers.
Better measures for individual risk: waist circumference, waist-to-hip ratio, and body fat percentage. BMI works well as a quick screen and to track changes in your own body over time.
BMI doesn't tell you what to do
If your BMI is "overweight" or "obese", the actionable next step isn't necessarily weight loss. Some questions to consider:
- How is my waist circumference? (More predictive of cardiometabolic risk than BMI.)
- What's my body fat percentage? (More accurate measure of body composition.)
- What's my fitness level? (Higher fitness reduces the health risks of higher BMI.)
- What does my doctor say? (Especially if you have other health conditions.)
Mental shortcut
If you know your weight in kg and height in m, the math is fast: square the height, divide weight. For most adult heights (1.5-2.0 m), squared values are between 2.25 and 4. So BMI ≈ kg / (height_m)².
For imperial: a 70 kg person is 154 lb. A 1.75 m person is 69 in. The math works either way.
Calculate yours
Our BMI calculator handles both metric and imperial inputs, gives you the category, and shows where your BMI sits in your demographic's distribution. Useful for tracking changes over time.