Unity & In-App UA
Fill Rate
The share of ad requests a network actually serves an ad against, an inventory-availability metric that shapes how much volume an in-app campaign can reach.
A high fill rate means the network has demand and inventory matched for your targeting; a low one means your requests go unfilled and volume is capped no matter how good the creative is. For a buyer it is a read on whether a GEO or format can actually deliver scale.
Fill rate varies by GEO, format, and season, so thin Tier-3 inventory or a crowded tournament window can throttle delivery. The trap is blaming the campaign for low volume when the real ceiling is inventory that simply is not there to buy.
In buyer speech
“Volume stalled on that GEO because fill rate on the format was low, not because the bid was wrong.”