An original method is presented to measure time resolved unsteady flow rate. From the kinetic energy theorem, the balance equation of the kinetic energy is integrated on a conical control volume (Venturi type). A non-linear ODE is obtained, simple enough to be resolved in real-time (that is to say in less than one time step). The two coefficients of the ODE depend only on the geometry of the device and are hence known by construction, it is therefore not necessary to calibrate the device. Experimental measurements at the intake of a combustion engine show that the device provides unsteady flow rate. By temporal integration, the unsteady measurements give a mean flow rate close to the reference flowmeter.The same analysis is then applied to a cylindrical control volume with a singular pressure drop (induce by a grid for example). In this case, one of the two coefficients of the ODE must be determined experimentally. Experimental measurements carried out with this device at the intake of a combustion engine show that this second method follows the flow fluctuations. A good agreement is found between unsteady measurements integrated over a whole number of periods and reference mass flowmeter. The inversions of direction of the flow are taken into account by the prototype. This device, more compact than the conical version (Venturi type), allows real-time measurements of the unsteady flow rate.