Purpose: Telecobalt machines are widely used in radiotherapy treatment units in developing countries, which have a Markus chamber to measure surface and build-up doses without applying the over-response correction factor (ξ), therefore, the aim of this study is to introduce the formulas for predicting the ξ of the Markus for 60Co beam in the present and previous studies. Methods: The percentage depth dose in build-up region was measured by a Markus, which was irradiated by a 60Co beam. Irradiations were performed in solid water equivalent phantom at various depths and fields. Results: The percent over-response of Gerbi and Khan (%ξ_GK) values were higher than that for Rawlinson (〖%ξ〗_R) for both chamber models at surface of phantom and reverse at all depth beyond surface, additionally, the measurements of %ξ were larger than that predicating for both formulas. Conclusion: Surface dose measurement is one of the most challenging issues for clinical dosimetry in radiotherapy. Accurate knowledge of surface and build-up region doses is very important. Therefore, this study recommends using the extrapolation chambers for measuring surface and build-up doses accurately. Although, these chambers are impractical because of very laborious and time-consuming procedures, so, this study also recommends using any formula of them to determine the dose over-response of Markus in build-up. These formulas can be easily implemented and allow the clinician and medical physicist to assess the accurate surface dose of patient. The percentage surface doses show a strong correlation to the structure of ionization chamber to minimize the over-response of chamber.