Method to determine the carbon equivalent content of a cast iron alloy which comprises the following steps: a) Previously determining (1.1) the maximum carbon equivalent value for the validity of a linear thermal analysis model for hypoeutectic and eutectic zones that relates the carbon equivalent with the liquidus temperature and the eutectic minimum temperature of a white iron solidification curve; b) Previously determining (1.2) the minimum carbon equivalent value for the validity of a linear thermal analysis model for hypereutectic zone that relates the carbon equivalent with the liquidus temperature of a graphitic solidification curve and the eutectic minimum temperature of a white iron solidification curve; c) Pour a liquid first sample (1.3) of the iron alloy in a composition cup and a liquid second sample (I.4) of the iron alloy in a thermal cup; d) Obtain the solidification curve for both the first sample (1.5) and the second sample (1.6) and hence the white iron solidification curve and the graphitic solidification curve, respectively; e) Obtain (1.7) from the solidification curve of the first sample the liquidus temperature and the eutectic minimum temperature, and with these two values, define the carbon equivalent content for the iron alloy by means of the linear thermal analysis model for hypoeutectic and eutectic zones; f) If the carbon equivalent content for the iron alloy obtained in step e) is lower than the maximum carbon equivalent value defined in a), then establish (1.8) the carbon equivalent content of the iron alloy by using the linear model for hypoeutectic and eutectic zones; g) If (1.8) the carbon equivalent content for the iron alloy obtained in step e) is higher than the maximum carbon equivalent value defined in a), then obtain (1.9) the liquidus temperature of the second sample and check (1.10) if it is higher than the value of the liquidus temperature corresponding to the minimum carbon equivalent value defined in b); h) If the comparisons of step g) are positive, then establish the carbon equivalent content of the cast iron alloy by using the linear thermal analysis model for hypereutectic zone (1.11); i) If the second comparison (1.10) of step g) is negative, then establish the carbon equivalent as the one obtained in e). a) Previously determining (1.1) the maximum carbon equivalent value for the validity of a linear thermal analysis model for hypoeutectic and eutectic zones that relates the carbon equivalent with the liquidus temperature and the eutectic minimum temperature of a white iron solidification curve; b) Previously determining (1.2) the minimum carbon equivalent value for the validity of a linear thermal analysis model for hypereutectic zone that relates the carbon equivalent with the liquidus temperature of a graphitic solidification curve and the eutectic minimum temperature of a white iron solidification curve; c) Pour a liquid first sample (1.3) of the iron alloy in a composition cup and a liquid second sample (I.4) of the iron alloy in a thermal cup; d) Obtain the solidification curve for both the first sample (1.5) and the second sample (1.6) and hence the white iron solidification curve and the graphitic solidification curve, respectively; e) Obtain (1.7) from the solidification curve of the first sample the liquidus temperature and the eutectic minimum temperature, and with these two values, define the carbon equivalent content for the iron alloy by means of the linear thermal analysis model for hypoeutectic and eutectic zones; f) If the carbon equivalent content for the iron alloy obtained in step e) is lower than the maximum carbon equivalent value defined in a), then establish (1.8) the carbon equivalent content of the iron alloy by using the linear model for hypoeutectic and eutectic zones; g) If (1.8) the carbon equivalent content for the iron alloy obtained in step e) is higher than the maximum carbon equivalent value defined in a), then obtain (1.9) the liquidus temperature of the second sample and check (1.10) if it is higher than the value of the liquidus temperature corresponding to the minimum carbon equivalent value defined in b); h) If the comparisons of step g) are positive, then establish the carbon equivalent content of the cast iron alloy by using the linear thermal analysis model for hypereutectic zone (1.11); i) If the second comparison (1.10) of step g) is negative, then establish the carbon equivalent as the one obtained in e).