A method for determining anatomical dead space in a respiratory tract of a living organizm, include the steps of continuously and simultaneously measuring flow (F) and respiratory air density (D) during exhalation (EX) over time (T). The time is measured from the start of exhalation, in which the flow is greater than zero, until the dead space point at which, after significant decreases following the start, the respiratory air density merges to an approximately constant value. The integral of the flow is formed from the start point until the dead space end point, the measurements of the respiratory air density and the flow from the start until the dead space end point and during the short time, as that time span in which the respiratory air density assumes an approximately constant value, taken place multiple times in each case.