Convert pixels to kilometer square

I have some monthly .nc files (one is attached). I need to find the pixels which are above threshold three (3) for all the months. And convert it to kilometere square for every month. Therefore, instead of getting the total number of pixels for every month, I will have total km square (area) of the pixels of greater than threshold 3 for every month. Spatial resolution of the data is 0.5 x 0.625