User and system behaviour is difficult to predict for novel systems, and this affects the capacity of the system (the number of active users that can be supported with acceptable response delay). This leads to a range of values, in the form of a feasible or acceptable region for the potential capacity, conditional on the uncertain parameters. This work considers uncertainties in the delay between requests (user think time), network latency, and cache behaviour due to users? locality of reference. The acceptable region is shown to be bounded approximately by linear constraints which are easy to derive. This simple result is useful for sensitivity and scalability analysis, and appears to have been overlooked. It is applied to a web-based system for telephony, using VoiceXML for service ranging from interactive voice response, to voice-based e-mail.