A method for manufacturing a balance-spring of a clockwork regulating organ, comprising the following steps: - determining the geometry of the balance-spring; - manufacturing the balance-spring with the geometry thus determined; characterized in that the step for determining the geometry of the balance-spring consists in defining at least one continuous polynomial function characterizing the entire length of the balance-spring, the representative curve of which passes through at least three points fixed by the position and the geometry of fastening organs of the balance-spring, the three points being: - the point defined by the fastening of the balance-spring to a collet, - the point of the next-to-last coil intended to be situated in front of a balance-spring stud, and - the point of the last coil intended to be fastened to said balance-spring stud, said polynomial function being a function that characterizes the radius of the balance-spring and is expressed as follows: R s θ = a i θ i +... + a 2 θ 2 + a 1 θ + a 0 avec i > 3 with R S [m] Radius of the balance-spring θ [ rad] Angle of the balance-spring from the collet a [-] Polynomial coefficient and in that it further consists in defining a second function defining the thickness of the balance-spring e s over the entire length thereof, said second function being: e s θ = b j θ j +... + b 1 θ + b 0 avec j ≥ 2 with e s θ = es θmax − es 0 R 0 − R θmax R 0 − R θ + es 0 or alternatively, said thickness being defined by the following three equations: β = e αθ max eslog θmax / es θmax − 1 and eslog θ = e aθ β + 1 es θmax − eslog 0 R 0 − R θmax R 0 − R θ + eslog 0 while guaranteeing that es moy = eslog moy where eslog moy is the mean thickness obtained for eslog(θ) with θ comprised between θ min and θ max, and es moy is the mean thickness obtained for es(θ) with θ comprised between θ min and θ max; and in that it consists in optimizing the coefficients of said polynomial and said second function by calculations and simulations by calculating the movement of the center of gravity of the obtained balance-spring and decreasing this movement, and/or by calculating the reaction force at the pivot of the obtained balance-spring and decreasing the variations of this reaction force. - determining the geometry of the balance-spring; - manufacturing the balance-spring with the geometry thus determined; characterized in that the step for determining the geometry of the balance-spring consists in defining at least one continuous polynomial function characterizing the entire length of the balance-spring, the representative curve of which passes through at least three points fixed by the position and the geometry of fastening organs of the balance-spring, the three points being: - the point defined by the fastening of the balance-spring to a collet, - the point of the next-to-last coil intended to be situated in front of a balance-spring stud, and - the point of the last coil intended to be fastened to said balance-spring stud, said polynomial function being a function that characterizes the radius of the balance-spring and is expressed as follows: R s θ = a i θ i +... + a 2 θ 2 + a 1 θ + a 0 avec i > 3 with R S [m] Radius of the balance-spring θ [ rad] Angle of the balance-spring from the collet a [-] Polynomial coefficient and in that it further consists in defining a second function defining the thickness of the balance-spring e s over the entire length thereof, said second function being: e s θ = b j θ j +... + b 1 θ + b 0 avec j ≥ 2 with e s θ = es θmax − es 0 R 0 − R θmax R 0 − R θ + es 0 or alternatively, said thickness being defined by the following three equations: β = e αθ max eslog θmax / es θmax − 1 and eslog θ = e aθ β + 1 es θmax − eslog 0 R 0 − R θmax R 0 − R θ + eslog 0 while guaranteeing that es moy = eslog moy where eslog moy is the mean thickness obtained for eslog(θ) with θ comprised between θ min and θ max, and es moy is the mean thickness obtained for es(θ) with θ comprised between θ min and θ max; - the point defined by the fastening of the balance-spring to a collet, - the point of the next-to-last coil intended to be situated in front of a balance-spring stud, and - the point of the last coil intended to be fastened to said balance-spring stud, R S [m] Radius of the balance-spring θ [ rad] Angle of the balance-spring from the collet a [-] Polynomial coefficient