Embodiments relate to idle time accumulation in a multithreading computer system. According to one aspect, a computer-implemented method for idle time accumulation in a computer system is provided. The computer system includes a configuration having a plurality of cores and an operating system (OS)-image configurable between single thread (ST) mode and a multithreading (MT) mode in a logical partition. The MT mode supports multiple threads on shared resources per core simultaneously. The method includes executing a query instruction on an initiating core of the plurality of cores. The executing includes obtaining, by the OS-image, a maximum thread identification value indicating a current maximum thread identifier of the cores within the logical partition. The initiating core also obtains a multithreading idle time value for each of the cores indicating an aggregate amount of idle time of all threads enabled on each of the cores in the MT mode.