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Python BMI Calculator Program

By (LinkedIn) · Every code example is run and checked on Python 3.13 (how we test) · Last updated

Short answer
A BMI calculator computes weight_kg / height_m ** 2 and uses if/elif to name the category: below 18.5 underweight, below 25 normal, below 30 overweight, otherwise obese.

Body Mass Index (BMI) is a simple number that relates weight to height. The program is small, but it practises everything a real application needs: reading input, converting numbers, applying a formula, rounding the result, making a decision with if/elif/else and validating bad values.

BMI is only a rough screening number. It does not measure body fat and says nothing about an individual's health, so treat the program as a coding exercise, not as medical advice.

InputWeight in kilograms and height in metres
OutputThe BMI rounded to one decimal place and its category
ApproachApply weight / height ** 2, then compare with the category limits
Time complexityO(1)

Approach: how to calculate BMI in Python step by step

  1. Read the weight and the height and convert both with float().
  2. Check that both values are greater than zero.
  3. Calculate bmi = weight / height ** 2.
  4. Round it with round(bmi, 1) (see round()).
  5. Compare the value with the category limits from top to bottom and return the first matching label.

Because the checks run in order, each elif only needs the upper limit; the lower limit is implied by the earlier condition.

How the BMI calculator code works

height_m ** 2 squares the height. calculate_bmi() only does the maths and bmi_category() only decides the label, so each function does one job and can be tested alone (see functions). An f-string such as f"{bmi:.1f}" formats the number with one decimal place for display.

Python 
weight = 70
height = 1.75
bmi = weight / height ** 2
print(bmi)
print(round(bmi, 1))
print(f"{bmi:.1f}")





      
      
    

    
    
    

      
BMI categories (WHO adult ranges)

      
      
      
      

        Adult BMI ranges used by the World Health Organization
        
BMICategoryCondition in code

        
Below 18.5Underweightbmi < 18.5
18.5 to 24.9Normal weightbmi < 25
25.0 to 29.9Overweightbmi < 30
30.0 and aboveObeseelse

      

      
    

    
    
    

      
Variations: pounds and inches, and input validation

      
For imperial units the formula is 703 * weight_lb / height_in ** 2. A real program must also reject zero or negative numbers, because a height of 0 would cause a ZeroDivisionError and negative values give meaningless results. Children and teenagers use age-based percentile charts, not these adult limits.

      
      
    

    
    

    
    

      
Examples you can run

      
Press Run, then edit the code and run it again.

      
      

        
BMI and category for one person

        
70 kg at 1.75 m gives a BMI of 22.9, which is in the normal range.

        

  

    BMI and category for one person
    

      
      
      
      
    

  

  
def bmi_category(bmi):
    if bmi < 18.5:
        return "Underweight"
    elif bmi < 25:
        return "Normal weight"
    elif bmi < 30:
        return "Overweight"
    return "Obese"

def calculate_bmi(weight_kg, height_m):
    return weight_kg / (height_m ** 2)

bmi = calculate_bmi(70, 1.75)
print(f"BMI: {bmi:.1f}")
print("Category:", bmi_category(bmi))

  
  
  
  

    
      
      
    
    
  

  
Expected outputBMI: 22.9
Category: Normal weight




      

      
      

        
Interactive BMI calculator

        
The user types weight and height; the program prints the result in one line.

        

  

    Interactive BMI calculator
    

      
      
      
      
    

  

  
def bmi_category(bmi):
    if bmi < 18.5:
        return "Underweight"
    elif bmi < 25:
        return "Normal weight"
    elif bmi < 30:
        return "Overweight"
    return "Obese"

def calculate_bmi(weight_kg, height_m):
    return weight_kg / (height_m ** 2)

weight = float(input("Weight (kg): "))
height = float(input("Height (m): "))
bmi = calculate_bmi(weight, height)
print(f"Your BMI is {bmi:.1f} ({bmi_category(bmi)})")

  

  
  
  

    
      
      
    
    
  

  
Expected outputWeight (kg): 58
Height (m): 1.68
Your BMI is 20.5 (Normal weight)




      

      
      

        
Alternative: imperial units

        
The same idea with pounds and inches, multiplied by the conversion factor 703.

        

  

    Alternative: imperial units
    

      
      
      
      
    

  

  
def bmi_category(bmi):
    if bmi < 18.5:
        return "Underweight"
    elif bmi < 25:
        return "Normal weight"
    elif bmi < 30:
        return "Overweight"
    return "Obese"

def calculate_bmi(weight_kg, height_m):
    return weight_kg / (height_m ** 2)

def calculate_bmi_imperial(weight_lb, height_in):
    return 703 * weight_lb / height_in ** 2

bmi = calculate_bmi_imperial(180, 70)
print(round(bmi, 1))
print(bmi_category(bmi))

  
  
  
  

    
      
      
    
    
  

  
Expected output25.8
Overweight




      

      
      

        
Edge case: invalid input

        
Zero or negative values are rejected before any division happens.

        

  

    Edge case: invalid input
    

      
      
      
      
    

  

  
def bmi_category(bmi):
    if bmi < 18.5:
        return "Underweight"
    elif bmi < 25:
        return "Normal weight"
    elif bmi < 30:
        return "Overweight"
    return "Obese"

def calculate_bmi(weight_kg, height_m):
    return weight_kg / (height_m ** 2)

weight = float(input("Weight (kg): "))
height = float(input("Height (m): "))
if weight <= 0 or height <= 0:
    print("Weight and height must be positive numbers")
else:
    print(round(calculate_bmi(weight, height), 1))

  

  
  
  

    
      
      
    
    
  

  
Expected outputWeight (kg): 65
Height (m): 0
Weight and height must be positive numbers




      

      
    

    

    
    

      
Common mistakes

      
      

        
Entering height in centimetres

        

          
✗ Wrong
weight = 70
height = 175
print(round(weight / height ** 2, 1))
0.0 (no error, but the result is meaningless because the formula expects metres)

          
✓ Right
weight = 70
height_cm = 175
height = height_cm / 100
print(round(weight / height ** 2, 1))

        

        
Convert centimetres to metres with height / 100, or ask for metres and say so in the prompt.

      

      
      

        
Writing the formula without the exponent

        

          
✗ Wrong
weight = 70
height = 1.75
print(round(weight / height * 2, 1))
80.0 (no error: `* 2` doubles instead of squaring, so the BMI is wrong)

          
✓ Right
weight = 70
height = 1.75
print(round(weight / height ** 2, 1))

        

        
Use the power operator **: height ** 2.

      

      
    

    

    
    

      
Python BMI Calculator Program: questions people ask

      
What is the BMI formula in Python?
bmi = weight_kg / height_m ** 2. For pounds and inches use 703 * weight_lb / height_in ** 2.
How do I round the BMI to one decimal place?
Use round(bmi, 1) for a number or an f-string such as f"{bmi:.1f}" for text, which keeps trailing zeros (for example 22.00).
What BMI ranges should my program use?
The WHO adult ranges: under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, and 30 or more obese. They do not apply to children.

    

    

    
    

      
Keep learning

      
Step-by-step lessons:
Python NumbersPython if, elif and elsePython Input and Output

      
Related:
Python Simple Calculator ProgramPython Tip Calculator Program (Split the Bill)Python round() FunctionPython float() Function

      
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