The rule everyone repeats
Search a “meta description length” and you’ll get the same answer from a hundred different sites: keep it under about 155 to 160 characters, or Google will cut it off with an ellipsis. It’s good advice, as far as it goes. It’s also not quite true, because character count was never the actual rule. Pixel width was, the whole time.
What Google actually measures
Google doesn’t count letters. It renders your description in a search result and cuts it off when the rendered text runs out of room in the space it has, measured in pixels, not characters. Most of the time those two things line up closely enough that a character-count rule works fine as an estimate. But letters aren’t all the same width. A lowercase “i” takes up a sliver of space. A capital “M” takes up nearly ten times as much. A description packed with narrow letters can run well past 160 characters and still fit. A description built from wide letters can get cut off sooner than the character count suggests it should.
This is easiest to see with two strings of identical length. “Illinois” and eight capital M’s in a row both have eight characters. Rendered in the same font, they come out to very different widths, because the letters themselves are different shapes. A character-count limit treats both strings identically. A pixel-width limit, which is what Google is actually working from, does not.
Desktop and mobile use different limits
The available space also changes by device. Mobile search results have a wider container but a smaller font, so the practical limit for a description runs longer on mobile than on desktop — somewhere around 1,300 pixels on mobile against roughly 920 on desktop, by current observation. Neither number is published by Google, and both can shift without notice. Treat them as the best current estimate, not a guarantee.
Why “just count characters” still mostly works
None of this means the 155-character rule is useless. Most ordinary English sentences use a mix of narrow and wide letters that averages out close enough to make a character count a reasonable approximation. The rule breaks down at the edges: a description full of narrow letters like “i,” “l,” and “t,” or one full of wide letters like “m,” “w,” and capital letters, will diverge from what a plain character count predicts. If your description sits right at the boundary, character count alone won’t tell you which side of the line you’re actually on.
Google will rewrite it anyway, regardless of length
Length is only one reason Google replaces what you wrote. It also swaps in its own snippet when it decides a different passage from the page answers the search query better than your description does, when the description looks like generic boilerplate repeated across many pages, or when the query itself doesn’t match what you wrote closely enough. Studies of large sets of search results have found Google rewrites the majority of meta descriptions it’s given, for reasons that have nothing to do with truncation. Getting the length right improves your odds. It doesn’t guarantee Google will use your description at all. See why Google rewrites your title tag for the same pattern playing out with titles, where it’s been studied more closely.
Writing to fit the space you actually have
Front-load the part that matters. Put your strongest, most specific claim in the first sentence, because that’s the part most likely to survive if the rest gets trimmed. Avoid restating your title tag word for word — a description that just repeats the title wastes the extra space you were given. Write it as a complete, natural sentence or two rather than a comma-separated pile of keywords; Google has gotten much better at recognizing keyword stuffing, and it reads badly to an actual visitor deciding whether to click. And don’t write the same description, or a lightly reworded version of it, across dozens of pages on your site — duplicate descriptions are one of the more common reasons Google decides to write its own instead.
Checking your own description
Character count can get you close, but it can’t tell you exactly where your description will cut off, because it isn’t measuring the thing that actually determines the cut. The meta length checker measures rendered pixel width directly, the same way a browser or a search result actually renders your text, and shows you a live preview of where the line lands for both desktop and mobile. It also flags emoji and non-Latin scripts, which measure differently than the plain Latin text these limits are calibrated for. Paste in a draft and see exactly how much room you actually have, instead of guessing from a character count that was never the real rule to begin with.