A method and device for evaluating the capacity of a compressor (1) by air being allowed to flow out from a pressure tank (3) through an opening of known geometry. Following a calculation, the quantity of evacuated air is obtained. After this, a compressor (1) pumps back up to the initial pressure in the pressure tank (3). The compressor capacity (1) is established by comparing the time it takes for the compressor (1) to pump back up to the initial pressure in the pressure tank (3) with the time it takes when an acceptable compressor (1) pumps the same quantity of air. By the compressor capacity (1) is here meant the quantity of air which the compressor (1) delivers per unit of time at a given compressor speed and counter-pressure.