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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Should you lower SQL Server fill factor to prevent page splits? Not by default. Microsoft says most workloads perform optimally with the default fill factor. A split is a reason to investigate an index’s workload—not, by itself, evidence that changing its fill factor will improve performance.
What a page split animation shows—and what it doesn’t
SQL Server database pages are 8 KiB. When a B-tree index page does not have room for an inserted row, SQL Server adds a page and moves approximately half the original page’s data to it. That is structural work: it changes how index entries are arranged. It does not prove that the split caused a measurable query slowdown.
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Splits in the middle of an index can be resource-intensive and may contribute to fragmentation, which can reduce read-ahead effectiveness during large scans. But the cost to a workload depends on how often splits occur and what that workload does. An animation illustrates the space-management event; it cannot establish its performance impact. Microsoft’s page architecture guide describes SQL Server pages and the movement of data during a split.
What fill factor changes
Fill factor controls how full SQL Server makes an index’s leaf-level pages when the index is created or rebuilt. A lower value leaves room on each leaf page for possible growth; it does not reserve one shared block of space at the end of the index. For example, a fill factor of 80 leaves 20 percent free on each leaf page at that point in time.
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SQL Server’s server-wide default fill factor value of 0 means pages are filled to capacity and is equivalent to 100. Microsoft says most workloads perform optimally with the default. A lower setting is therefore a workload-specific tradeoff, not a general fix for splits. Microsoft’s fill-factor documentation explains the setting and its default.
Why fewer splits can still mean slower reads
Space left empty on leaf pages has an immediate cost: the index occupies more storage, takes more memory to cache, and can require more disk I/O to read. Microsoft illustrates the tradeoff by noting that a fill factor of 50 doubles the disk I/O and memory required to read and cache the same amount of data. That is a documented illustration, not a benchmark result for every database.
Lower page density also means more pages for the same index data. Microsoft’s index-maintenance guidance warns that this can raise I/O, memory, CPU, and tree-level costs, and says increasing page density can often have a greater positive performance impact than reducing fragmentation. A fill factor that reduces some insert-related splits may still make read-heavy work more expensive.
Microsoft’s fill-factor guidance says database reads typically outnumber writes by a factor of five to ten, even for a write-intensive workload. That is the rationale given in the documentation, not a universal independently measured ratio. The useful question for a particular index is whether the write-side benefit of reserved space outweighs the read and storage costs. See Microsoft’s index reorganization and rebuild guidance for its discussion of density and fragmentation.
Check where new keys land
Reserved space helps only when future inserts or updates can use it. Consider the key pattern for the index you are changing:
- Keys inserted across the index: If new keys arrive throughout the key range, free space on leaf pages may accommodate some growth and reduce the need for splits.
- Keys added at the end: With an increasing key such as an
IDENTITY, new rows are typically added at the index’s right edge. Free space left on other leaf pages may go unused, so lowering fill factor may add read and storage costs without addressing the relevant insertion pressure.
Do not choose a lower setting from a split count alone. Establish that splits are materially affecting the workload, then assess whether the index’s insert pattern is likely to use the reserved space.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When and how SQL Server applies the setting
Fill factor is applied when an index is created or rebuilt; setting a value does not continuously keep pages at that density as rows change. Microsoft documents this as an example of a rebuild command with fill factor 80:
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ALTER INDEX index_name ON table_name REBUILD WITH (FILLFACTOR = 80);
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The value 80 is an example from the documentation, not a recommended target. There is no universally correct lower fill factor established for an index without evidence about its workload. If you test a change, evaluate both write behavior and read behavior rather than treating a lower split count as success by itself.
Don’t carry SQL Server’s setting over to PostgreSQL
Fill-factor defaults and behavior are engine-specific. PostgreSQL 18 documents a default B-tree fill factor of 90 and a selectable range of 10–100; its documentation says values from 50 to 90 may help some indexes expecting many inserts or updates by smoothing early-life splits. Those are PostgreSQL B-tree figures, not SQL Server recommendations. Consult the relevant engine and index-method documentation before transferring a setting: PostgreSQL 18 CREATE INDEX.
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