A calibration method of a valve, wherein a valve seat houses a movable element which extends along an axis, a spring associated with the movable element and a ball carried by the movable element and arranged, in use, in a closing position, in abutment against an abutment seat housed in the valve seat and provided with a channel having a first end configured to be closed by the ball and a second end communicating with an inlet of the valve; the ball separates the channel from a chamber which houses the spring, the movable element and the ball and communicates with an outlet of the valve; said method using a calibration tool, provided with a pusher element movable, in use, along said axis, the method comprising a calibration phase wherein the following steps are carried out: - axially moving the pusher element so that the pusher element abuts on the abutment seat and displaces it with respect to the valve seat and subsequently stopping the motion of the pusher element; - supplying fluid under pressure to the inlet of the valve being calibrated; - creating a narrowing at the outlet of the valve and measuring the pressure inside the chamber detecting a pressure caused by the fluid supplied at the inlet to the valve being calibrated; - carrying out an adjustment step for the flow of the fluid entering the valve wherein the pressure inside said chamber is controlled so that it is close to a set fixed value (P out); - acquiring the supply pressure (P mis) of the valve during the adjustment step; - comparing the pressure (P mis) with a valve setting target pressure (Ptarget) calculating the pressure difference ΔP; and calculating the displacement (ΔX) to be applied to the seat in order to reduce the pressure difference (ΔP) to a minimum value; - controlling the displacement of the pusher element to physically perform the displacement (ΔX) of the seat terminating the adjustment step. the ball separates the channel from a chamber which houses the spring, the movable element and the ball and communicates with an outlet of the valve; said method using a calibration tool, provided with a pusher element movable, in use, along said axis, the method comprising a calibration phase wherein the following steps are carried out: - axially moving the pusher element so that the pusher element abuts on the abutment seat and displaces it with respect to the valve seat and subsequently stopping the motion of the pusher element; - supplying fluid under pressure to the inlet of the valve being calibrated; - creating a narrowing at the outlet of the valve and measuring the pressure inside the chamber detecting a pressure caused by the fluid supplied at the inlet to the valve being calibrated; - carrying out an adjustment step for the flow of the fluid entering the valve wherein the pressure inside said chamber is controlled so that it is close to a set fixed value (P out); - acquiring the supply pressure (P mis) of the valve during the adjustment step; - comparing the pressure (P mis) with a valve setting target pressure (Ptarget) calculating the pressure difference ΔP; and - axially moving the pusher element so that the pusher element abuts on the abutment seat and displaces it with respect to the valve seat and subsequently stopping the motion of the pusher element; - supplying fluid under pressure to the inlet of the valve being calibrated; - creating a narrowing at the outlet of the valve and measuring the pressure inside the chamber detecting a pressure caused by the fluid supplied at the inlet to the valve being calibrated; - carrying out an adjustment step for the flow of the fluid entering the valve wherein the pressure inside said chamber is controlled so that it is close to a set fixed value (P out); - acquiring the supply pressure (P mis) of the valve during the adjustment step; - comparing the pressure (P mis) with a valve setting target pressure (Ptarget) calculating the pressure difference ΔP; and calculating the displacement (ΔX) to be applied to the seat in order to reduce the pressure difference (ΔP) to a minimum value; - controlling the displacement of the pusher element to physically perform the displacement (ΔX) of the seat terminating the adjustment step. - controlling the displacement of the pusher element to physically perform the displacement (ΔX) of the seat terminating the adjustment step.