Алексей Полоз

function getTextWidth(text, font) {
	var canvas = getTextWidth.canvas || (getTextWidth.canvas = document.createElement("canvas"));
	var context = canvas.getContext("2d");
	context.font = font;
	var metrics = context.measureText(text);
	return metrics.width;
}

var text = $('input').val();
if (!text) text = $('input').attr('placeholder');

var width_text = getTextWidth(text, $('body').css('font'));
alert(width_text);