Does anyone know how to make a 20px or 1em gutter for the grid system below?

I got all the divs to all go in a row but I want to know how to add a gutter in between each div. I'm doing this to learn how grids are made. jsFiddle Here.

body {
    font:20px/1.2 Verdana, Arial; padding:0px; margin:0px;
}

*, *:after, *:before {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing:border-box;
}

.row { width:100%; }

.row > [class*='col-'] { /* ... */ }
.row > [class*='col-']:last-of-type { /* ... */ }

[class*="col-"] {
    float:left; 
    height:200px;
    background-color: #dedede;
    border: 1px solid #000;
    padding-right:20px;
}

[class*=col-]:last-of-type {
    padding-right:0px;
}

.col-1-12 {
    width: calc(100% / 12);
}

.col-2-12 {
    width: calc(100% / 12 * 2);
}

.col-3-12 {
    width: calc(100% / 12 * 3);
}
.col-4-12 {
    width: calc(100% / 12 * 4);
}

HTML:

<div class="row">
    <div class="col-4-12">
        Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
    </div>
    <div class="col-2-12">
        Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. 
    </div>      
    <div class="col-3-12">
        Duis aute irure dolor in reprehenderit in voluptate velit esse
        cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
    </div>  
    <div class="col-3-12">
        Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. 
    </div>
</div>

Well, here is the calculation of the columns' width, base on the column number considering the gutter of 20px between each column.

For instance, col-2-12:

width: calc( (100% - (12/2 - 1) * 20px) / 12 * 2 );

Explanation:

width:
    (100%   /* Total width */
      - (12/2 - 1) * 20px /* Number of gutters between col-2  x  gutter width */
    ) / 12 /* Total columns */
      * 2  /* Size of the current column which is col-2 */

Also, instead of using margin for the first and the last column, you can use padding for the container .row and set the margin for that columns to 0.

In addition, as the columns are floated, you should clear the float at the bottom of the .row element.

.row {
    padding: 0 20px;
    *zoom: 1;
}

.row:after, .row:before {
    content: ' ';
    display: table;
}

.row:after { clear: both;}

.row > [class*='col-']:first-child { margin-left: 0; }
.row > [class*='col-']:last-child { margin-right: 0; }

WORKING DEMO.

Sassy CSS

Using CSS preprocessors such as Sass, makes the calculation of grid systems fun!

Here is the Sassy way of a fluid grid system:

$total_columns : 12;
$total_width   : 100%;
$gutter_width  : 2%;

.row {
    padding: 0 $gutter_width;
    *zoom: 1;

    &:after, &:before { content: ' '; display: table; }
    &:after { clear: both; }

    & > [class*='col-']:first-child { margin-left: 0; }
    & > [class*='col-']:last-child { margin-right: 0; }
    & > [class*='col-'] { margin: 0 $gutter_width/2; }
}

[class*="col-"] {
    float:left; min-height:200px;
    background-color: #dedede; border: 1px solid #000;
}

@for $i from 1 through $total_columns {
    .col-#{$i}-#{$total_columns} {
        width: (100% - ($total_columns/$i - 1) * $gutter_width) / $total_columns * $i;
    }
}

ONLINE DEMO.