Simple Calculator Project Using Bootstrap and JavaScript in HTML/CSS

Key features

Bootstrap 5 responsive layout
Vanilla JavaScript with safe evaluation (basic sanitization)
Keyboard support
Clear, backspace, percentage, decimals
Accessible buttons (ARIA labels & focus states)
Mobile-friendly

html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
  <title>Simple Calculator</title>
  <!-- Bootstrap 5 CDN -->
  <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-..." crossorigin="anonymous">
  <style>
    body {
      background: #f0f2f5;
      min-height: 100vh;
      display: flex;
      align-items: center;
      padding: 1rem;
    }
    .calc {
      max-width: 380px;
      margin: 0 auto;
    }
    .display {
      font-size: 1.75rem;
      text-align: right;
      padding: 0.75rem;
      background: #ffffff;
      border-radius: 0.5rem;
      box-shadow: inset 0 0 4px rgba(0,0,0,0.08);
      min-height: 3.2rem;
      word-wrap: break-word;
      letter-spacing: 0.5px;
    }
    .btn-calculator {
      font-size: 1.1rem;
      padding: 0.75rem 0.5rem;
      border-radius: 0.5rem;
      min-width: 0;
    }
    .btn-operator {
      background-color: #ffb400;
      color: #fff;
    }
    .btn-equals {
      background-color: #28a745;
      color: #fff;
    }
    .btn-secondary-custom {
      background-color: #6c757d;
      color: white;
    }
    .visually-hidden-focus:focus {
      outline: 3px solid #0d6efd;
      outline-offset: 2px;
    }
  </style>
</head>
<body>
  <div class="calc card shadow-sm">
    <div class="card-body">
      <div class="mb-3">
        <div id="display" class="display" role="textbox" aria-label="Calculator display" aria-live="polite">0</div>
      </div>
      <div class="gx-1 gy-1 row row-cols-4">
        <!-- First row -->
        <div class="col mb-1">
          <button class="btn btn-secondary btn-calculator w-100 visually-hidden-focus" data-action="clear" aria-label="Clear">C</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-secondary btn-calculator w-100 visually-hidden-focus" data-action="back" aria-label="Backspace">←</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-secondary btn-calculator w-100 visually-hidden-focus" data-value="%" aria-label="Percent">%</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-operator btn-calculator w-100 visually-hidden-focus" data-value="/" aria-label="Divide">÷</button>
        </div>
        <!-- Second row -->
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="7" aria-label="7">7</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="8" aria-label="8">8</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="9" aria-label="9">9</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-operator btn-calculator w-100 visually-hidden-focus" data-value="*" aria-label="Multiply">×</button>
        </div>
        <!-- Third row -->
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="4" aria-label="4">4</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="5" aria-label="5">5</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="6" aria-label="6">6</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-operator btn-calculator w-100 visually-hidden-focus" data-value="-" aria-label="Subtract">−</button>
        </div>
        <!-- Fourth row -->
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="1" aria-label="1">1</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="2" aria-label="2">2</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="3" aria-label="3">3</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-operator btn-calculator w-100 visually-hidden-focus" data-value="+" aria-label="Add">+</button>
        </div>
        <!-- Fifth row -->
        <div class="col-6 mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="0" aria-label="0">0</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-light btn-calculator w-100 visually-hidden-focus" data-value="." aria-label="Decimal point">.</button>
        </div>
        <div class="col mb-1">
          <button class="btn btn-equals btn-calculator w-100 visually-hidden-focus" data-action="equals" aria-label="Equals">=</button>
        </div>
      </div>
    </div>
    <div class="card-footer text-muted small">
      MIT Licensed — original by Rhalp Darren Cabrera. Keep attribution when redistributing.
    </div>
  </div>

  <script>
    (() => {
      const displayEl = document.getElementById('display');
      let expr = '';

      function updateDisplay(text) {
        displayEl.textContent = text || '0';
      }

      function sanitize(input) {
        // Allow digits, operators, dot, parentheses, percent, spaces
        return /^[0-9+\-*/%.() ]*$/.test(input);
      }

      function evaluateExpression(input) {
        if (!sanitize(input)) return 'Error';
        try {
          // Convert percentages like 25% to (25/100)
          const replaced = input.replace(/(\d+(\.\d+)?)%/g, '($1/100)');
          // eslint-disable-next-line no-eval
          let result = eval(replaced);
          if (typeof result === 'number') {
            if (!isFinite(result)) return 'Error';
            // Trim floating point artifacts
            result = Math.round((result + Number.EPSILON) * 1e12) / 1e12;
            return result;
          }
          return 'Error';
        } catch {
          return 'Error';
        }
      }

      function appendValue(v) {
        // Prevent two operators in a row (simple)
        if (/[\+\-*/%.]$/.test(expr) && /[\+\-*/%.]/.test(v)) {
          expr = expr.slice(0, -1) + v;
        } else {
          expr += v;
        }
      }

      document.querySelectorAll('button').forEach(btn => {
        btn.addEventListener('click', () => {
          const val = btn.getAttribute('data-value');
          const action = btn.getAttribute('data-action');
          if (action === 'clear') {
            expr = '';
          } else if (action === 'back') {
            expr = expr.slice(0, -1);
          } else if (action === 'equals') {
            const res = evaluateExpression(expr);
            expr = (res === 'Error') ? '' : String(res);
          } else if (val !== null) {
            appendValue(val);
          }
          updateDisplay(expr);
        });
      });

      // Keyboard support
      document.addEventListener('keydown', e => {
        const key = e.key;
        if (/[\d.+\-*/%()]/.test(key)) {
          appendValue(key);
        } else if (key === 'Enter' || key === '=') {
          const res = evaluateExpression(expr);
          expr = (res === 'Error') ? '' : String(res);
        } else if (key === 'Backspace') {
          expr = expr.slice(0, -1);
        } else if (key.toLowerCase() === 'c') {
          expr = '';
        } else {
          return;
        }
        e.preventDefault();
        updateDisplay(expr);
      });

      updateDisplay(expr);
    })();
  </script>
</body>
</html>

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<!DOCTYPE html>

<head>

<title>Simple Calculator</title>

<style>

body {

background: #f0f2f5;

min-height: 100vh;

display: flex;

align-items: center;

padding: 1rem;

}

.calc {

max-width: 380px;

margin: 0 auto;

}

.display {

font-size: 1.75rem;

text-align: right;

padding: 0.75rem;

background: #ffffff;

border-radius: 0.5rem;

box-shadow: inset 0 0 4px rgba(0,0,0,0.08);

min-height: 3.2rem;

word-wrap: break-word;

letter-spacing: 0.5px;

}

.btn-calculator {

font-size: 1.1rem;

padding: 0.75rem 0.5rem;

border-radius: 0.5rem;

min-width: 0;

}

.btn-operator {

background-color: #ffb400;

color: #fff;

}

.btn-equals {

background-color: #28a745;

color: #fff;

}

.btn-secondary-custom {

background-color: #6c757d;

color: white;

}

.visually-hidden-focus:focus {

outline: 3px solid #0d6efd;

outline-offset: 2px;

}

</style>

</head>

<body>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

</div>

MIT Licensed — original by Rhalp Darren Cabrera. Keep attribution when redistributing.

</div>

(() => {

const displayEl = document.getElementById('display');

let expr = '';

function updateDisplay(text) {

displayEl.textContent = text || '0';

}

function sanitize(input) {

// Allow digits, operators, dot, parentheses, percent, spaces

return /^[0-9+\-*/%.() ]*$/.test(input);

}

function evaluateExpression(input) {

if (!sanitize(input)) return 'Error';

try {

// Convert percentages like 25% to (25/100)

const replaced = input.replace(/(\d+(\.\d+)?)%/g, '($1/100)');

// eslint-disable-next-line no-eval

let result = eval(replaced);

if (typeof result === 'number') {

if (!isFinite(result)) return 'Error';

// Trim floating point artifacts

result = Math.round((result + Number.EPSILON) * 1e12) / 1e12;

return result;

}

return 'Error';

} catch {

return 'Error';

}

function appendValue(v) {

// Prevent two operators in a row (simple)

if (/[\+\-*/%.]$/.test(expr) && /[\+\-*/%.]/.test(v)) {

expr = expr.slice(0, -1) + v;

} else {

expr += v;

}

document.querySelectorAll('button').forEach(btn => {

btn.addEventListener('click', () => {

const val = btn.getAttribute('data-value');

const action = btn.getAttribute('data-action');

if (action === 'clear') {

expr = '';

} else if (action === 'back') {

expr = expr.slice(0, -1);

} else if (action === 'equals') {

const res = evaluateExpression(expr);

expr = (res === 'Error') ? '' : String(res);

} else if (val !== null) {

appendValue(val);

}

updateDisplay(expr);

});

// Keyboard support

document.addEventListener('keydown', e => {

const key = e.key;

if (/[\d.+\-*/%()]/.test(key)) {

appendValue(key);

} else if (key === 'Enter' || key === '=') {

const res = evaluateExpression(expr);

expr = (res === 'Error') ? '' : String(res);

} else if (key === 'Backspace') {

expr = expr.slice(0, -1);

} else if (key.toLowerCase() === 'c') {

expr = '';

} else {

return;

}

e.preventDefault();

updateDisplay(expr);

});

updateDisplay(expr);

})();

</script>

</body>

</html>

What to do with it

Use as-is: Save above as calculator.html and open in a browser.
Extend: Add memory buttons (M+, MR), scientific functions, theme toggles.
Embed: Drop the inner card into a larger dashboard or convert it to a component.

License reminder

You can modify/sell/distribute this freely. If you distribute this version (or substantial parts), include the MIT notice (as shown in the footer) to satisfy the license attribution requirement.

Download Full Code: Calculator Master

Simple Calculator Project Using Bootstrap and JavaScript in HTML/CSS

Key features

What to do with it

License reminder

Related Posts:

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