Red blood cells (RBCs) are more deformable than most other cells in the body, and any change in the deformability of RBCs can have major physiological effects. Here, we report a high-throughput micro- fluidic device to determine the Young's modulus of single RBCs. Our device consists of a single channel opening into a funnel, with a semi-circular obstacle placed at the mouth of the funnel. As an RBC passes the obstacle, it deflects from its original path. Using populations of artificially stiffened RBCs, we show that the stiffer RBCs deflect more compared to the healthy RBCs. We then generate a calibration curve that maps each RBC trajectory to its Young's modulus, obtained using an atomic force microscope. Finally, we sort a mixed population of RBCs based on their deformability alone. Our device could potentially be further miniaturized to sort and obtain the elastic constants of nanoscale objects, such as exosomes, whose shape change is difficult to monitor by optical microscopy.