Center (proportional font) text in an HTML5 canvas

If you don't necesserilly need a width of the text but just want to center text you can do

  canvas_context.textBaseline = 'middle';
  canvas_context.textAlign = "center";

Which should put a text centered both vertically and horizontally.

You can do this by using measureText

var canvas = document.getElementById("canvas"),
    ctx = canvas.getContext("2d")

canvas.width = 400;
canvas.height = 200;

ctx.fillStyle = "#003300";
ctx.font = '20px sans-serif';

var textString = "Hello look at me!!!",
    textWidth = ctx.measureText(textString ).width;


ctx.fillText(textString , (canvas.width/2) - (textWidth / 2), 100);

Live Demo

More elaborate demo