It took me two months to work up the courage to purchase a mattress online! I bought two twin-size mattresses for the twin size upholstered day bed with trundle (manufacturer Latitude Run), and I have to say its a tight fit on the trundle, but it works! And my goodness, this mattress is a DREAM! Super soft but supportive. I have a senior pup that fell asleep on it the moment the bed was put together ❤️