The BM25 term-frequency saturation parameter (commonly written k1) that sets how quickly repeated occurrences of a keyword stop adding to a document’s relevance score.
| Term | Saturation Parameter |
|---|---|
| Category | Statistics and Evidence |
| Also known as | k1, BM25 K1 |
| Where it appears | Settings → Factors and BM25 math |
What it means in RankGear
RankGear exposes the saturation parameter in Settings → Factors, alongside the rest of the BM25 math. BM25 scores a page partly on how often a query term appears, but each additional occurrence contributes less than the one before it — k1 controls how steep that tapering is. A low value makes the term-frequency contribution plateau after only a few mentions; a higher value lets further repetitions keep adding before the curve flattens. RankGear applies the same value everywhere it computes a BM25 or BM25F figure, so relevance scores stay comparable from one page and one run to the next.
How to interpret it
k1 is a positive tuning constant, usually in the 1.2 to 2.0 range in standard BM25 implementations. Read it as a diminishing-returns dial: smaller means term frequency saturates faster, so the first repetitions do most of the work and later ones barely move the number; larger means the reward for repetition fades more slowly. It is not a score or a ranking signal on its own — changing it rescales how heavily RankGear weighs keyword frequency inside its relevance math. Leave it fixed when you want two runs to be directly comparable, and re-baseline if you ever change it.
| k1 value | What it tells you |
|---|---|
| Lower (around 1.2) | Term frequency saturates quickly — the first few mentions carry most of the contribution and further repetition adds little. |
| Higher (around 2.0) | Repetition keeps contributing longer before the score flattens, so heavier term frequency is rewarded more. |
Example
Say a page uses its target phrase eight times. With k1 tuned low, the second and third occurrences carry almost the whole term-frequency contribution, and occurrences four through eight add almost nothing — repeating the phrase further will not lift the BM25 component. Raise k1 and those later mentions still nudge the score upward before it flattens. Score the same unchanged page under both settings and the relevance figure moves purely because the math changed, not because the page did.
Important considerations
- k1 shapes RankGear’s internal BM25 relevance math. It is not a Google ranking factor, and Google neither exposes nor uses this value.
- A BM25 figure is comparative evidence, not proof of causation — a higher relevance score does not by itself make a page rank.
- Change k1 only deliberately; altering it partway through a project breaks comparability between earlier and later runs.
- Small SERP samples can be unstable, so read BM25-derived comparisons across enough competitors before drawing conclusions.
Related terms
Part of the RankGear glossary · how RankGear measures · the 870 factors.