An estimator identifying the flow rate through a rotary blood pump was developed. This estimation algorithm uses the pump current and speed signals to estimate the blood flow rate through the pump. The algorithm was derived based on the force balance between the electric torque generated by the motor and the load torque, including the mechanical losses. The corresponding estimator parameters were identified using experimental data in the laboratory. The estimator performed well in mock circulatory loop experiments simulating different physiologic conditions. This algorithm will be implemented in a single board computer to estimate the flow rate of the rotary pump in an implantable left ventricular assist device without invasive sensors.