I am currently treating my decks with Penofin. The manufacturer says coverage will range from 175 to 300 square feet per gallon. I have about 1100 square feet of redwood deck PLUS a lot of deck rail and spindles. Its looking like about a 5-gallon job at this moment. Without knowing anything about your deck, its condition or wood species, you should figure on about 3 gallons, and maybe 2 for less porous woods.