<html>
<body>
<form oninput="result.value = 2 * 3.14159 * parseFloat(r.value);">
<label>Enter radius:</label>
<input type="number" name="r" value="1" />
<p>
<label>Circumference is:</label>
<output name="result"></output>
</p>
</form>
</body>
</html>
The output element represents the result of a calculation.
A live example:
N of