The device has a support (1) with a measuring face (5) and a contact face (4) to be applied on skin (6) of a person or an animal, and an inflating element (2) by absorbing sweat and secured to the support. The inflating element is arranged to be in direct contact or fluid communication with the skin, and measuring units (3) e.g. photodetector, measure increase of dimension of the inflating element, where increase of dimension is caused by absorption of sweat. The inflating element is made of hydrophilic polymer. An independent claim is also included for a method for estimating the water lost by a person or an animal.
