BMI Calculator

August 30, 2013

A person’s body mass index is a simple calculation based on height and weight that classifies the person as underweight, overweight, or normal. The formulas, for imperial and metric units, are:

BMI (imperial) = weight in pounds / (height in inches)2 * 703

BMI (metric) = weight in kilograms / (height in meters)2

The usual categories are:

BMI <= 18.5: underweight

18.5 < BMI < 25: normal

25 <= BMI < 30: overweight

30 <= BMI < 40: obese

40 <= BMI: morbidly obese

For example, the BMI of a person who is 5′ 4″ and 125 pounds is 21.5 (normal) and the BMI of a person who is 190 centimeters and 95 kilograms is 26.3 (overweight).

Your task is to write a program that calculates the BMI and category given a person’s height and weight. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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3 Responses to “BMI Calculator”

  1. Paul said

    My version in Python. I cheated a little with the categories. According to specs underweight is when bmi <= 18.5. Here it is if bmi < 18.5.

    CATEGORIES = zip([18.5, 25, 30, 40, 1000], ["underweight", "normal",
    "overweight", "obese", "morbidly obese"])
    WEIGHT_CONV = dict(zip(["kg", "lbs", "ounce"], [1, 0.45359237, 0.0283495231]))
    LENGTH_CONV = dict(zip(["m", "cm", "inch", "ft"], [1, 0.01, 0.0254, 0.3048]))

    def val_metric(value, conv):
    """convert to metric units (kg, m)
    allowed: 5 ft 8 inch or 100 lbs 3 ounce
    """
    elements = value.split()
    total = 0
    while elements:
    total += float(elements.pop(0)) * conv[elements.pop(0)]
    return total

    def bmi(weight, length):
    w = val_metric(weight, WEIGHT_CONV)
    l = val_metric(length, LENGTH_CONV)
    bmi = w / l ** 2
    for c, txt in CATEGORIES:
    if bmi < c:
    cat = txt
    break
    return bmi, cat

    print "bmi=", bmi("160 lbs", "5 ft 7 inch")

  2. Paul said

    Hopefully this time with better formatting:

    CATEGORIES = zip([18.5, 25, 30, 40, 1000], ["underweight", "normal", 
                   "overweight", "obese", "morbidly obese"])
    WEIGHT_CONV = dict(zip(["kg", "lbs", "ounce"], [1, 0.45359237, 0.0283495231]))
    LENGTH_CONV = dict(zip(["m", "cm", "inch", "ft"], [1, 0.01, 0.0254, 0.3048]))
    
    def val_metric(value, conv):
        """convert to metric units (kg, m)
           allowed: 5 ft 8 inch   or 100 lbs 3 ounce
        """
        elements = value.split()
        total = 0
        while elements:
            total += float(elements.pop(0)) * conv[elements.pop(0)]
        return total
            
    def bmi(weight, length):
        w = val_metric(weight, WEIGHT_CONV)
        l = val_metric(length, LENGTH_CONV)
        bmi = w / l ** 2
        for c, txt in CATEGORIES:
            if bmi < c:
                cat = txt
                break
        return bmi, cat
        
    print "bmi=", bmi("160 lbs", "5 ft 7 inch")
    
  3. Jamie Hope said

    Kawa has built-in support for Quantities (numbers with units), so here’s a solution using them instead of case-lambda:

    ;;; Kawa knows about the units g, m, and cm, but not kg, ft, or lb, so add those.
    (define-unit ft 12in)
    (define-unit kg 1000g)
    (define-unit lb 0.453592kg)
    
    (define (bmi (weight ::quantity) (height ::quantity)) ::double
      (let* ((weight ::quantity (+ 0.0kg weight)) ; force to kg
             (height ::quantity (+ 0.0m height))  ; force to m
             (bmi-with-units ::quantity (/ weight (* height height))))
        (*:number bmi-with-units))) ; drop the units
    

    The bmi-class function is the same as above.

    (bmi 95kg 190cm) =>
    26.31578947368421
    (bmi 125lb (+ 5ft 4in)) =>
    21.4559633220829

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