One Uptime guarantee is a service provider's promise that its online services will be available for a certain period of time. Uptime is usually measured as a percentage and indicates how often the service was available during a certain period of time. For example, a 99.9% uptime guarantee means that the service will not be offline for more than 0.1% (about 43 minutes per month) during the agreed period. Note, however, that the Uptime guarantee does not guarantee the performance or speed of the service.