<p>In-situ stresses are among the most important inputs in all deep underground engineering analysis. In the oil and gas industry, in-situ stresses are critical to the integrity of boreholes during drilling, completion, and production. Uncertainty in stress determination can result in costly failures, such as borehole collapse, sand production, liner deformation, etc., especially when the safe operation window is narrow, like in underbalanced drilling. In this work, we validated two improved methods for accurately determining the maximum horizontal stress from borehole breakout. Compared with the original breakout method, the major difference is the net borehole pressure term, i.e., in an impermeable borehole, the effect of net borehole pressure is magnified by friction angle thus the original breakout method tends to underestimate the maximum horizontal stress; in a permeable borehole, the effect of net borehole pressure is scaled by Poisson’s ratio of the formation. The predicted maximum horizontal stress is further constrained by the limit imposed by the stress regime where the borehole is located. The improved methods are verified with a numerical simulation and shown to be capable of improving the accuracy of the determined maximum horizontal stress from breakout width. The improved methods, together with the original method, have been implemented in an in-house wellbore mechanical software with an illustrative example provided. Three borehole breakout models are ready for field applications, e.g., to support the design of some new operation projects, or diagnose observations and failures of some existing operation projects.</p>