The instrument
The CPM formula, worked to death
By Oliver Wakefield-Smith · updated October 2026
The short answer
CPM equals spend divided by impressions, multiplied by 1,000. The two rearrangements: impressions equal spend divided by CPM times 1,000, and spend equals CPM times impressions divided by 1,000. Keep impressions as raw counts, not thousands, and the formula never lies to you.
CPM = spend ÷ impressions × 1,000
$750 ÷ 62,000 × 1,000 = $12.10
impressions = spend ÷ CPM × 1,000
$750 ÷ $12.10 × 1,000 = 61,983
spend = CPM × impressions ÷ 1,000
$12.10 × 62,000 ÷ 1,000 = $750.20
Why divide by 1,000 at all
Mille is Latin for thousand; the metric dates from print rate cards quoted per thousand readers. Google's own definition is the modern authority: CPM is the cost per 1,000 impressions. Quoting per single impression would put four zeros after the decimal point, and nobody negotiates in hundredths of a cent. More on the letters at what does CPM stand for.
The two unit traps
- Thousands vs raw counts. If a report already shows impressions in thousands and you multiply by 1,000 again, your CPM is off by 1,000x. If your computed CPM is $0.01 or $10,000, you have hit this trap, not a market anomaly.
- Impressions vs reach. Impressions count deliveries; reach counts people. One person seeing your ad five times is five impressions, one reach. Dividing spend by reach gives cost per person, a different (and bigger) number. The reach side is worked at reach vs frequency.
Six worked substitutions at sourced rates
A flat $1,000 budget, run through the formula at every sourced rate on this site (retrieved October 2026). Same arithmetic as the calculator.
| Channel | CPM | $1,000 buys | Source |
|---|---|---|---|
| Meta, Dec 2025 | $10.02 | 99,800 impressions | Gupta Media tracker |
| Meta, Jun 2025 | $8.06 | 124,069 impressions | Gupta Media tracker |
| TikTok, Dec 2025 | $2.84 | 352,113 impressions | Gupta Media tracker |
| Podcast 30s host-read | $18.00 | 55,556 impressions | Libsyn Ads, low end of $18-22 |
| Podcast 60s host-read | $24.00 | 41,667 impressions | Libsyn Ads, low end of $24-26 |
| Podcast programmatic | $12.00 | 83,333 impressions | Libsyn Ads, low end of $12-15 |
Related: eCPM applies the same formula to earnings instead of spend. The $1,000 test budget scenario runs this table against a real campaign plan.