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HTML Calculator Tutorial for Beginners — Build a Calculator with HTML, CSS & JavaScript

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HTML Calculator Tutorial for Beginners — Build a Calculator with HTML, CSS & JavaScript

Learn to create a clean, responsive calculator step-by-step. Includes source code you can copy, a live demo, FAQ, and schema markup — optimized for Blogger.

⏱️ Estimated read: 6 min · 📁 Category: HTML / JavaScript
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Introduction

In this tutorial you'll build a simple but robust calculator using plain HTML, CSS and JavaScript. The app supports clicking buttons, keyboard input, decimal points, clear and backspace, and safe evaluation. Use the copy buttons to copy code blocks, or the run buttons to preview the app instantly.

Step 1 — HTML Structure

Create the basic markup for the calculator. This includes a display, buttons, and a small history area.


<div class="calc-container" role="application" aria-label="Calculator">
  <input id="display" class="calc-display" aria-label="Calculator display" disabled />
  <div class="calc-buttons">
    <button data-val="7">7</button>
    <button data-val="8">8</button>
    <button data-val="9">9</button>
    <button data-op="/" aria-label="divide">/</button>
    <button data-val="4">4</button>
    <button data-val="5">5</button>
    <button data-val="6">6</button>
    <button data-op="*" aria-label="multiply">*</button>
    <button data-val="1">1</button>
    <button data-val="2">2</button>
    <button data-val="3">3</button>
    <button data-op="-" aria-label="subtract">-</button>
    <button data-val="0">0</button>
    <button data-val=".">.</button>
    <button id="equal" aria-label="equals">=</button>
    <button data-op="+" aria-label="add">+</button>
    <button id="back" title="Backspace">⌫</button>
    <button id="clear" style="grid-column: span 3; background:#ef4444; color:#fff;">CLEAR</button>
  </div>
  <div id="history" aria-live="polite" style="margin-top:12px;font-size:13px;color:#555;"></div>
</div>

Step 2 — CSS Styling

Add modern, responsive styles that look good on desktop and mobile.


.calc-container {
  width: 100%;
  max-width: 320px;
  margin: 18px auto;
  background: #fff;
  padding: 18px;
  border-radius: 12px;
  box-shadow: 0 8px 30px rgba(2,6,23,0.06);
  font-family: Inter, Roboto, Arial, sans-serif;
}
.calc-display {
  width: 100%;
  height: 56px;
  font-size: 22px;
  text-align: right;
  padding: 10px;
  border-radius: 8px;
  border: 1px solid #e6edf3;
  margin-bottom: 12px;
  background:#f8fafc;
}
.calc-buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap:10px;
}
.calc-buttons button {
  height:46px;
  font-size:18px;
  border: none;
  border-radius:8px;
  background:#0ea5e9;
  color:#fff;
  cursor:pointer;
}
.calc-buttons button:active { transform: scale(0.98); }
#back { background:#f59e0b; }
#equal { background:#10b981; }
@media (max-width:420px){
  .calc-container { padding:14px; }
  .calc-display { height:48px; font-size:20px; }
}

Step 3 — JavaScript Logic

This script handles button clicks, keyboard support, safe evaluation, and a small history.


(function(){
  const display = document.getElementById('display');
  const buttons = document.querySelectorAll('.calc-buttons button[data-val], .calc-buttons button[data-op]');
  const equal = document.getElementById('equal');
  const clearBtn = document.getElementById('clear');
  const backBtn = document.getElementById('back');
  const history = document.getElementById('history');

  function safeEval(expr){
    if(!/^[0-9+\-*/.()\s]+$/.test(expr)) throw new Error('Invalid characters');
    return eval(expr);
  }

  buttons.forEach(btn => {
    btn.addEventListener('click', () => {
      const v = btn.getAttribute('data-val') || btn.getAttribute('data-op');
      display.value += v;
    });
  });

  equal.addEventListener('click', () => {
    try{
      const expr = display.value.trim();
      if(!expr) return;
      const result = safeEval(expr);
      history.innerText = expr + ' = ' + result;
      display.value = String(result);
    } catch(err){
      alert('Invalid expression');
    }
  });

  clearBtn.addEventListener('click', () => display.value = '');
  backBtn.addEventListener('click', () => display.value = display.value.slice(0,-1));

  document.addEventListener('keydown', e => {
    if(e.key === 'Enter') { equal.click(); e.preventDefault(); }
    else if(e.key === 'Backspace') backBtn.click();
    else if(e.key === 'Escape') clearBtn.click();
    else if(/^[0-9+\-*/().]$/.test(e.key)) display.value += e.key;
  });
})();

Live Demo

FAQ

Can I extend this to a scientific calculator?

Yes — add buttons for sin, cos, tan, log, sqrt and implement them using JavaScript Math functions.

Is using eval() safe here?

We use a small safeEval() that first validates the expression to contain only numbers, parentheses, spaces, and operators before calling eval(). For large apps, consider implementing a proper expression parser.

How to add history or memory?

You can push results to an array and render them beneath the calculator or persist them with localStorage.

Schema & SEO

JSON-LD HowTo schema is included at the top. Set your Blogger post's Title and Search Description for best SEO results.

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