What density actually tells you
Keyword density is the share of your text taken up by a word or phrase. It was once a ranking factor in the crude sense — repeat a phrase enough and you ranked for it — and that era ended two decades ago. Google has never published a target, and no threshold triggers a penalty.
What density still measures well is repetitiveness. A phrase at 6% of a page is a phrase a reader trips over every other sentence, and that is a real problem: people leave, and the page reads like it was written for a machine. The number is a proxy for prose quality, not a ranking dial.
The other honest use is absence. Writers routinely produce a page targeting a phrase that never appears in it, because they wrote around the idea in synonyms. If your focus keyword shows zero uses, that is worth knowing before anything else on the page.
Why the arithmetic here differs from other tools
Density needs a denominator, and tools quietly disagree about it. Here it is always total words: a three-word phrase used four times in a 400-word page is 4 ÷ 400, or 1%. Some tools instead divide by the number of three-word positions, which is roughly a third of the word count, and report the same page as 3%.
That is not a rounding difference. It is the difference between a page that looks fine and a page that looks stuffed, and it sends people rewriting copy that was never a problem. The same denominator across all three phrase lengths is what makes the numbers comparable to each other.
There is a second choice worth stating: phrases here never begin or end on a filler word. Without that rule, "the seo audit", "seo audit is" and "a seo audit" are counted as three different phrases, and the one you actually care about is split across all of them.
What to do with the result
If your focus phrase appears zero times, add it where it belongs — the title, the opening paragraph, one subheading. If it appears once in 1,500 words, the page probably is not really about it, which is a content question rather than an editing one.
If something sits above 5%, read those sentences aloud. Repetition that survives reading aloud is usually fine — a page about GST will say GST often, and that is the subject, not stuffing. Repetition that makes you wince is the kind Google's spam policies are aimed at.
And then stop. There is no optimal figure to tune towards, and no version of this tool that can give you one, because the number that matters is whether the page answers the question better than the ones currently ranking.