Balance with inertia adjustment for timepieces, including a hub defining the pivot axis of the balance, a rim, a web extending between a lower plane and an upper plane on either side of a median plane, the web including at least one arm connecting the rim to the hub, the arm including a housing configured to receive and hold in place at least one inertia block. According to the invention, the inertia block has a front face and a rear face and is integrated in the balance arm, so that the front face and rear face are substantially in the upper plane and the lower plane.