The estimation problem of carbon content of molten steel in a Basic Oxygen Furnace is addressed in this work. It is assumed that only measurements of carbon monoxide content of off-gas are available. Given that such measurements are corrupted with noise, determining the carbon from the thermodynamic equilibrium relationship yields poor results. Due to this, an integral estimator with noise attenuation is proposed. Results of numerical simulations show a good agreement between the actual carbon content and the estimated value.