The flag of Tonga features a red field with a white, slightly off-centered square in the canton. Within the white square is a red cross bordered in yellow that divides the square into four triangles. The dimensions of the flag are in a ratio of 1:2, and the width of the cross is one-third of the flag's height. The color codes for the flag are:

Red: Pantone PMS186C; RGB 221,37,28; HEX #DD251C White: Pantone White; RGB 255,255,255; HEX #FFFFFF Yellow: Pantone PMS116C; RGB 255,211,0; HEX #FFD300

The geometric properties of the flag are as follows:

• Aspect ratio: 1:2
• Total area of the flag: twice the area of the height of the hoist (the side attached to the pole) multiplied by the length of the fly (the free edge of the flag).
• The diagonal of the white square in the canton is equal to the height of the hoist.

These specifications were officially adopted on November 4, 1875, and have remained unchanged since then.