• British flags (like Union Jack/Irish/Canadian and more) have a 1:2 ratio.
• The Star-sprangled Banner uses a 10:19 ratio.
• Latin flags (like the French, Italian, Spanish and most Latin-American) have a 2:3 ratio.
• Germany, Finland and Sweden (and most likely more countries) use "The Divine Proportion"/"Golden Ratio"/Fibonacci sequence rule where the shorter side is to the longer as the longer is to the shorter
plus the longer, i.e. (1, 2,) 3, 5, 8, 13, 21, 34 a.s.o.) – 5:8 would be a good measurement (although 8:13 would of course be more exact
).
• Denmark uses an odd 28:37 ratio.
• Belgium has a 13:15 ratio.
• Indonesia and Monaco actually have the same flag design, differing only in proportions (2:3 vs 4:5).
/Tiny Ratios