Measure the frame or existing mattress to be sure the new mattress will fit on the frame.
Measure the thickness of the existing futon mattress, if you have one. If the mattress you purchase is too thick, the futon will not fold up properly.
Choose the firmness that you need for support, depending on your preferences. Futon mattresses are available in a range from super soft to ultra firm.
Choose a mattress with a warranty included in the purchase price.
Consider a foam mattress. It's the least expensive option and may work well if you don't plan on using the futon extensively. For futons that will be used on a regular basis, consider a mattress with coils. Solid mattresses that are filled with cotton are fine for medium use.
Purchase a washable cover to protect your investment as well as accent your décor.