 <p>If you're using em's then you first need to define a base value for the font. Ex.:</p> <p><code>body { font-size: 16px }</code></p> <p>Now 1em = 16px.</p> <p>Most browsers have a default user-agent font-size of 16px but you shouldn't rely on it. Set the base value then start using em's.</p>