Myth: more ad slots always lift page RPM

Myth: more ad slots always lift page RPM

The oldest reflex: add a unit, make more money. Past a point, you're cannibalizing yourself.

What it does — extra in-content slots raise impressions per pageview.
Setup effort — 1/5 to add, 4/5 to know when to stop.
The correction — page RPM is impressions × per-impression value. Stacking units splits the same advertiser demand, drops each unit's competition, and your average eCPM sinks faster than impressions rise. Session RPM (per visitor) is the honest metric — it usually peaks 1-2 slots before page RPM does.
RPM lift seen — trimming from 7 to 5 in-content units raised session RPM 8% on a test site.
Pros of trimming — faster page, better viewability per slot.
Cons — fewer impressions to sell.

Lift: ★★★★☆ — Risk: ★★☆☆☆
Best for: cluttered templates. Skip if you're already under 4 slots.