Many methods for estimating rock strength for drilling rate optimization were developed for the oil and gas industry, some of which are being used in geothermal fields. We have developed a methodology for using estimated rock strength for drillability assessment in the hydrothermally altered lithologies encountered in the geothermal industry. The Modified Alteration Strength Index (mASI) provides rock strength estimates using simple intact rock properties observable in drill cuttings, such as mineralogy and fractures. We show that rock strengths for hydrothermally altered rock estimated using mASI correlate better to drillability indicators than convetional methods from the oil and gas industry.