Ratio is ratio is ratio, no matter what units you measure in. 24:1 is 24 parts gasoline to one part oil.
But here's a handy calc which will do imperial or metric:
Gas Oil Mixture Ratio Calculator
Speaking of converter programs, grab Convert here
Programs
It's a single convert.exe windows utility you can just drop on your desktop and use whenever you need it.
Free, and it will convert a value from any given measure to another on the fly.
The easiest way to figure ratio is to use what your oil is measured in, in the US it's usually ounces, so in the US Gallon example above, a gallon is 126 ounces.
at 24:1, that is 126/24 so you would add 5.33 ounces of 2-stroke oil to a gallon of gas.
A UK gallon on the other hand is 153.72 ounces, so 24:1 would be 6.38 ounces of oil to a UK gallon, unless of course you are using UK ounces...
A UK gallon is 160 UK ounces, then it would be 6.66 UK ounces of oil to a UK gallon which seems almost evil to me hehehe ;-}
Very handy little util ,and as you see by the tabs it does 21 different types of measure.
While you are on my server you might as well grab the free non-expiring fully functional version of Paint Shop Pro ver 4.21
I have just just about every fancy wiz-bang paint/photo edit program there is and it's all I used for the last 10 years.
It's small, fast, and does just about anything you want with a digital pic in just a couple of clicks, like picture re-sampling for forum display for example.
Just 2 handy programs that find a place on all my computers, so help yourself.