Summary: It is often necessary to calculate MTBF (mean time between failures) quickly in order to make timely design decisions. An important system for which such calculations must be made is a k-out-of-n: G parallel system with unlimited repair and exponential interfailure and repair times at the unit level. Although a general formula is known, it is not easily remembered nor derived. This paper presents a method for deriving a formula for MTBF in this situation that is easily reproduced quickly by remembering a few simple concepts.