The flag of Saint Pierre and Miquelon has an aspect ratio of 2:3. It consists of three vertical stripes and a square in the upper hoist-side corner.

1. The left stripe is blue with a width of 1/3 of the flag's length.
2. The middle stripe is red with a width of 1/3 of the flag's length.
3. The right stripe is white with a width of 1/3 of the flag's length.

The square covers 2/5th of the flag's width and 2/3rd of its height, and contains smaller elements within.

Color codes (hex values) for the colors used in the flag: Blue: #002395 Red: #ED2939 White: #FFFFFF

For a detailed illustration and exact measurements of the flag, you can refer to official government sources or vexillological references.