We have developed a tracking algorithm to determine the speeds of supra-arcade downflows (SADs) based on the Farneback optical flow method. We show that the algorithm is effective by implementing it in several flares. The calculated heating terms related to velocity (adiabatic compression, viscous heating) showed consistency with previous works. In addition, the flow calculation helps us semi-automatically detect the sizes of SADs. In the one flare we have analyzed so far, the derived size results showed a SAD size distribution with small sizes that are unrevealed in previous works, thanks to the high spatial resolution of SDO/AIA. We are going to conduct an analysis on a large sample of flares observed by SDO/AIA and compare it with previous statistical works done in the era without SDO/AIA.