Two plans with the same impressions can do opposite things
A million impressions can mean 500,000 people seeing an ad twice, or 50,000 people seeing it twenty times. Both cost the same at the same CPM; one builds awareness and the other annoys a small crowd that already decided. Impressions tell you how much you bought. Reach and frequency tell you what it did.
Reading frequency in the results
Frequency is rarely wrong on its own; it is wrong relative to what the audience does next. The pattern to watch is click-through rate falling week over week while frequency rises and CPM holds steady. Nothing changed about the audience or the price — they have simply seen it enough. The fixes, in order: new creative, a broader audience, then less budget.
Planning a campaign with this calculator
Start from the audience, not the budget. Decide how many people the campaign has to reach and how often, solve for impressions here, then price those impressions with the CPM calculator. Working the other way — budget first — tends to produce a plan that reaches whoever was cheapest rather than whoever mattered.
For a warm audience of recent visitors, the retargeting calculator runs the same arithmetic from your site traffic and adds the return side.