A system for balancing upright piano key mechanisms consists in weighting the key (1) in its front portion (at 11), opposite to the portion (5) where the key downweight is usually placed, and consequently in weighting (at 15), by means of a weight simulating the weight of the hammer, also the portion where the hammer (7) rests, the hammer in turn resting, through the wippen (9), on the rear portion of the key, so as to create a balance between the front portion and the rear portion of the key itself.