Here's an example of a basic calculator implemented using HTML:

<!DOCTYPE html>
<html>
<head>
<title>Calculator</title>
<style>
.calculator {
width: 200px;
border: 1px solid #ccc;
padding: 10px;
}

.calculator input[type="text"] {
width: 100%;
margin-bottom: 10px;
}

.calculator input[type="button"] {
width: 48%;
margin-bottom: 5px;
}
</style>
<script>
function calculate() {
var num1 = parseFloat(document.getElementById("num1").value);
var num2 = parseFloat(document.getElementById("num2").value);
var operator = document.getElementById("operator").value;
var result;

if (operator === "+") {
result = num1 + num2;
} else if (operator === "-") {
result = num1 - num2;
} else if (operator === "*") {
result = num1 * num2;
} else if (operator === "/") {
result = num1 / num2;
}

document.getElementById("result").value = result;
}
</script>
</head>
<body>
<div class="calculator">
<input type="text" id="num1" placeholder="Enter number 1">
<input type="text" id="num2" placeholder="Enter number 2">
<select id="operator">
<option value="+">+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
</select>
<input type="button" value="Calculate" onclick="calculate()">
<input type="text" id="result" placeholder="Result" readonly>
</div>
</body>
</html>