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Using ems to set font size

From: Typography for Web Designers

Video: Using ems to set font size

Up until now we've set our text using pixels. In this lesson we're going to set our font size using ems. Ems are relative unit of measurement, based on the default font size of the page. Let's start by finding the right size for our text. We'll be working on the sans serif version of a bibliography, so we'll be looking at PT Sans. The recommended size is 12 to 16 pixels. 12 pixels isn't bad, because it has a larger x-height than Georgia, but it's still a little small.

Using ems to set font size

Up until now we've set our text using pixels. In this lesson we're going to set our font size using ems. Ems are relative unit of measurement, based on the default font size of the page. Let's start by finding the right size for our text. We'll be working on the sans serif version of a bibliography, so we'll be looking at PT Sans. The recommended size is 12 to 16 pixels. 12 pixels isn't bad, because it has a larger x-height than Georgia, but it's still a little small.

14 pixels is more legible. PT Sans bold is a little narrow. This reduces the amount of space in the letters. Bumping the text up two pixels helps with readability. Now let's work on the line height. A recommended measurement for line height is 150% of the font size. PT Sans has a larger x-height, so I thought it would need a little more space between the lines of text, because the larger x-height takes up some of the internal space and makes the lines of text look closer together. But 150% looks fine.

This line height helps the lines stand apart from each other a bit, without them feeling like they're floating away from each other. So it looks like we can use 14 pixels for the font size and 21 pixels for the line height. But we're using ems. To find the number of ems, we need to set the font size at. We take the size we want the font to be and divide it by the default size. Thus 14 pixels, our target size, divided by 16 pixels, our base-size, is .875 ems.

We can write ems out to three decimal points, so we can use the whole measurement. We'll set the font-size in our CSS in a minute, but first, now that we have that number, I want to tell you about a trick. Working with ems would be way easier if the base font-size was 10 pixels instead of 16 pixels. Why? Well, we use the base-ten system in math. If the default font-size was 10pixels, we divide the number of pixels by 10, which is much easier than deciding by 16. For example, we want a 14 pixel font size.

If we take 14 pixels, our target size, and divided by 10 pixels, our dream base font-size, our target number of ems would be 1.4ems. 1.4 ems is just one decimal point away from 14 pixels. That reduces the amount of math needed to work with ems. The trick is we can make the base font size 10 pixels and I'll show you how in a minute. But first let's figure out the relative measurement for our 21 pixel line height.

If our font size is flexible, our line height has to be too. That way your line height will change along with the font size if a reader changes their font size in the browser. There are two ways we can set a relative line height. We can simply set it at 150%. This measure will set to 150% of the font size, which is what we want, or we could set the line-height to 1.5ems. Since line height is relative to font size, it will set to 1.5 times the font size, and not the base font size.

So the line-height will basically set to 150% of the font size. I'm going to recommend we use 150%. Using a percentage will work whether you set your font size in pixels or ems. If you get used to using percentage for your line height, you won't have to switch back and forth between different units of measurement if you end up doing one project in ems and another project in pixels. Now that we've figured out our measurements let's apply them to our sans serif bibliography.

You can see we haven't set a font size or a line height, so the text is coming in with default measurements. The default font size is usually 16 pixels, so it's coming in a bit bigger than we wanted to be. We want to set the font size for the text and we'll do that in our CSS file. First let's make our base font size equal 10 pixels. We're going to create a body selector and make sure we have our curly brackets, and then we will put in the property, font-size: 62.5%.

This works, because the default size is 16 pixels and 62.5% of 16 pixels is 10 pixels. You might be tempted to just set the body font size to 10 pixels, but don't do it. Pixels are not a relative unit of measurement and if you set the body font size using pixels, you'll lose your flexible font sizing. Now let's go to our p. I'm going to type in font-size: 1.4em. I always say ems, but there is no 's' on it when you're typing it in the CSS, and we also have to apply the chosen line-height, so we'll type in line-height:150%.

We can save our changes and view the page in the browser. I'll refresh. The text is now set to the size and line height we want. To help you get used to using ems and percentages, we'll be using both throughout the rest of this course. Since we'll be moving back and forth between two versions of the same bibliography, we'll continue using pixels for the Georgia version and ems for the PT Sans version. I just want to be clear that pixels and ems have nothing to do with what font you use.

We just happen to be exploring typography using two different fonts. You might as well get practice using two different systems of measurements too. In the meantime, if you're interested in learning more about sizing fonts with ems, I highly recommend the article "How to Size Text in CSS" by Richard Rutter on alistapart.com

Show transcript

This video is part of

Image for Typography for Web Designers
Typography for Web Designers

74 video lessons · 13541 viewers

Laura Franz
Author

 
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  1. 6m 18s
    1. Welcome
      1m 9s
    2. Using the exercise files
      1m 57s
    3. Things to consider before starting this course
      3m 12s
  2. 41m 3s
    1. Understanding how good typography promotes reading
      2m 9s
    2. Understanding legibility
      4m 41s
    3. Understanding how fonts convey meaning
      5m 19s
    4. Choosing web-safe fonts to convey meaning
      6m 13s
    5. Using font size, case, style, letter spacing, weight, and color to convey meaning
      6m 22s
    6. Choosing web fonts to convey meaning
      6m 23s
    7. Downloading web fonts
      4m 9s
    8. Applying web fonts in CSS with @font-face
      5m 47s
  3. 38m 0s
    1. Choosing a web-safe font for use in text
      4m 13s
    2. Applying the web-safe font to the text and the heading
      3m 4s
    3. Setting a class for the resource titles in the text
      3m 45s
    4. Choosing a second web-safe font for the heading
      2m 42s
    5. Applying the second font to the heading
      2m 16s
    6. Choosing a web font from the Google Font API for use in text
      5m 44s
    7. Adding and applying the Google Font API syntax
      4m 29s
    8. Choosing a second web font from the Google Font API for the heading
      2m 56s
    9. Adding and applying the second font to the heading
      4m 52s
    10. Analyzing the fonts on some professional sites
      3m 59s
  4. 55m 31s
    1. Understanding how we read
      4m 34s
    2. Finding and applying a good font size and line height
      4m 50s
    3. Finding and applying a good line length
      8m 6s
    4. Understanding ems
      6m 17s
    5. Using ems to set font size
      6m 9s
    6. Using ems to set line length
      3m 40s
    7. Understanding how color affects readability
      3m 58s
    8. Improving a color palette by improving contrast
      5m 39s
    9. Improving a color palette by reducing optical vibration
      4m 59s
    10. Analyzing text readability on the professional sites
      7m 19s
  5. 1h 11m
    1. Understanding how we "chunk" visual elements
      3m 59s
    2. Developing a system of hierarchy
      2m 17s
    3. Applying hierarchy in HTML and CSS
      7m 16s
    4. Developing a system to help chunk text for readers
      6m 1s
    5. Applying the system in the CSS
      4m 19s
    6. Changing an element by creating and applying a class
      5m 0s
    7. Using multiple columns to create hierarchy
      4m 12s
    8. Building a two-column system in HTML and CSS
      10m 56s
    9. Refining the horizontal space in a two-column layout
      6m 1s
    10. Adding rule lines to improve chunking
      5m 50s
    11. Adding emphasis within a heading
      4m 36s
    12. Analyzing the chunking on the professional sites
      11m 18s
  6. 17m 57s
    1. Understanding classic and modernist typographic pages
      7m 3s
    2. Understanding how to create rhythm and tension
      6m 0s
    3. Applying typography skills when making design decisions
      4m 54s
  7. 55m 47s
    1. Designing typographic links for the traditional page
      5m 54s
    2. Adding a list of links to the traditional page
      8m 44s
    3. Describing the link states in CSS
      6m 30s
    4. Returning links to their original "unvisited" style
      2m 38s
    5. Using different CSS for different kinds of links
      7m 28s
    6. Using CSS notation to organize syntax
      5m 34s
    7. Choosing a background color or image
      4m 0s
    8. Applying a repeating background image
      2m 58s
    9. Shaping the traditional page layout
      6m 38s
    10. Analyzing the traditional typographic elements on the professional sites
      5m 23s
  8. 43m 0s
    1. Designing typographic links for the modernist page
      6m 47s
    2. Making a list of links run across the page
      2m 14s
    3. Adding and removing space between the navigation links
      6m 50s
    4. Styling the inline links on the modernist page
      5m 33s
    5. Choosing a background color or image for the modernist bibliography
      4m 4s
    6. Applying a no-repeat background image
      4m 13s
    7. Shaping the modernist page layout
      6m 58s
    8. Analyzing the modernist typographic elements on the professional sites
      6m 21s
  9. 52m 53s
    1. Fixing quotation marks and apostrophes
      6m 59s
    2. Fixing dashes
      6m 33s
    3. Working with lining figures (numbers) and acronyms
      9m 28s
    4. Fixing characters that don't look right
      8m 19s
    5. Hanging punctuation
      2m 54s
    6. Applying typographic accents
      2m 36s
    7. Vertically centering text
      5m 18s
    8. Creating drop caps
      5m 59s
    9. Analyzing the typographic details on the professional sites
      4m 47s
  10. 3m 9s
    1. Additional resources
      3m 9s

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