The paper advances an amalgamated model of software usability in conjunction with a method for improving the usability of commercial software products. The model organises the substantial amount of prior research on software usability into seven basic principles, their underlying attributes, and associated relative weights. This model of software usability for human-computer interaction has two primary goals: first, to allow software designers to make quantitative decisions about which usability attributes should be included in a design; and second, to provide a usability metric by which software designs can be consistently rated and compared. The paper focuses on the second goal. Since it is ultimately the users of a software system who decide its usability, the method proposed suggests that the users be made an integral part of the software design and development process. It recommends that the users of current and future software products be asked, through questionnaires, how well a product meets the principles and underlying attributes of usability as defined by the model. Only if the ultimate users of a product are pleased is a product likely to succeed. The paper reviews the model and its evaluation. It then illustrates the method by describing its use in evaluting a particular wordprocessing product, WordPerfect. In that evaluation, 988 questionnaires were sent to users of the product.