Python Simple Calculator Example

A simple calculator is a great beginner Python project. It helps you practice several important basics in one small script:

getting user input
storing values in variables
converting text to numbers
using if, elif, and else
doing basic math
handling simple errors

In this example, you will build a calculator that asks for two numbers and an operator, then prints the result.

num1 = float(input("Enter first number: "))
operator = input("Enter operator (+, -, *, /): ")
num2 = float(input("Enter second number: "))

if operator == "+":
    print(num1 + num2)
elif operator == "-":
    print(num1 - num2)
elif operator == "*":
    print(num1 * num2)
elif operator == "/":
    if num2 == 0:
        print("Cannot divide by zero")
    else:
        print(num1 / num2)
else:
    print("Invalid operator")

Use this as the main working example. The rest of the page explains each part step by step.

What this example teaches

This small project shows you how to:

get values from the user with input()
convert text input into numbers with float()
choose an action with if, elif, and else
perform +, -, *, and / operations
handle invalid operators and division by zero

What the calculator should do

The calculator should:

ask for the first number
ask for the operator
ask for the second number
run the correct math operation
print the result or a clear error message

Step 1: Read user input

The first part of the program gets values from the user.

num1 = float(input("Enter first number: "))
operator = input("Enter operator (+, -, *, /): ")
num2 = float(input("Enter second number: "))

Important idea: `input()` always returns text

Even if the user types 12, Python reads it as a string, not a number.

That is why the code uses float() around the input() calls for num1 and num2.

input() gets the user's typed value
float() converts that text into a number with decimals

For example:

value = input("Enter a number: ")
print(type(value))

Output:

<class 'str'>

After conversion:

value = float(input("Enter a number: "))
print(type(value))

Output:

<class 'float'>

If you want more practice with this, see how to get user input in Python and how to convert user input to numbers in Python.

Step 2: Check the operator

Next, the program checks which operator the user typed.

if operator == "+":
    print(num1 + num2)
elif operator == "-":
    print(num1 - num2)
elif operator == "*":
    print(num1 * num2)
elif operator == "/":
    print(num1 / num2)
else:
    print("Invalid operator")

This works by comparing the operator variable to different strings.

if the user enters "+", the program adds
if the user enters "-", the program subtracts
if the user enters "*", the program multiplies
if the user enters "/", the program divides
anything else goes to else

The else part is useful for unsupported input such as:

%
?
add

Step 3: Handle division safely

Division needs one extra check.

If you try to divide by zero, Python raises an error. To avoid that, check num2 before dividing.

elif operator == "/":
    if num2 == 0:
        print("Cannot divide by zero")
    else:
        print(num1 / num2)

This makes the program friendlier for beginners and users.

Without the check, code like this would fail:

print(8 / 0)

That causes a ZeroDivisionError. If you want to understand that error in more detail, see ZeroDivisionError: division by zero fix.

Full calculator example

Here is the full program again:

num1 = float(input("Enter first number: "))
operator = input("Enter operator (+, -, *, /): ")
num2 = float(input("Enter second number: "))

if operator == "+":
    print(num1 + num2)
elif operator == "-":
    print(num1 - num2)
elif operator == "*":
    print(num1 * num2)
elif operator == "/":
    if num2 == 0:
        print("Cannot divide by zero")
    else:
        print(num1 / num2)
else:
    print("Invalid operator")

How it works

num1 stores the first number
operator stores the math symbol
num2 stores the second number
the if structure chooses the correct calculation
the result is printed to the screen

Expected output examples

Here are some sample runs.

Example 1

Enter first number: 12
Enter operator (+, -, *, /): +
Enter second number: 3
15.0

Example 2

Enter first number: 10
Enter operator (+, -, *, /): /
Enter second number: 2
5.0

Example 3

Enter first number: 8
Enter operator (+, -, *, /): /
Enter second number: 0
Cannot divide by zero

Example 4

Enter first number: 4
Enter operator (+, -, *, /): ?
Enter second number: 2
Invalid operator

Beginner improvements

Once this version works, you can improve it in small steps.

Try one of these:

round the result with round()
repeat the calculator in a loop
let users quit with q
use int() instead of float() if you only want whole numbers

For example, rounding the result:

num1 = float(input("Enter first number: "))
operator = input("Enter operator (+, -, *, /): ")
num2 = float(input("Enter second number: "))

if operator == "/":
    if num2 == 0:
        print("Cannot divide by zero")
    else:
        print(round(num1 / num2, 2))

This prints the division result rounded to 2 decimal places.

When this example is useful

This project is useful:

as a first small Python project
to practice conditionals and input
to combine multiple beginner concepts in one script
to prepare for larger menu-based programs

It is a good next step after learning variables, input(), and basic if statements.

Common mistakes

Beginners often run into the same problems when building this calculator.

Forgetting to convert `input()` values to numbers

This is very common. If you skip float(), the values stay as strings.

Wrong:

num1 = input("Enter first number: ")
num2 = input("Enter second number: ")
print(num1 + num2)

If the user enters 2 and 3, this prints:

That happens because Python joins the strings instead of adding numbers.

Using `=` instead of `==` in an `if` statement

Wrong:

if operator = "+":
    print(num1 + num2)

Use == when comparing values.

Correct:

if operator == "+":
    print(num1 + num2)

Not checking for division by zero

If num2 is 0, division will fail unless you handle it first.

Typing an unsupported operator

Your program only supports:

If the user types something else, the program should show Invalid operator.

Misspelling variable names

For example, writing nub2 instead of num2 will cause errors or wrong behavior.

FAQ

Why use `float()` instead of `int()`?

float() allows decimal numbers like 2.5. int() only works for whole numbers.

Why does `input()` need conversion?

input() returns a string. Math needs numbers, so convert with int() or float().

Why does division return a decimal?

In Python, / returns a float result even when both numbers are whole numbers.

Can I make the calculator run again without restarting?

Yes. Put the code inside a while loop and ask the user whether to continue.