How to Calculate BMI in Code

Python

CATEGORIES = [(18.5, "Underweight"), (25, "Healthy weight"), (30, "Overweight"),
              (35, "Obesity class 1"), (40, "Obesity class 2"), (float("inf"), "Obesity class 3")]

def bmi_metric(weight_kg: float, height_cm: float) -> float:
    return weight_kg / (height_cm / 100) ** 2

def bmi_imperial(weight_lb: float, height_in: float) -> float:
    return 703 * weight_lb / height_in ** 2

def category(bmi: float) -> str:
    rounded = round(bmi, 1)
    return next(name for limit, name in CATEGORIES if rounded < limit)

bmi = bmi_metric(65, 170)
print(round(bmi, 1), category(bmi))  # 22.5 Healthy weight

JavaScript

const CATEGORIES = [[18.5, 'Underweight'], [25, 'Healthy weight'], [30, 'Overweight'],
  [35, 'Obesity class 1'], [40, 'Obesity class 2'], [Infinity, 'Obesity class 3']];

const bmiMetric = (kg, cm) => kg / (cm / 100) ** 2;
const bmiImperial = (lb, inches) => (703 * lb) / inches ** 2;
const round1 = n => Math.round(n * 10) / 10;
const category = bmi => CATEGORIES.find(([limit]) => round1(bmi) < limit)[1];

const bmi = bmiImperial(150, 67);
console.log(round1(bmi), category(bmi)); // 23.5 'Healthy weight'

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PHP

function bmi_metric(float $kg, float $cm): float
{
    return $kg / (($cm / 100) ** 2);
}

function bmi_category(float $bmi): string
{
    $b = round($bmi, 1);
    return match (true) {
        $b < 18.5 => 'Underweight',
        $b < 25 => 'Healthy weight',
        $b < 30 => 'Overweight',
        default => 'Obesity',
    };
}

echo round(bmi_metric(65, 170), 1); // 22.5

SQL

SELECT
  person_id,
  ROUND(weight_kg / POWER(height_cm / 100.0, 2), 1) AS bmi,
  CASE
    WHEN ROUND(weight_kg / POWER(height_cm / 100.0, 2), 1) < 18.5 THEN 'Underweight'
    WHEN ROUND(weight_kg / POWER(height_cm / 100.0, 2), 1) < 25 THEN 'Healthy weight'
    WHEN ROUND(weight_kg / POWER(height_cm / 100.0, 2), 1) < 30 THEN 'Overweight'
    ELSE 'Obesity'
  END AS category
FROM measurements
WHERE height_cm > 0;

Use 100.0, not 100, so databases that do integer division don't truncate the height.

Test cases for your own code

These are worked out by hand on this site and make good unit tests:

Things to watch for

• Categorizing the unrounded value. 24.96 displays as 25.0 but would be "healthy" if you test the raw number. Round first.
• Integer division. In some languages and databases, 170 / 100 is 1. Use decimals.
• Children. Under 20, BMI needs an age- and sex-specific percentile, not these categories. How BMI percentiles are calculated.