Fill rate is the share of ad requests that returned an actual ad. If a slot is requested 20,000 times a month and an ad appears 12,000 times, your fill rate is 60%.
It is the most ignored number in small-publisher advertising, and it quietly determines what your inventory is worth.
Why it is ignored
Because the empty case is invisible. When no ad is available, a well-behaved slot collapses and the page looks normal. You never see the gap. Your traffic report looks healthy. Meanwhile 40% of your inventory produced nothing.
What it costs you
Two things.
Revenue you never invoiced. Those 8,000 empty requests were real people looking at a real space on your page. If your slot is worth $10 per thousand, that is $80 a month evaporating.
Credibility with sponsors. If you quoted 20,000 impressions and delivered 12,000, you have under-delivered — even though your traffic was exactly as promised. The sponsor will not accept "the slot was empty" as an explanation. Nor should they.
What causes a low fill rate
No campaign booked for part of the month. The most common cause by far. A sponsorship ends on the 20th and nothing replaces it until the 1st.
Targeting that excludes most visitors. A campaign restricted to one country on a site with a global audience can only fill the share of traffic that matches.
Frequency caps. Cap a campaign at three views per visitor per day and your regulars stop seeing it — which is correct behaviour but shows up as unfilled requests.
A budget or impression cap hit early. A campaign that exhausts its cap on the 12th leaves the slot empty for the rest of the month.
How to fix it
Run a house ad. The single highest-return change. Fill unsold inventory with a promotion of your own — a newsletter signup, your own product, an affiliate offer. Fill rate goes to 100% and the empty space starts earning.
Overbook slightly. Two sponsors sharing one slot at a 60/40 weighting fills more of the month than one sponsor with gaps.
Sell in overlapping cycles. Stagger start dates so sponsorships do not all end on the same day.
Loosen targeting. Check whether the restrictions on a campaign are actually earning their keep. Country targeting in particular tends to be set once and never reviewed.
Judge yourself on filled impressions
The number to quote a sponsor, and to track yourself, is filled impressions — not requests. That is what was actually seen.
When you report, show both. "Your ad was requested 20,000 times and displayed 19,400 times" is a transparent way of saying your fill rate is 97%, and it makes the times you deliver 100% look deliberate rather than lucky.
What a good number looks like
With a house ad configured, fill rate should be essentially 100% — there is always something to show. Without one, anything above 80% is healthy for a small site, and below 60% means you have a scheduling problem rather than a traffic problem.