Chemical analysis has gained in importance in recent years because of an increase in international trade. Hence the quality of the results of chemical analysis and the extent of their reliability for the purpose for which they are used has become extremely important. This requirement is fulfilled by indicating the uncertainty of the measurement, which gives a measure of the confidence that can be placed on the result irrespective of the method used. In this paper, we evaluate the uncertainty of measurement for the determination of manganese in steel by spectrophotometry, following the internationally accepted guidelines brought out by ISO. Standard steel no. 20.02, obtained from NML, Jamshedpur, India, has been taken as the unknown sample and the concentration of manganese in it has been measured against certified reference steel BCS No. 232 (UK). The percentage of Mn in the NML steel sample is estimated as (0.56 ± 0.03) %m/m. The reported expanded uncertainty of the measurement is stated as the standard uncertainty of measurement multiplied by a coverage factor of 2, which for a normal distribution corresponds to a coverage probability of approximately 95%.