I am not a fan of the "Inches of fish per gallon" theory as it is too full of variables (fish size and type, filtration, maintenance routine and shape of the tank for a start). If you want the metric version for tropicals, multiply length x breadth of the tank and divide the result by 25 to supposedly give you the maximum amount of fish in cm.
example: a 60L x 30W tank = 1800 divide by 25 = 72cm total
This is probably ok if you have a tank full of small to medium sized fish like the smaller barbs, rasboras and tetras etc, but 1 large 10cm full bodied fish (like an oscar or other cichlids) produces a lot more waste than 2 5cm fish.
For those of you wanting the cold water stocking theory, multiply the length and breadth of the tank and divide the result by 75 to give you the amount of fish you can keep in cm.
example 60L x 30W tank = 1800 divide by 76 = 23.6cm total of fish.
You have to remember to bear in mind the final adult size of the fish, not the current size of your cute little young addition.