17.5 actually lol. But you never truly fill up your tank as there's still room on the top so it's not actually 17.5. It's maybe a gallon less. You have to account for vapor. I'm too concerned about how much I have when my light comes on, I just wanna know how far the needle goes down before it dies. It went below the E line yesterday and it still ran, so I think I'm good lol.