In HTML 5, you can just use the Canvas.measureText method (further explanation here).

Try this fiddle:

/**
  * Uses canvas.measureText to compute and return the width of the given text of given font in pixels.
  * 
  * @param {String} text The text to be rendered.
  * @param {String} font The css font descriptor that text is to be rendered with (e.g. "bold 14px verdana").
  * 
  * @see https://stackoverflow.com/questions/118241/calculate-text-width-with-javascript/21015393#21015393
  */
function getTextWidth(text, font) {
  // re-use canvas object for better performance
  const canvas = getTextWidth.canvas || (getTextWidth.canvas = document.createElement("canvas"));
  const context = canvas.getContext("2d");
  context.font = font;
  const metrics = context.measureText(text);
  return metrics.width;
}

function getCssStyle(element, prop) {
    return window.getComputedStyle(element, null).getPropertyValue(prop);
}

function getCanvasFont(el = document.body) {
  const fontWeight = getCssStyle(el, 'font-weight') || 'normal';
  const fontSize = getCssStyle(el, 'font-size') || '16px';
  const fontFamily = getCssStyle(el, 'font-family') || 'Times New Roman';
  
  return `${fontWeight} ${fontSize} ${fontFamily}`;
}

console.log(getTextWidth("hello there!", "bold 12pt arial"));  // close to 86

If you want to use the font-size of some specific element myEl, you can make use of the getCanvasFont utility function:

const fontSize = getTextWidth(text, getCanvasFont(myEl));
// do something with fontSize here...

Explanation: The getCanvasFontSize function takes some element’s (by default: the body‘s) font and converts it into a format compatible with the Context.font property. Of course any element must first be added to the DOM before usage, else it gives you bogus values.

More Notes

There are several advantages to this approach, including:

  • More concise and safer than the other (DOM-based) methods because it does not change global state, such as your DOM.
  • Further customization is possible by modifying more canvas text properties, such as textAlign and textBaseline.

NOTE: When you add the text to your DOM, remember to also take account of padding, margin and border.

NOTE 2: On some browsers, this method yields sub-pixel accuracy (result is a floating point number), on others it does not (result is only an int). You might want to run Math.floor (or Math.ceil) on the result, to avoid inconsistencies. Since the DOM-based method is never sub-pixel accurate, this method has even higher precision than the other methods here.

According to this jsperf (thanks to the contributors in comments), the Canvas method and the DOM-based method are about equally fast, if caching is added to the DOM-based method and you are not using Firefox. In Firefox, for some reason, this Canvas method is much much faster than the DOM-based method (as of September 2014).

Performance

This fiddle compares this Canvas method to a variation of Bob Monteverde’s DOM-based method, so you can analyze and compare accuracy of the results.