Expect BestMeta description length is one of the most asked and least important questions in search, and the reason it keeps coming up is that people assume it affects rankings. It does not, and it has not for well over a decade.
What it does affect is whether anyone clicks, which is not nothing.
The meta description length that actually displays
Google truncates by pixel width rather than by character count, but as a practical rule it shows roughly 155 to 160 characters on desktop and closer to 120 on mobile.
Write for 120 to 155. Front-load the important part so that a mobile truncation still leaves a coherent, persuasive sentence. Anything past 160 is being written for nobody.
The lower bound matters too. A thirty-character description looks thin in the results and wastes space a competitor is using to make their case.
What it is actually for
Think of it as the only advertising copy you get to write that appears directly in Google's results, free of charge, next to your competitors' attempts at the same thing.
That framing changes how you write it. A summary of the page is a waste of the slot. What earns the click is a reason to choose this result over the four around it: a specific benefit, a price, a location, a differentiator, or a direct answer to the question the person just typed.
Compare these two for a local plumber. A description reading "We are a family-run plumbing company established in 1998 offering a range of plumbing services" is a summary. One reading "Emergency plumber covering Poole and Bournemouth. No call-out fee, same-day appointments, Gas Safe registered. Call before 4pm for today." is an advert.
Why Google rewrites most of them
Studies consistently find Google replaces the majority of meta descriptions with its own snippet, and that proportion has grown as search has become better at matching intent.
This is not a failure on your part. Google rewrites when it thinks a different passage from the page matches the specific search more closely, and with one page ranking for dozens of different queries, it is often right.
The practical consequence is that the description is not a guarantee. It is a strong suggestion, most likely to be honoured when the search closely matches what you wrote. That is another argument for writing it against one specific search rather than describing the page in general.
Writing one that works
Start with the search the page is actually for and answer it in the first clause. Include the thing that differentiates you, whether that is location, price, speed or specialism. Add a light call to action if there is room, and there usually is not.
Avoid three things. Keyword stuffing, which reads badly and persuades nobody. Duplicating descriptions across pages, which tells Google the pages are interchangeable. And leaving the default text from your theme in place, which happens more often than anyone would like to admit.
Meta description length in the order of priorities
Low, honestly. If your pages are slow, thin or badly structured, rewriting descriptions is rearranging furniture.
But it is quick, it is entirely within your control, and on a page already ranking in positions four to ten a better description can lift click-through measurably without moving the position at all. That makes it one of the better returns on twenty minutes of work available in search, as long as the bigger things are already handled.
Bear in mind too that one page usually ranks for dozens of different searches, and you are writing a single description for all of them. Pick the one that matters commercially and write for that, accepting that Google will substitute its own text for the rest. Trying to write something that covers every possible query produces a sentence that persuades nobody.
A sensible order of work: fix the pages that rank between four and fifteen first, because a click-through gain there is worth the most. Leave page-one-position-one alone, since it is already getting the clicks. Ignore anything on page three entirely until the position improves, because nobody is reading the description of a result nobody sees.
Our SEO service treats it that way: worth doing properly, never worth doing first.