Power transformers (PT) are the most important components in electrical power systems; their failures generate considerable losses. Worldwide, many PT are in their final stages of life; their owners invest only what is necessary in maintenance and operation and, as they wish to know the actual condition of the equipment at all times, in order to reduce their failures, a method, using the available operating, maintenance and diagnostic information, should be used to estimate the health condition of the PT by obtaining the Health Index (HI). This becomes more complex in developing countries, with economic problems that hinder the acquisition of diagnostic equipment, so it should be checked whether the methods used internationally, which involve the mathematical processing of the available information, are usable in the conditions of these countries. The authors have found no references indicating that this aspect has been addressed previously. The HI of the investigated PT was calculated by Excel, using the data provided by the diagnostic tests that could be carried out; the results correspond with the general state of each equipment and the opinion of the experts consulted, so it was verified that one of the methods, used internationally to calculate the HI, can be used in conditions where diagnostic tests are poorly carried out. Subsequently, a web page was set up to calculate the HI of the PT and to store the information, which ensures independence from methods used in other latitudes.