Isnt PBW just really Souped up Oxyclean? Essentially an alkaline cleaner?
Sorry, totally missed beersk's post a few above this one when I posted the recipe below. Same thing. I usually measure it by volume. It's not an exact science. You can play with the ratio a bit to find something that works well for you.
My original post:
If you want to make some homemade PBW, use 3 parts Oxyclean (no dyes) or a generic subsitute like Sun Oxygen Cleaner (Sodium percarbonate) with 1 part Red Devil TSP-90 (Sodium Metasilicate) available at most hardware stores. Mix well and you can have almost 9 lbs of homemade PBW for less than $10.
This recipe is derived from the description of the make up of PBW given by Charlie Talley (owner of 5 Star) on the Brewing Network. I have used it for several years with great success.