Javascript convert width and height to font-size

I'm not sure if this question has already been asked and answered but I haven't found any relevant result.

I would like to know if it's possible to calculate and convert specific dimensions (width/height) to a font-size ?

For example, I have :

var textWidth = 150;
var textHeight = 30;

And I would like to get a font-size from these dimensions (pixels)?

EDIT : Thanks dystroy, I finally managed to resolve this issue with your help. I used the function posted here and after that, it was not very complicated :

$(function(){

            $( "#height, #width" ).keyup(function(){

                var varHeight = $( "#height" ).val();
                var varWidth = $( "#width" ).val();

                if(  varHeight != "" && varWidth != "" ){

                    $( "#text_container" ).css( "width", varWidth );    
                    $( "#text_container" ).css( "height", varHeight );

                    $( "#text_container" ).textfill({ maxFontPixels: 500 });

                    var getFontsize = $( "#text_container > span" ).css( "font-size" );
                    var fontSize = parseInt( getFontsize.slice(0, -2) );

                    $( "#fontsize" ).val( fontSize );

                }

            });

        });

Thanks again.

I've made a small interface for what I understand is your question : you input the max width and max height for some text and it automatically computes the size of the font to adapt the text : http://jsfiddle.net/kPZ3E/

HTML :

Max width : <input id=width value=200> px
<br>Max height : <input id=height value=20> px
<br><br>
<table border=1><tr><td id=txtcell nowrap><span id=txt style=>Some sample text.</span></td></tr></table>
<br>Font Size : <span id=mes></span>

JS :

var fontSize = 100;
var reduce = function() {
    $('#txt').css('font-size', fontSize);
    $('#mes').html(fontSize);
    if ($('#txt').width()>$('#width').val() || $('#txt').height()>$('#height').val()) {
       fontSize -= 1;
       reduce();
    }
};
reduce();
$('#width, #height').change(function(){fontSize = 100;reduce()});

Note : the timeout in the fiddle is only here to show the reduction process with an animation.