It doesn't rank you. It decides whether the ranking is worth anything.
Google has stated plainly and repeatedly that the meta description is not used as a ranking signal. That fact gets quoted as a reason to ignore it, which is the wrong conclusion. Position three with a snippet that answers the question outperforms position two with a snippet that reads like a company boilerplate.
So treat it as ad copy for a slot you already won. It costs nothing, it ships with the page, and it is the only part of your search result you can write freely — the title tag is squeezed by pixel width and the URL is fixed.
Why Google swaps in its own snippet
Google's own documentation is explicit that snippets are generated to suit the query. That single sentence explains almost every rewrite you've ever seen. A page that ranks for forty different phrasings has one meta description; Google would rather show a passage from the page that contains the words the searcher actually typed, with those words bolded.
Google has never published a rewrite rate, and any number you see quoted comes from third-party sampling, not from Google. What's consistently observable is that rewriting is common, not exceptional, and it climbs as a page ranks for a wider spread of queries.
What makes Google keep yours
- It contains the query phrase, or a close variant of it, in natural sentences.
- It genuinely summarises the page rather than pitching the company.
- It fits the display window without being cut mid-thought.
- The page ranks for a narrow, well-defined set of queries — a definition page, a product page, a service page.
What gets it thrown out
- Duplicated across a template, so hundreds of URLs share one description.
- Written about the brand instead of the page: "We are a leading agency offering a range of services."
- Stuffed with commas and keyword variants rather than written as sentences.
- Wildly mismatched with the query — a description about pricing on a page ranking for a how-to.
The 140–155 character window, and why it's a convention
Like titles, descriptions are truncated by rendered width rather than a hard character count, and Google has changed the display length more than once — it briefly ran much longer snippets in 2017 before pulling back. Treat 140–155 characters as a house rule that survives most of that churn, and expect mobile to show less.
Front-load the meaning. Write the sentence that would make someone click in the first eighty characters, and let the rest be the part you can afford to lose. Do not write to the character count by padding; a 120-character description that says something beats a 155-character one that trails off into filler.
Pages where you should skip it entirely
This is the part most guides won't say: on some pages, leaving the description blank produces better snippets than writing one. Google's own guidance acknowledges that a good generated snippet can beat a poor hand-written one, and a blank field lets Google pick a passage matched to each individual query.
Skip it on: large ecommerce catalogues where you'd be generating descriptions programmatically anyway, deep blog archives that pull long-tail traffic across dozens of phrasings, and any page whose queries you genuinely can't predict. Write it deliberately on: the homepage, service pages, location pages, comparison pages and anything with a commercial query you're actively targeting.
Whichever you choose, check the result. Search Console's performance report shows you which queries a page earns clicks for — if the click rate is far below its neighbours at the same average position, the snippet is the suspect.