Although important properties such as strength, toughness, and weldability can be easily measured in the laboratory, there is no generally accepted laboratory test for determining the atmospheric corrosion resistance of low-alloy weathering steels. Because many years of outdoor testing are needed to develop corrosion data, there is a need for reliable methods of estimating corrosion performance. The relative corrosion resistance of weathering steels can be estimated from the corrosion index calculated from composition by use of the ASTM Standard Guide for Estimating the Atmospheric Corrosion Resistance of Low-Alloy Steels (G 101). This paper describes a new, alternate method for estimating corrosion resistance from composition that has been recently added to G 101. The new method involves calculation of a corrosion index based on historical data recently published by Bethlehem Steel. It overcomes several limitations of the original G 101 corrosion index. Specifically, more elements are considered. In addition to the five elements (copper, nickel, phosphorus, chromium, and silicon) considered in G 101, the new method also takes account of the effects of several other elements, including carbon, molybdenum, sulfur, and tin. Because the method is based on data from three test sites, as compared to only one site for G 101, the results should be applicable to a wider range of environmental conditions. Also, the ranges of the elements are generally larger than those of G 101. Finally, the effects of the elements increase monotonically. As a result, the new method is free of anomalies resulting from quadratic terms, such as a maximum effect of copper at about 0.25% that is predicted by G 101. The overall absence of quadratic terms allows for more reliable extrapolation beyond the ranges of the original data. To facilitate computation of the G 101 corrosion indices, a calculator is available on the ASTM website at http://www.astm.org/COMMIT/G01_G101Calculator.xls.