gallons needed = 12.6 gallons. Well, none of the answers are exactly 12.6. The closest one is 13 gallons but!!!! we know that they won't sell you 0.6 of a gallon. I guess you can buy a quart here and there but you will always have some left over. Plus you always use more than you need to so, isn't it better to round it up to 13 gallons just to be on the safe side and avoid another trip to the hardware store?