Graphic composites made in a program like Photoshop, Illustrator, Fireworks (RIP), or Sketch all have fixed pixel dimensions. At some point, the designs need to be converted to proportional dimensions by a developer when recreating the design as a fluid layout in a browser.

There is a beautifully simple formula for making this conversion that the father of responsive web design, Ethan Marcotte, set down in his 2009 article, Fluid Grids (http://alistapart.com/article/FLUIDGRIDS):

target / context = result

If anything resembling math makes you quiver, think of it this way: divide the units of the thing you want, by the thing it lives in. Let's put that into practice as understanding it will enable you to convert any fixed dimension layouts into responsive/fluid equivalents.

Consider a very basic page layout intended for desktop. In an ideal world we would always be moving to a desktop layout from a smaller screen layout, but for the sake of illustrating the proportions we will look at the two situations back to front.

Here's an image of the layout:

The layout is 960px wide. Both header and footer are the full width of layout. The left hand side area is 200px wide, the right hand area is 100px wide. Even with my mathematically challenged brain I can tell you the middle section will be 660px wide. We need to convert the middle and side sections to proportional dimensions.

First up, the left hand side. It's 200 units wide (target). Divide that size by 960 units (the context) and we have a result: .208333333. Now, whenever we get our result with that formula we need to shift the decimal point two to the right. That would give us 20.8333333%. That's 200px described as a percentage of 960px.

OK, what about the middle section? 660 (target) divided by 960 (context) gives us .6875. Move the decimal two points to the right and we have 68.75%. Finally, the right hand section. 100 (target) divided by 960 (context) gives us .104166667. Move the decimal point and we have 10.4166667%. That's as difficult as it gets. Say it with me: target, divided by context, equals result.

To prove the point, let's quickly build that basic layout as blocks in the browser. You can view the layout as example_03-01. Here is the HTML:

<div class="Wrap">
    <div class="Header"></div>
    <div class="WrapMiddle">
        <div class="Left"></div>
        <div class="Middle"></div>
        <div class="Right"></div>
    </div>
    <div class="Footer"></div>
</div>

And here is the CSS:

html,
body {
    margin: 0;
    padding: 0;
}

.Wrap {
    max-width: 1400px;
    margin: 0 auto;
}

.Header {
    width: 100%;
    height: 130px;
    background-color: #038C5A;
}

.WrapMiddle {
    width: 100%;
    font-size: 0;
}

.Left {
    height: 625px;
    width: 20.8333333%;
    background-color: #03A66A;
    display: inline-block;
}

.Middle {
    height: 625px;
    width: 68.75%;
    background-color: #bbbf90;
    display: inline-block;
}

.Right {
    height: 625px;
    width: 10.4166667%;
    background-color: #03A66A;
    display: inline-block;
}

.Footer {
    height: 200px;
    width: 100%;
    background-color: #025059;
}

If you open the example code in a browser and resize the page you will see the dimensions of the middle sections remain proportional to one another. You can also play around with the max-width of the .Wrap values to make the bounding dimensions for the layout bigger or smaller (it's set in the example to 1400px).

Now, let's consider how we would have the same content on a smaller screen that flexes to a point and then changes to the layout we have already seen. You can view the final code of this layout in example_03-02.

The idea is that for smaller screens we will have a single 'tube' of content. The left hand side area will only be viewable as an 'off canvas' area; typically an area for a menu area or similar, that sits off the viewable screen area and slides in when a menu button is pressed. The main content sits below the header, then the right hand section below that, and finally the footer area. In our example, we can expose the left hand menu area by clicking anywhere on the header. Typically, when making this kind of design pattern for real, a menu button would be used to activate the side menu.

As you would expect, when combining this with our newly mastered media query skills we can adjust the viewport and the design just 'responds'—effortlessly moving from one layout to another and stretching between the two.

I'm not going to list out all the CSS here, it's all in example_03-02. However, here's an example—the left hand section:

.Left {
    height: 625px;
    background-color: #03A66A;
    display: inline-block;
    position: absolute;
    left: -200px;
    width: 200px;
    font-size: .9rem;
    transition: transform .3s;
}

@media (min-width: 40rem) {
    .Left {
        width: 20.8333333%;
        left: 0;
        position: relative;
    }
}

You can see that up first, without a media query, is the small screen layout. Then, at larger screen sizes, the width becomes proportional, the positioning relative and the left value is set to zero. We don't need to re-write properties such as the height, display, or background-color as we aren't changing them.

This is progress. We have combined two of the core responsive web design techniques we have covered; converting fixed dimensions to proportions and using media queries to target CSS rules relevant to the viewport size.