If you are using 65-68% cal hypo then one pound is the equivalent of 1 gallon of 10-12.5% liquid chlorine (not exact but close enough!)

Beware, much of the cal hypo out there, particularly the HTH stuff is now only 45-48% becuase Walmart had an explosion and fire from it in one of their warehouses and wanted the strength reduced! (When Walmart speaks vendors listen!) HTH is about to reformulate their shock again and include phosphonic acid, a seqesterant. This could increase the phosphate content of your water. We just had a meeting about it at work today (We carry the HTH line) and the regional Mgr. wants us to try and go exclusively to the ProTeam products which are still 68% cal hypo without any additives except for their top of the line shock which also has borax in it (not really a bad thing). Now it will just take convincing our customers who are used to HTH which costs less (but you have to use more to get the same ppm chlorine so it actually costs more to use!)