1/4" or so. Just kidding. I hiked with a woman that told me her sister never slept on a bed. She just used a pad on the floor. And in many countries they do not have big fluffy beds. So for 3 months I slept on a pad on a carpeted concrete floor. It took a week to get used to it, but after that it was no problem. The effect on my camping sleep was obvious. I slept much better and felt more rested on camping trips. I finally gave up on the pad at home - my wife and kids were starting to wonder if about my sanity - they just don't understand.