Calculator using HTML CSS and JavaScript

If you are looking for a calculator program written in HTML, CSS, and JavaScript then you are in the right place. Here you will find the complete source code for the calculator with step by step implementation guide.

How to Create Calculator in HTML CSS & JavaScript

First of all, create the HTML structure for the calculator as follows:

<div id="calculator">
	<!-- Screen and clear key -->
	<div class="top">
		<span class="clear">C</span>
		<div class="screen"></div>
	</div>
	
	<div class="keys">
		<!-- operators and other keys -->
		<span>7</span>
		<span>8</span>
		<span>9</span>
		<span class="operator">+</span>
		<span>4</span>
		<span>5</span>
		<span>6</span>
		<span class="operator">-</span>
		<span>1</span>
		<span>2</span>
		<span>3</span>
		<span class="operator">÷</span>
		<span>0</span>
		<span>.</span>
		<span class="eval">=</span>
		<span class="operator">x</span>
	</div>
</div>

You can place the above HTML code anywhere in your project where you want to add this calculator widget.

Styling Calculator with CSS

After creating the HTML structure, now it’s time to style the calculator using CSS. So, target the main container with the #calculator id selector. Define its 325px fixed width along with the “auto” height. Similarly, specify the padding, background, border-radius, and box-shadow property as mentioned below:

#calculator {
	width: 325px;
	height: auto;
	margin: 100px auto;
	padding: 20px 20px 9px;
	background: #9dd2ea;
	background: linear-gradient(#9dd2ea, #8bceec);
	border-radius: 3px;
	box-shadow: 0px 4px #009de4, 0px 10px 15px rgba(0, 0, 0, 0.2);
}

Now style the top portion of the calculator that contains the screen and “C” clear button.

.top span.clear {
	float: left;
}
/* Clear floats */
.keys, .top {overflow: hidden;}

Likewise, specify the CSS styles for the calculator screen. Target the ".top.screen" class, define its height and width as 40px and 212px respectively. Float it to the right, define the background, padding. font-size, etc as described:

/* Inset shadow on the screen to create indent */
.top .screen {
	height: 40px;
	width: 212px;
	float: right;
	padding: 0 10px;
	background: rgba(0, 0, 0, 0.2);
	border-radius: 3px;
	box-shadow: inset 0px 4px rgba(0, 0, 0, 0.2);
	/* Typography */
	font-size: 17px;
	line-height: 40px;
	color: white;
	text-shadow: 1px 1px 2px rgba(0, 0, 0, 0.2);
	text-align: right;
	letter-spacing: 1px;
}

Now, style the calculator keys as described in the below CSS snippet. If you want to increase/decrease the button size, you can set the custom values for the width and height property. Likewise, you can also set the custom background color and font-size in order to make it different from the demo calculator.

.keys span, .top span.clear {
	float: left;
	position: relative;
	top: 0;
	cursor: pointer;
	width: 66px;
	height: 36px;
	background: white;
	border-radius: 3px;
	box-shadow: 0px 4px rgba(0, 0, 0, 0.2);
	margin: 0 7px 11px 0;
	color: #888;
	line-height: 36px;
	text-align: center;
	
	/* prevent selection of text inside keys */
	user-select: none;
	
	/* Smoothing out hover and active states using css3 transitions */
	transition: all 0.2s ease;
}

After that, remove the right margins from operator keys and style operators/evaluate/clear keys differently.

.keys span.operator {
	background: #FFF0F5;
	margin-right: 0;
}
.keys span.eval {
	background: #f1ff92;
	box-shadow: 0px 4px #9da853;
	color: #888e5f;
}
.top span.clear {
	background: #ff9fa8;
	box-shadow: 0px 4px #ff7c87;
	color: white;
}

Define the hover effects for the calculator buttons.

.keys span:hover {
	background: #9c89f6;
	box-shadow: 0px 4px #6b54d3;
	color: white;
}

.keys span.eval:hover {
	background: #abb850;
	box-shadow: 0px 4px #717a33;
	color: #ffffff;
}

.top span.clear:hover {
	background: #f68991;
	box-shadow: 0px 4px #d3545d;
	color: white;
}

At last, specify the active effect for the buttons as follows:

.keys span:active {
	box-shadow: 0px 0px #6b54d3;
	top: 4px;
}

.keys span.eval:active {
	box-shadow: 0px 0px #717a33;
	top: 4px;
}

.top span.clear:active {
	top: 4px;
	box-shadow: 0px 0px #d3545d;
}

Now you have everything ready to program the calculator. At the last stage, you need to add the JavaScript function to make the calculator functional.

JavaScript Program for Calculator

Finally, add the following JavaScript function between the "<script></script>" tag and place it before the closing of body tag.

// Get all the keys from document
var keys = document.querySelectorAll('#calculator span');
var operators = ['+', '-', 'x', '÷'];
var decimalAdded = false;

// Add onclick event to all the keys and perform operations
for(var i = 0; i < keys.length; i++) {
	keys[i].onclick = function(e) {
		// Get the input and button values
		var input = document.querySelector('.screen');
		var inputVal = input.innerHTML;
		var btnVal = this.innerHTML;
		
		// Now, just append the key values (btnValue) to the input string and finally use javascript's eval function to get the result
		// If clear key is pressed, erase everything
		if(btnVal == 'C') {
			input.innerHTML = '';
			decimalAdded = false;
		}
		
		// If eval key is pressed, calculate and display the result
		else if(btnVal == '=') {
			var equation = inputVal;
			var lastChar = equation[equation.length - 1];
			
			// Replace all instances of x and ÷ with * and / respectively. This can be done easily using regex and the 'g' tag which will replace all instances of the matched character/substring
			equation = equation.replace(/x/g, '*').replace(/÷/g, '/');
			
			// Final thing left to do is checking the last character of the equation. If it's an operator or a decimal, remove it
			if(operators.indexOf(lastChar) > -1 || lastChar == '.')
				equation = equation.replace(/.$/, '');
			
			if(equation)
				input.innerHTML = eval(equation);
				
			decimalAdded = false;
		}
		
		// Basic functionality of the calculator is complete. But there are some problems like 
		// 1. No two operators should be added consecutively.
		// 2. The equation shouldn't start from an operator except minus
		// 3. not more than 1 decimal should be there in a number
		
		// We'll fix these issues using some simple checks
		
		// indexOf works only in IE9+
		else if(operators.indexOf(btnVal) > -1) {
			// Operator is clicked
			// Get the last character from the equation
			var lastChar = inputVal[inputVal.length - 1];
			
			// Only add operator if input is not empty and there is no operator at the last
			if(inputVal != '' && operators.indexOf(lastChar) == -1) 
				input.innerHTML += btnVal;
			
			// Allow minus if the string is empty
			else if(inputVal == '' && btnVal == '-') 
				input.innerHTML += btnVal;
			
			// Replace the last operator (if exists) with the newly pressed operator
			if(operators.indexOf(lastChar) > -1 && inputVal.length > 1) {
				// Here, '.' matches any character while $ denotes the end of string, so anything (will be an operator in this case) at the end of string will get replaced by new operator
				input.innerHTML = inputVal.replace(/.$/, btnVal);
			}
			
			decimalAdded =false;
		}
		
		// Now only the decimal problem is left. We can solve it easily using a flag 'decimalAdded' which we'll set once the decimal is added and prevent more decimals to be added once it's set. It will be reset when an operator, eval or clear key is pressed.
		else if(btnVal == '.') {
			if(!decimalAdded) {
				input.innerHTML += btnVal;
				decimalAdded = true;
			}
		}
		
		// if any other key is pressed, just append it
		else {
			input.innerHTML += btnVal;
		}
		
		// prevent page jumps
		e.preventDefault();
	} 
}

That’s all! hopefully, you have successfully created the calculator in HTML, CSS, and JavaScript. You can also download the complete source code given at the top of this tutorial. If you have any questions or need further help, let us know by comment below.