It depends. For pontoon carpet, you can generally buy the length plus a little extra. For bass boat carpet, you may want to buy the length of the carpeted area plus 20% to allow extra for wrapping lids. Some people even make a chart of the area to more accurately estimate. Just MAKE SURE that all the carpet runs in the same direction – if you spin a piece of boat carpet sideways or backward, it will look a different color than the other pieces of marine carpet.