# Fixed Element Width: Percent of a Percent

On a recent project, and again while building v3.0 of this site, I needed to calculate the percent width of an element set to `position: fixed`

. The task was tricky because all of the column widths were percents.

## The Riddle

`.container`

is 70% of the window, and `.sidebar`

is 20% of `.container`

. The percent of a fixed element is calculated from the window, not the parent. Find the percent which makes `.nav_fixed`

the same width as `.sidebar`

.

## The Answer

Setting `.sidebar`

to 20% won’t work; it’s too large at 20% of the windows width. We need to figure out the size of `.sidebar`

in relation to the window, which is 20% of 70%.

The quick answer: **multiply 20×70, then move the decimal left two places**. 14% is the width of `.nav_fixed`

.

And, here’s the math behind the answer:

Credit goes to Michael Cohen and his video on Percent of Percent, in which he gives a better explanation.