In one line
CPC (Cost Per Click) is the cost of a single click. It is less a standalone metric than the result of CPM ÷ CTR, so when it rises the first question is which of those two moved.
Why it matters
At the same CPM, higher CTR means lower CPC. So when CPC rises, figure out whether the platform got more expensive (CPM up) or your creative stopped working (CTR down) — the fix is completely different depending on which it is.
Go deeper
What to do when CTR is low is covered in Ad Performance Diagnosis.
Reading it in order
If CPC rose since last week, put CPM and CTR side by side for the same period and channel. CPM alone rising points at bid competition, audience expansion or contraction, or seasonality — media price. CPM steady while CTR falls points at creative fatigue, a mismatch between message and audience, or a placement shift. If both moved, break it down by channel and creative rather than concluding from one average.
Splitting it with numbers
CPC is CPM ÷ (CTR × 1,000). At a $2 CPM and 1% CTR, that is a $0.20 CPC.
Move CPM alone to $3 and CPC becomes $0.30; drop CTR alone to 0.5% and CPC becomes $0.40. The same "CPC is up" report has two completely different fixes behind it, and CPC on its own cannot tell you which — so open CPM and CTR alongside it every time.
Go deeper
What to fix when CTR is low is covered in Ad Performance Diagnosis.