Rather than trying to compute it, why not just tell the SVG text to center itself? I found a reference here:
http://www.w3.org/TR/SVG/text.html#TextAlignmentProperties.

...roboticus

When your only tool is a hammer, all problems look like your thumb.

Update: s/it/itself/ to clarify intent.

Use the attribute text-anchor="middle" instead of computing the width yourself (which is hard to do as you usually wouldn't easily get at the individual glyphs' widths of the particular font being used).

Here's an example that centers text above a rectangle:

<?xml version="1.0" encoding="UTF-8"?>
<svg version="1.2" width="100%" height="100%" viewBox="0 0 1000 1000"
    xmlns="http://www.w3.org/2000/svg">

 <g transform="translate(100 100)">
   <rect width="200" height="50" x="-100" y="10" fill="lightyellow"/>
   <text text-anchor="middle">Some centered text</text>
 </g>
</svg>
[download]

(tested with Firefox 3.6.8 and Opera 10.63)

This is indeed the best solution, though one has to be aware that client support for text anchors isn't generally very good - last I tried inkscape didn't recognize it.

While SVG is a very nice format, and easy to generate, client side support is as unreliable as web programming in the days where you had to support IE 4 to 6 and all the "normal" browsers too. (I've ranted about this before).

Another possible approach (but clearly inferior to text-anchor) is to estimate the length of the rendered text (for example with help of Font::TTFMetrics), and then fixing the length to the calculated value with the textLength attribute. But then again this often results in ugly output, and the client support for textLength is equally bad.

Perl 6 - second systems done right

Thanks! That did the trick.

You don't say in what unit you want to measure the width. Using Inkscape, you could render (the relevant part of) your SVG into a bitmap and then look at how wide it is. There also is SVG::Rasterize, but I haven't used it.

I hear Inkscape has tools to measure angles and distance