DIY? heres a math calculator, project , make a chart,

count the teeth on the crankset chainrings. and count the teeth
on each cog on the rear wheel .. old math put the tooth numbers on a grid .

one vertical line on the side, the chainring tooth numbers (A),
across the top write the cog tooth count (B).

and divide A by B, and write it in the block on the chainring line , under the cog number..

old standard way of speaking of what gear you are in , is to just say what the tooth counts are.
48 /12 , 52 /14, 36 / 22 , , 42/20, and so forth...

1, 2 , 3 , 4 .. is not really applicable to a derailleur bike because you may have 2 cog/chainring proportions
that may describe the same ratio ... gear redundancys ..

The Late Sheldon Brown and friends worked out a computer calculator program

http://sheldonbrown.com/gears/ and http://sheldonbrown.com/gears/internal.html