JPEG: what the quality number controls

What it is

JPEG divides a picture into eight-by-eight blocks, transforms each block into a set of frequency coefficients, and then divides those coefficients by numbers taken from a table before rounding them. The quality setting chooses the table. A high setting divides by small numbers and keeps most of the detail; a low setting divides by large ones and rounds most of the fine structure to zero.

That is the whole mechanism, and it explains both of the format’s habits. Loss lands on high-frequency detail first, which is why a face survives what a page of text does not. And because the rounding happens per block, a picture pushed too far shows its block grid rather than degrading smoothly.

Where it opens

The rows below are about software a file will actually be handed to, not about a specification. A format is supported when the person you sent it to can open it.

Where	What happens
Every browser	Opens everywhere, and has since before any of them were named.
Windows and macOS, out of the box	Opens in the file viewer, the mail client and the print dialog with nothing installed.
Email clients	The only image format no mail client has ever had an opinion about.
Print workflows	Accepted by every layout and print-ordering system, though a lossless source is better where one exists.
Anything expecting transparency	There is no alpha channel. Transparent pixels become whatever colour was painted behind them.

Last read against vendor documentation on 19 September 2026. Reviewed quarterly; if that date has slipped, treat the rows above as history rather than fact.

What it costs to write

Native to every browser here: a 12-megapixel photograph writes in tens of milliseconds, which is why this bench can search seven settings for it without anyone noticing.

Where it goes wrong

Every save is another pass through the encoder. Open, edit, save, repeat, and the loss accumulates in a way no later save can undo.
Chroma subsampling throws away colour resolution in a way that shows on saturated red text and thin coloured lines long before it shows on a face.
Flat areas band. A gradient sky that looked smooth at quality 95 can show steps at quality 80 while the rest of the picture looks untouched.
A CMYK JPEG, which some print software writes, decodes with its colours inverted in more than one browser.

When this is the right answer

A photograph that has to be opened by software you do not control. A file going to a print shop, an insurance form, a mail client of unknown vintage, or anyone whose computer you have never seen. There is no format with a longer tail of support and there will not be one for a long time.

It is the wrong answer for anything drawn — screenshots, charts, logos, line art — and for anything that needs transparency, which it cannot store at all.

Questions about JPEG

Is quality 80 in one encoder the same as quality 80 in another?

No. The setting indexes that encoder's own quantisation tables and its own tuning, so two encoders at the same number produce visibly different pictures at different sizes. That is the reason this site searches for a measured result instead of trusting the number.

What does chroma subsampling do?

It stores colour at half the resolution of brightness, horizontally or in both directions, on the basis that human vision is less sensitive to fine colour detail. It usually costs nothing visible on a photograph and is obvious on saturated red text, thin coloured lines and hard colour edges.

Does opening and saving a JPEG damage it?

Opening it does nothing. Saving it runs the pixels through the encoder again, and that second pass rounds coefficients that were already rounded. The damage is small per save and cumulative across them, and no later save recovers what an earlier one discarded.

Why is my JPEG bigger after this bench encoded it?

Because your original was encoded at a lower quality than the target you selected, so matching that target required a larger file. The size shown is a fact about a file the bench made; when it is larger than yours, the recommendation says to keep yours.

What happens to a transparent picture saved as JPEG?

The transparency is flattened onto a colour you choose before the encode runs, defaulting to white. It cannot be undone afterwards, which is why the choice is offered first rather than reported later.

Encoding happens on this machine; the file stays where it is.