Basics
Fill rate
Fill rate is the percentage of ad requests that are actually filled with an ad, calculated as impressions served divided by ad requests, multiplied by 100.
Every time an ad slot asks for an ad, that is an ad request. If an ad comes back and is shown, the request was "filled". Fill rate is simply filled requests divided by total requests. A 90% fill rate means one in ten opportunities went unused.
Publishers watch fill rate by demand partner, format and geography. Low fill can mean too-high floors, weak demand in a country, blocked categories, latency or auction timeout issues, or ads that failed to render. In app ad mediation, each network has its own fill rate, and mediation works down the list until an ad is found.
High fill is not automatically good: you can reach 100% by accepting very low bids or low-quality ads, which hurts eCPM and user experience. Fill rate also differs from sell-through rate, which measures how much of a publisher's forecast inventory was sold in advance through direct deals. Sudden jumps in requests without matching fill can also be a sign of bot traffic inflating requests.
Think of it like this
Fill rate is like the share of restaurant tables that are occupied on a given night: empty tables earn nothing, but cramming in anyone at any price is not the goal either.
An example
An app sends 1,000,000 ad requests and gets 820,000 ads back that display. Fill rate = 820,000 ÷ 1,000,000 = 82%.
Related terms
Ad request
An ad request is the signal a web page, app or video player sends to an ad server or ad platform asking for an ad to fill a specific slot.
eCPM (effective CPM)
eCPM (effective CPM) is the revenue or cost per thousand impressions calculated after the fact, which lets you compare ads sold on CPM, CPC, CPA or other models.
Ad mediation
Ad mediation is software that lets app developers connect many ad networks and bidders through one SDK and chooses which network fills each ad request to maximise revenue.
Sell-through rate
Sell-through rate is the percentage of a publisher's available inventory that has been sold, usually through direct or guaranteed deals, over a given period.
Floor price (reserve price)
A floor price is the minimum bid a publisher will accept for an impression; bids below the floor are rejected, even if they are the highest bid in the auction.