A method for measuring the equivalent diameter ( D eq) of a drop of water, said method comprising: - a step (E100, F100) of obtaining a first light intensity signal (SIG 1, SIG 1 ') at a determined angle ( θ) in a horizontal plane defined by the axis (D L) of a laser source and a step (E200, F200) of obtaining a second light intensity signal (SIG 2, SIG 2 ') at said determined angle ( θ) in a vertical plane defined by said axis of said laser source, said light intensities being modulated by the passage of said drop of water through at least two laser beams (f 1, f 2) emitted by said laser source so that said at least two beams are horizontal, parallel and vertically spaced apart by a predetermined distance ( d), said intensity signals being acquired by optical receivers (3, 4) positioned so that the angles formed by the axis (DO 1, DO 2) of said optical receivers and the axis of said laser source (D L) are equal to said determined angle ( θ); - a step (E300, F300) of measuring a first time-shift (Δ 1) between the two largest intensity maxima of one of said intensity signals; - a step (E400, F400) of measuring a second time-shift (Δ 2) between said first (SIG 1, SIG 1 ') and said second (SIG 2, SIG 2 ') intensity signal; - a step (E500, F500-F600) of calculating the equivalent diameter ( D eq) of said drop of water from an ellipsoidal model of said drop of water in the form of an ellipsoid whose major axis is parallel to the horizontal emission plane of said at least two laser beams, a law connecting the ratio (a) of the two axes (A, B) of said ellipsoidal model to the equivalent diameter of said ellipsoidal model, said law associating no constant to said ratio (a) regardless of said equivalent diameter of said ellipsoidal model, said first (Δ 1) and said second (Δ 2) time-shift. - a step (E100, F100) of obtaining a first light intensity signal (SIG 1, SIG 1 ') at a determined angle ( θ) in a horizontal plane defined by the axis (D L) of a laser source and a step (E200, F200) of obtaining a second light intensity signal (SIG 2, SIG 2 ') at said determined angle ( θ) in a vertical plane defined by said axis of said laser source, said light intensities being modulated by the passage of said drop of water through at least two laser beams (f 1, f 2) emitted by said laser source so that said at least two beams are horizontal, parallel and vertically spaced apart by a predetermined distance ( d), said intensity signals being acquired by optical receivers (3, 4) positioned so that the angles formed by the axis (DO 1, DO 2) of said optical receivers and the axis of said laser source (D L) are equal to said determined angle ( θ); - a step (E300, F300) of measuring a first time-shift (Δ 1) between the two largest intensity maxima of one of said intensity signals; - a step (E400, F400) of measuring a second time-shift (Δ 2) between said first (SIG 1, SIG 1 ') and said second (SIG 2, SIG 2 ') intensity signal; - a step (E500, F500-F600) of calculating the equivalent diameter ( D eq) of said drop of water from an ellipsoidal model of said drop of water in the form of an ellipsoid whose major axis is parallel to the horizontal emission plane of said at least two laser beams, a law connecting the ratio (a) of the two axes (A, B) of said ellipsoidal model to the equivalent diameter of said ellipsoidal model, said law associating no constant to said ratio (a) regardless of said equivalent diameter of said ellipsoidal model, said first (Δ 1) and said second (Δ 2) time-shift.