Gear ratios

syncro

Hi all.
Just a quick question, when gear ratio specs are written in inches, how do you work this out?
Cheers.

wiffachip

number of teeth on front cog divided by number on back cog multiplied by wheel diameter

so 30 " would be for going up steep hills, 100 " would be for bombing along

peanut

this might help
Sheldon Brown gear inches