Quantum volume is a metric that measures the performance of a quantum computer’s capabilities and error rates. IBM’s quantum computer Raleigh, achieved a score of 32 in January 2020. In March 2020 Honeywell’s quantum computer achieved a volume of 64 with just 6 qubits In August 2020 IBM announced it had also achieved a volume of 64 in a 27-qubit system.
Quantum computers are difficult to compare. Quantum volume is a single number designed to show all round performance. It is calculated by taking into account several features of a quantum computer, starting with its number of qubits—other measures used are gate and measurement errors, crosstalk and connectivity.
IBM introduced the Quantum Volume metric  because a classical computer’s transistor count and a quantum computer’s quantum bit count aren’t the same. Qubits decohere with a resulting loss of performance so a few fault tolerant bits are more valuable as a performance measure than a larger number of noisy, error-prone qubits. 
Generally, the larger the quantum volume, the more complex the problems a quantum computer can solve.
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