The flag of Brunei has an aspect ratio of 1:2. It consists of four horizontal stripes and a diagonal stripe overlaying them. The top and bottom stripes are yellow (#FCD116), while the middle two stripes are white (#FFFFFF) and black (#000000). The diagonal stripe is also yellow, running from the bottom left corner to the upper right corner.

The width of each horizontal stripe is equal, with the total height of the three stripes being half the flag's height. The diagonal stripe has a width equal to the height of one horizontal stripe.

In the center of the flag, there is a red (#BF0A30) crest with intricate details, which is placed on the intersection point of the diagonal stripe and the middle two horizontal stripes.