Cycles per instruction ($\mathrm{CPI}=C/N$)

Cycles per instruction is the average number of processor clock cycles spent per counted instruction for a specified execution interval and counting convention. If $N$ instructions complete in $C$ cycles, $\mathrm{CPI}=C/N$. At constant clock frequency $f$, execution time satisfiesFor example, $10^9$ completed instructions at CPI 1.5 and $3\times 10^9$ hertz take 0.5 seconds.

The value depends on instruction mix, dependencies, cache misses, branch behavior and the chosen microarchitecture. It can be less than one when several instructions complete per cycle. A long-latency instruction need not prevent other independent work from overlapping, so individual instruction latencies cannot simply be added to predict elapsed time. Comparing processors by frequency alone ignores both CPI and the number of instructions needed for the same task.