The flag of Saint Helena, Ascension and Tristan da Cunha has an aspect ratio of 1:2. It is a British Blue Ensign with a blue field (Pantone 280C) and the Union Jack in the upper hoist-side quadrant. In the fly part, there is a shield from the coat of arms that is divided into three parts:

1. The top segment represents Saint Helena, featuring a rocky coastline with a three-masted sailing ship.
2. The middle segment represents Ascension Island, displaying a volcanic landscape with a bird flying above.
3. The bottom segment represents Tristan da Cunha, showing four yellow birds around a green mountain.

The colors used in the shield are:

• Green (Pantone 355C)
• Yellow (Pantone 116C)
• Brown (Pantone 469C)
• White (Pantone Safe)

The exact geometric properties and measurements of the elements within the flag may vary depending on the specific manufacturing or rendering process.