Materiel Availability (Am) is the percentage of the total inventory of a system operationally capable (ready for tasking) of performing an assigned mission at a given time, based on materiel condition. This can be expressed mathematically as:
Key Performance Parameter (KPP) 
Sustainability (Availability) KPP Consists of two components; Materiel Availability and Operational Availability (AO). These components provide availability percentages from a corporate, fleet-wide perspective and an operational unit perspective, respectively.
- Materiel Availability: Must address the total population of end items planned for operational use, including those temporarily in a non-operational status once placed into service (such as for depot-level maintenance). The total life cycle timeframe, from placement into operational service through the planned end of service life, must be included. Development of the Materiel Availability metric is a Program Manager (PM) responsibility.
- Operational Availability (AO): Percentage of time that a system or group of systems within a unit are operationally capable of performing an assigned mission and can be expressed as uptime/(uptime + downtime). AO can also be expressed in terms of logistics parameters. Development of the AO metric is a requirements manager responsibility. (See Introduction to Operational Availability)
Determining the optimum value for Materiel Availability requires a comprehensive analysis of the system and its planned use, including the planned operating environment, operating tempo, reliability alternatives, maintenance approaches, and supply chain solutions. It’s primarily determined by system downtime, both planned and unplanned, requiring the early examination and determination of critical factors, such as the total number of end items to be fielded and the major categories and drivers of system downtime. 
AcqLinks and References:
-  Website: ACQuipedia – Materiel Availability
- Introduction to Operational Availability
-  DAU Glossary