There is provided a device for measuring a user's oxygen-consumption. The device includes a venturi tube. The venturi tube has a first tapered portion, a second tapered portion that is more tapered compared to the first tapered portion, and a constriction between the portions thereof. The device includes at least one pressure sensor in communication with the constriction and the first tapered portion of the venturi tube. The device includes an oxygen sensor in communication with the constriction and the first tapered portion of the venturi tube.