Python BMI Calculator

 

Code:

import tkinter as tk from tkinter import messagebox def calculate_bmi(): try: w = float(entry_weight.get()) h = float(entry_height.get()) / 100 # convert cm → meters bmi = w / (h*h) bmi = round(bmi, 2) # Category check if bmi < 18.5: status = "Underweight 😕" color = "blue" elif bmi < 25: status = "Normal 😊" color = "green" elif bmi < 30: status = "Overweight 😐" color = "orange" else: status = "Obese 😧" color = "red" label_result.config(text=f"Your BMI: {bmi}\nStatus: {status}", fg=color) except: messagebox.showwarning("Input Error", "Please enter valid numbers!") # --- GUI Window --- root = tk.Tk() root.title("BMI Calculator") root.geometry("360x350") root.config(bg="#E8F6EF") root.resizable(False, False) # Title title = tk.Label(root, text="💪 BMI Calculator 💪", font=("Arial", 18, "bold"), bg="#E8F6EF", fg="#2C3A47") title.pack(pady=15) # Frame Inputs frame = tk.Frame(root, bg="#E8F6EF") frame.pack(pady=10) tk.Label(frame, text="Weight (kg):", font=("Arial",12), bg="#E8F6EF").grid(row=0, column=0, padx=10, pady=5) entry_weight = tk.Entry(frame, width=12, font=("Arial",12)) entry_weight.grid(row=0, column=1) tk.Label(frame, text="Height (cm):", font=("Arial",12), bg="#E8F6EF").grid(row=1, column=0, padx=10, pady=5) entry_height = tk.Entry(frame, width=12, font=("Arial",12)) entry_height.grid(row=1, column=1) # Calculate Button btn_calc = tk.Button(root, text="Calculate BMI", font=("Arial", 12, "bold"), bg="#45CE30", fg="white", width=18, command=calculate_bmi) btn_calc.pack(pady=15) # Result Label label_result = tk.Label(root, text="", font=("Arial", 14, "bold"), bg="#E8F6EF") label_result.pack(pady=20) # Footer footer = tk.Label(root, text="Healthy BMI = 18.5 – 24.9", font=("Arial",10), bg="#E8F6EF", fg="#6D214F") footer.pack() root.mainloop()

Output:

Code Explanation:

Importing Required Libraries

import tkinter as tk

from tkinter import messagebox

What this means:

tkinter as tk — imports the Tkinter module and assigns it a short name tk to make widget creation easier.

messagebox — a Tkinter sub-module that allows pop-up alert dialogs.

You use it here to show an Input Error warning when invalid data is entered.

Defining the BMI Calculation Function

def calculate_bmi():

This function executes when the Calculate BMI button is clicked.

Inside the function, we wrap everything in a try / except block:

try:

    w = float(entry_weight.get())

    h = float(entry_height.get()) / 100

entry_weight.get() pulls the typed text from the weight input box.

float(...) converts text to a number.

entry_height.get() reads height in centimeters, so dividing by 100 converts it to meters, which is required for BMI formula.

If the conversion fails (text is empty or contains letters), the code jumps to the except block.

Applying the BMI Formula

bmi = w / (h*h)

bmi = round(bmi, 2)

Explanation:

BMI formula = weight (kg) / height(m)²

The result is rounded to 2 decimals for clarity.

Example:

70kg & 175cm → 70 / 1.75² = 22.86

Checking BMI Category

After calculating BMI, the code checks which WHO health range it belongs to:

if bmi < 18.5:

    status = "Underweight 😕"

    color = "blue"

elif bmi < 25:

    status = "Normal 😊"

    color = "green"

elif bmi < 30:

    status = "Overweight 😐"

    color = "orange"

else:

    status = "Obese 😧"

    color = "red"

What it does:

Uses if − elif − else to compare BMI against ranges.

Assigns:

status text describing health category

emoji for emotion

color for text display

Categories logic:

BMI Range Category Color

< 18.5 Underweight Blue

18.5–24.9 Normal Green

25–29.9 Overweight Orange

≥ 30 Obese Red

This makes the output visual, emotional, and health-interpretable.

Displaying the Result on the Label

label_result.config(text=f"Your BMI: {bmi}\nStatus: {status}", fg=color)

This dynamically updates an existing Label widget by:

putting BMI value + category text

coloring the letters based on status (fg=color)

adding \n for line break

So the interface changes instantly without creating a new widget.

Error Handling

except:

    messagebox.showwarning("Input Error", "Please enter valid numbers!")

This runs only if:

inputs are blank

non-numeric characters are entered

showwarning() opens a yellow warning popup window.

Creating the GUI Window

root = tk.Tk()

root.title("BMI Calculator")

root.geometry("360x350")

root.config(bg="#E8F6EF")

root.resizable(False, False)

Explanation:

tk.Tk() creates the main application window

title() sets the window title bar text

geometry() sets pixel size (width × height)

config(bg=...) changes background color

resizable(False, False) prevents resizing horizontally & vertically

This results in a fixed-size clean window.

Creating the App Title Label

title = tk.Label(root, text="💪 BMI Calculator 💪", font=("Arial", 18, "bold"), bg="#E8F6EF", fg="#2C3A47")

title.pack(pady=15)

A Label widget displays text on screen

Font size 18 bold makes it prominent

Emojis add personality

.pack(pady=15) spaces it vertically

Creating an Input Frame

frame = tk.Frame(root, bg="#E8F6EF")

frame.pack(pady=10)

A Frame groups widgets together, making layout easier.

Weight Input Row

tk.Label(frame, text="Weight (kg):", font=("Arial",12), bg="#E8F6EF").grid(row=0, column=0, padx=10, pady=5)

entry_weight = tk.Entry(frame, width=12, font=("Arial",12))

entry_weight.grid(row=0, column=1)

Here:

A Label displays "Weight (kg):"

An Entry box allows user input

We use .grid(row,column) for neat placement inside the frame

Padding adds space around elements

Height Input Row

tk.Label(frame, text="Height (cm):", font=("Arial",12), bg="#E8F6EF").grid(row=1, column=0, padx=10, pady=5)

entry_height = tk.Entry(frame, width=12, font=("Arial",12))

entry_height.grid(row=1, column=1)

Similar to weight input but for Height in centimeters.

The Calculate Button

btn_calc = tk.Button(root, text="Calculate BMI", font=("Arial", 12, "bold"),

                     bg="#45CE30", fg="white", width=18, command=calculate_bmi)

btn_calc.pack(pady=15)

Explanation:

Creates a Button widget

Green button with white text

Width 18 makes it visually wide

Most important part:

command=calculate_bmi

— means this function is called when clicked.

Output Result Label

label_result = tk.Label(root, text="", font=("Arial", 14, "bold"),

                        bg="#E8F6EF")

label_result.pack(pady=20)

This blank label will later display:

BMI value

Category

Emoji

Color

Initially empty.

Footer Text

footer = tk.Label(root, text="Healthy BMI = 18.5 – 24.9", font=("Arial",10),

                  bg="#E8F6EF", fg="#6D214F")

footer.pack()

Simply shows helpful advice to guide users.

Event Loop (Runs the App Forever)

root.mainloop()

This line:

Starts Tkinter GUI event system

Keeps window open

Listens for button clicks, input typing, etc.

Without mainloop(), the window would close immediately.