What is the standard flag ratio?

There is no single standard. The most common ratio for national flags is 2:3, meaning the flag is 1.5 times as long as it is tall. 1:2 and 3:5 are the next most common. France and Japan use 2:3, Canada uses 1:2, and Germany uses 3:5.

Ratios are written height to length, so a 1:2 flag is twice as long as it is tall. The United States sets its flag at 10:19 for federal use, and Belgium uses 13:15. Switzerland and Vatican City are square, which you can read about in which countries have square flags.

For your own design, 2:3 is a safe default, and 3:5 matches the common 3x5 ft size sold in the US. The flag maker has presets for 2:3, 1:2, 3:5, 10:19, 1:1 and more. The flag ratios guide lists more countries and pixel sizes, and the glossary explains flag proportion.