Compress pictureMake your photos smaller in seconds

The details

Fewer bytes, the same pixel dimensions. Set a quality, type a cap in kilobytes, or take a percentage off — then read what it cost at 400%.

What compression takes away

A JPEG is stored as blocks of eight pixels by eight, each turned into a set of frequency coefficients, each coefficient divided by a number from a quantisation table and rounded. The quality setting chooses that table. Rounding is where the bytes go, and what it rounds away is the fine end of the picture: the edge of small text, single-pixel highlights, film grain, the texture of skin. None of it comes back on the way out.

A PNG works the other way round. Nothing is discarded unless you ask for it: the lossless pass encodes the picture under more than one row filter, keeps whichever attempt came out shortest, and hands back pixels that are bit-for-bit the pixels that went in. Ask for a palette instead and every pixel is remapped to the nearest of the colours you allowed, which is why a screenshot survives it and a sunset does not.

Neither mechanism changes how big the picture is. A 4032 by 3024 photograph that leaves here at 180 KB is still 4032 by 3024 — though read that figure for what it is: twelve million pixels carrying about one byte for every sixty-six of them, which is a hard squeeze and a visible one, not a free result you should expect on every file. Dimensions surviving is the whole promise of this page, and it is the reason the one operation that would change them — reducing the long edge when a cap is out of reach — is a question rather than a step.

The pass, step by step

1. 1The file is read from disk by this page. Its first megabyte is walked segment by segment for metadata blocks, and each block's length is added up — that is where the itemised figures in the ledger come from.
2. 2Orientation is resolved once. The browser is tested with a two-pixel JPEG that carries a rotation flag, and the rotation is applied by hand only if the browser did not already apply it, so a phone photograph never comes out sideways or twice-rotated.
3. 3You choose the mechanism: a quality, a byte cap, or a percentage off. The picture is encoded at quality 80 the moment it opens, and again every time a setting comes to rest, so the size on the ruler is always a measured one.
4. 4The encode runs in a worker. For a byte cap, the first trial is the format's floor — if even that overshoots, you know after one encode instead of eight — and the remaining trials climb back up looking for the best setting that still fits. Every trial is printed with its size as it lands.
5. 5The result is decoded again for the comparison. What you split, set side by side or magnify to 400 percent is the file itself, not a preview of it, so the bytes you approve are the bytes you save.
6. 6The result downloads as one file. If the re-encode came back larger than what you opened, the tool says so and tells you to keep your original rather than offering you a worse file.

Formats, and where each one stops

A JPEG leaves as a JPEG and a PNG leaves as a PNG unless you choose otherwise in the Format panel. The extension never changes on its own for either of them.

• JPG and PNG are read and written on this page, and the two routes below go into each of them properly.
• JPEG, PNG, WebP and AVIF are written as well as read — WebP and AVIF by libwebp and libavif in a background worker. GIF, BMP, HEIC and — where this browser can decode it — TIFF are read but not written: a file in one of those leaves as PNG if it has transparency to keep and as JPG if it does not, unless you pick another format. An efficient WebP re-encoded as JPG can easily come back larger; that is reported rather than hidden.
• Animated GIF and animated WebP lose their animation on this page — the first frame is what gets decoded. For an animated GIF, use the GIF page, which keeps every frame.
• A CMYK JPEG is converted to sRGB by the browser's own decoder before this tool sees a pixel, and a 16-bit PNG drops to 8 bits per channel on the way through.
• Intake stops at 120 MB, and at one file: this page is an instrument, and opening a second picture asks whether to replace the first rather than starting a queue behind your back. Above roughly 32 megapixels — 16 on iOS — you are offered a lighter working copy as a question, because past that a canvas fails by handing back a blank image rather than by reporting an error.

When the format is doing the work

A file is sometimes large because it is in the wrong container rather than because it was written generously. A photograph saved as a PNG carries every pixel exactly, artefacts included, and commonly weighs eight times what the same picture weighs as a JPEG at a quality nobody could fault. A screenshot of black text saved as a JPEG is the same mistake pointing the other way: the letters get soft edges and the file is bigger than a palette PNG of it would have been.

Changing the container is a different job from making the container smaller, and this page does not do it behind your back — a PNG that arrives here leaves as a PNG unless you choose otherwise. The Format panel measures the alternatives on a small sample of your picture and may suggest one, with the figures; it switches only when you say so. Where the change of format is the job, choose it yourself in the Format panel once a picture is open, or start from the WebP route, which writes WebP from the first encode and prints the measured bytes beside each setting.

The three mechanisms, one page each

• Compress to a size in KBThe byte cap as the whole interface, with the caps that recruitment and university forms keep asking for.
• Compress a JPEGWhat the quality number selects, and where the floor of an already-compressed JPEG is.
• Compress a PNGThe lossless pass and the palette pass, kept apart, because only one of them is lossless.

Two things worth reading when a file will not move

Some photographs barely shrink and the control above will not explain why on its own. Two pieces of writing cover the cases where that happens, both short, both about files rather than about theory. The index of the guides says which is which and how long each takes.

• Why this JPEG will not get smallerQuantisation floors, what a second re-encode costs, and when the honest answer is to leave the file alone.
• Upload limits on Indian exam portalsWhat the photograph, signature and document fields measure, and the order to do the work in when a form caps both dimensions and bytes.