how to set font size based on container size? [duplicate]

If you want to set the font-size as a percentage of the viewport width, use the vwunit:

#mydiv { font-size: 5vw; }

The other alternative is to use SVG embedded in the HTML. It will just be a few lines. The font-size attribute to the text element will be interpreted as "user units", for instance those the viewport is defined in terms of. So if you define viewport as 0 0 100 100, then a font-size of 1 will be one one-hundredth of the size of the svg element.

And no, there is no way to do this in CSS using calculations. The problem is that percentages used for font-size, including percentages inside a calculation, are interpreted in terms of the inherited font size, not the size of the container. CSS could use a unit called bw (box-width) for this purpose, so you could say div { font-size: 5bw; }, but I've never heard this proposed.

Another js alternative:

Working Example

fontsize = function () {
    var fontSize = $("#container").width() * 0.10; // 10% of container width
    $("#container h1").css('font-size', fontSize);
};
$(window).resize(fontsize);
$(document).ready(fontsize);

Or as stated in torazaburo's answer you could use svg. I put together a simple example as a proof of concept:

SVG Example

<div id="container">
    <svg width="100%" height="100%" viewBox="0 0 13 15">
        <text x="0" y="13">X</text>
    </svg>
</div>

You may be able to do this with CSS3 using calculations, however it would most likely be safer to use JavaScript.

Here is an example: http://jsfiddle.net/8TrTU/

Using JS you can change the height of the text, then simply bind this same calculation to a resize event, during resize so it scales while the user is making adjustments, or however you are allowing resizing of your elements.

I used Fittext on some of my projects and it looks like a good solution to a problem like this.

FitText makes font-sizes flexible. Use this plugin on your fluid or responsive layout to achieve scalable headlines that fill the width of a parent element.

It cannot be accomplished with css font-size

Assuming that "external factors" you are referring to could be picked up by media queries, you could use them - adjustments will likely have to be limited to a set of predefined sizes.