calc.html

<!DOCTYPE html>
<html lang="pt-br">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <meta http-equiv="X-UA-Compatible" content="ie=edge">
        <link rel="stylesheet" href="css/style.css">
        <title>CALC()</title>
        <style>
            #calc{width: calc(100% - 16px);}
            #box{transform: translateX(-50%);position: relative;left: 50%;background-color: #ffffff;padding: 24px 0;text-align: center;border-radius: 4px;}
        </style>
    </head>
    <body>
        <input type="text" id="calc" placeholder="Coloque seu calculo aqui">
        <div id="box"></div>
        <script>
            calc.addEventListener('input', (e) => {
                box.style.width = `calc(${e.target.value})`;
                box.textContent = `${box.clientWidth}px`;
            });
        </script>
    </body>
</html>