For example = bike works of New York has a chart with all the gear inch combinations.

Being the 700c x 25 wheel +- 2105 mm in circumference, we translate it in inches = 82.874 inches

Now we divide that diameter by 3.14 so we get the diameter. 26.39 inches.

Being the wheel diameter 26.4 inches, multiplied by fron/rear cog (42/15) my gear inch ratio is

Old gear ratio 26.4*(42/13) = 85.2923077 gear inches.
New gear ratio 26.4*(42/15) = 73.9 gear inches.

I was about to change it a bit, but maybe I'll do it if I change my tires first.

Handy dandy formula, eh?