I've measured this out to be 7 tsp (teaspoon) = 1 oz. or 2.33333 tbsp (tablespoon) = 1 oz.
For fermenters and kegs PBW says .75 oz per gallon of water.
Using teaspoons; it would take 3.75 oz. for 5 gallons. I did some math and that would come out to 26.25 tsp. or 8.75 tbsp. Might as well make it 9 tbsp per 5 gallons.
That's getting a bit crazy, i know...but according to this.. I've been using about 1/4 the amount that it calls for per gallon.
Honestly. I've been using 5-6 tsp per 5 gallons and it works fine if you let it soak.