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.
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