While there is no limit, weight plays a role in the overall performance of the system due to the compression (caused by weight) of the silicone tubes that circulate the cold water throughout the pad.  Based on information from ChiliSleep, the Chilipad performs best under a net weight of 250 pounds (90kg). For reference, I weigh approximately 210 pounds and my buddy Bryan clocks in at 240 pounds. Neither of us have noticed a degradation in performance.