BMI Formula with Examples (Metric & US)

Quick answer: In metric, BMI = weight (kg) ÷ height² (m²). In U.S. units, BMI = 703 × weight (lb) ÷ height² (in²). Below you’ll find step-by-step examples, rounding rules, and tips. Prefer not to calculate by hand? Use our fast BMI Calculator.

The BMI formulas (metric & US)

SystemFormulaInputs
Metric BMI = kg ÷ (m × m) Weight in kilograms; height in meters
US / Imperial BMI = 703 × lb ÷ (in × in) Weight in pounds; height in inches

Why 703? It’s a unit-conversion factor that makes the US formula give the same result as the metric formula.

Example 1 — Metric (kg & cm)

Person A: 65 kg, 170 cm

  1. Convert height to meters: 170 cm = 1.70 m
  2. Square the height: 1.70 × 1.70 = 2.89 m²
  3. Apply the formula: BMI = 65 ÷ 2.89 = 22.5

Interpretation: 22.5 is within the typical “healthy weight” range for adults. See the full chart in our downloadable BMI chart (PDF).

Example 2 — US (lb, ft/in)

Person B: 160 lb, 5 ft 10 in

  1. Convert total height to inches: (5 × 12) + 10 = 70 in
  2. Square the height: 70 × 70 = 4,900 in²
  3. Apply the formula: BMI = 703 × 160 ÷ 4,900 = 112,480 ÷ 4,900 = 22.9

Optional: convert units before using the metric formula

If you only remember the metric formula, you can convert first:

  • lb → kg: kg = lb × 0.45359237
  • in → m: m = in × 0.0254

Using Person B again: 160 lb = 72.6 kg; 70 in = 1.778 m; BMI = 72.6 ÷ (1.778²) = 72.6 ÷ 3.162 = 22.9 (same result).

Rounding & common mistakes

  • Round BMI to 1 decimal place for reporting (e.g., 22.94 → 22.9).
  • Always square height before dividing.
  • Don’t mix units (e.g., kg with inches) unless you use the correct conversion.

What your number means (quick view)

Category (adults)BMI
Underweight< 18.5
Healthy weight18.5–24.9
Overweight25.0–29.9
Obesity (Class I)30.0–34.9
Obesity (Class II)35.0–39.9
Obesity (Class III)≥ 40.0

These cutoffs are adult general guidelines. For teens, BMI is interpreted using age- and sex-specific percentiles. See our notes in the calculator page.

Beyond BMI: BMI Prime & Ponderal Index

  • BMI Prime = BMI ÷ 25.0 → shows distance from the top of “healthy.” Example: BMI 22.5 → Prime 0.90.
  • Ponderal Index = weight (kg) ÷ height³ (m³) → can be more informative at height extremes.

You can see both of these instantly in our BMI Calculator after you enter your values.

More worked examples you can copy

Example 3 — Metric (80 kg, 180 cm)
Height = 1.80 m; height² = 3.24; BMI = 80 ÷ 3.24 = 24.7
Example 4 — US (195 lb, 5 ft 8 in)
Height = 68 in; height² = 4,624; BMI = 703 × 195 ÷ 4,624 = 137,085 ÷ 4,624 = 29.6

FAQ

Why does the US formula use 703?

Because the metric formula outputs kg/m². When using lb and in, multiplying by 703 converts the units so both systems match.

Is BMI different for men vs women?

The formula is the same. Interpretation can differ in context (e.g., body-fat distribution), but adult cutoffs above are commonly used for both sexes.

How many decimals should I keep?

One decimal place (e.g., 24.7) is standard for BMI reports and charts.

Where can I get a printable BMI chart?

Grab our one-page BMI Chart (PDF).

Try it now: Skip the math and get your BMI, category, healthy weight range, BMI Prime, and Ponderal Index instantly with our free BMI Calculator.