Measure your futon to get the correct size mattress. Measure the futon when it is completely extended as a bed. Measure from front to back and side to side. Write down the measurements.
Decide which size mattress will fit your futon and write it down. A twin mattress will fit your 39-inch by 75-inch frame. A full-sized mattress will fit your 54-inch by 75-inch frame. A queen-sized mattress will fit your 60-inch by 80-inch frame. A king-sized mattress will fit your 78-inch by 80-inch frame. A California king-sized mattress will fit your 72-inch by 84-inch frame.
Go to your local furniture store and view the futon mattresses available. Test the thickness of each mattress. If the mattress is too thin, you will be able to feel the bars of the frame as you sit or sleep. Choose a mattress that is at least five inches thick.
Choose a style of mattress that will fit the decor of the room it will be furnishing. If there aren't any designs that you like, ask the salesperson at the store if there are covers available as well.