Converting PNG to JPG: transparency, quality and file size

An official form that caps attachments at 2 MB, a gallery that takes eight seconds to appear, a Retina screenshot saved as PNG that weighs 9 MB on its own: in most of these situations, switching to JPG solves the problem in a single operation. But JPEG does not “compress” in the everyday sense of the word — it throws information away for good, and it has no transparency channel at all. Before converting in bulk, it is worth knowing what will disappear.

PNG and JPG do not compress the same thing

PNG uses lossless compression: the DEFLATE algorithm, applied to the whole stream after a predictive filter has run line by line. Every pixel comes out exactly as it went in. That behaviour is remarkably efficient on flat areas of colour: an interface, a logo, a chart, black text on a white background all compress extremely well, because hundreds of identical pixels boil down to a handful of bytes.

JPEG works the other way round. It cuts the image into 8×8-pixel blocks, transforms them into frequencies (DCT), then discards the components the eye barely notices. It also reduces the resolution of the colour information relative to the luminance — that is chroma subsampling, typically 4:2:0. On a photograph the trade-off is all but invisible and the saving is enormous; on fine text or a block of vivid colour it produces fringing and leaves a halo around the edges.

Hence the rule that settles almost every case:

• Photographs, gradients, textures: PNG is absurdly heavy here. A 12-megapixel photo saved as PNG often goes past 20 MB; the same shot as a quality-85 JPEG lands between 2 and 4 MB, with no difference you can see on screen.
• Interface screenshots, diagrams, logos: PNG is already both the smallest and the sharpest option. Converting to JPG can produce a file that is larger and uglier.
• A screenshot that contains a photo or a gradient background: here JPEG takes the lead again, sometimes by a factor of five to ten.

Transparency: the number one trap

A PNG-32 carries an alpha channel: every pixel has its own opacity value, which is what lets a cut-out logo sit on any background. JPEG has no equivalent. So you have to decide what replaces the empty space — and the default choice varies from one tool to the next.

Plenty of converters let the rendering engine fill transparent areas with black, which turns a cut-out logo into a dark rectangle. That is what a canvas gives you by default when no background has been painted, and there is no fixing it afterwards: the alpha information no longer exists in the output file. Convertu’s image format converter explicitly paints a white background before drawing whenever the output is JPEG, and shows the warning as soon as you pick that format from the list.

White suits a light-themed site or a printed document. It does not suit a logo that has to land on a coloured banner, a video or a dark background. In that case there are two ways out: keep the PNG, or export to WebP, which holds on to the alpha channel while compressing lossily. A cut-out logo saved as lossless WebP is generally about a quarter smaller than the same file as PNG, and far smaller again if you accept lossy mode, and every current browser displays it.

One more subtlety: a PNG-8 (indexed colour) has only 256 colours, but it can carry an alpha value per palette entry, so its edges can still be antialiased. What it will show is banding across a smooth gradient — and that banding is already there before the conversion, so there is no point blaming JPEG for it.

Choosing the right quality level

The JPEG quality slider runs from 10 to 100. It is not a percentage of fidelity but a quantisation index: each notch changes how aggressively the high frequencies are thrown away. A few useful landmarks:

• 95-100: no visible loss to speak of, but the file can weigh two or three times what it would at 85, for a gain nobody will notice. Keep it for images that will be edited again.
• 85-90: the sensible setting for a photo headed for the web or for an email. It is the format converter’s default (90).
• 70-80: heavy compression, acceptable for thumbnails or a gallery. Sky gradients start to show banding.
• Below 60: the 8-pixel blocks become visible across smooth areas, and any text baked into the image starts to smear.

If the only goal is to shed weight, the dedicated tool — Compress Image — starts at 75 and displays the size you end up with, so you can hit a precise ceiling in a couple of attempts.

One last point that often goes unnoticed: generation loss. Every time a JPEG is saved again, quantisation runs afresh and degrades the image a little more. Open a JPG, crop it, export it, then repeat three times over: the artefacts pile up. Keep the original PNG as your master and regenerate the JPG from it each time.

Converting in practice

The procedure comes down to five steps: drop the file in, choose JPEG as the output format, set the slider, click Convert, download. Three details are worth knowing.

The accepted input formats are not limited to PNG: JPG, WebP, GIF and BMP go through as well, which helps when an old BMP file is still sitting in a folder. On the way out there are three choices: JPEG, PNG and WebP. The quality slider disappears if you select PNG, since that compression is lossless and no setting would change anything.

Two separate limits apply, and only one of them is measured in bytes. The file has to stay under the maximum size shown on the tool page, which the upload check enforces before anything else happens. The second limit is a pixel count, and it is the one people rarely see coming: processing happens inside the browser through a canvas, and beyond roughly 40 megapixels (about 8000 × 5000) browser canvases return a blank image rather than an error — so rather than hand you an empty file, the tool refuses to run and warns that the image is too large for the browser to process. An A4 scan at 600 DPI sits just under that threshold, at around 35 Mpx, but an A3 scan at the same setting, or an A4 page at 1200 DPI, blows well past it: reduce the dimensions first with the image resizer, then convert. A phone photo of 8 to 12 Mpx causes no trouble at all.

One file at a time. Because the work is local there is no queue, but no batch mode either: twenty images means twenty runs. In return, none of those files ever leaves your device.

Metadata and colour: the side effects

A conversion through a canvas re-encodes the image pixel by pixel. The direct consequence: everything attached to the original file disappears. EXIF data (camera model, date, GPS coordinates), comment blocks and embedded profiles are not copied across into the JPEG that comes out.

That is good news for privacy — publishing a photo without its GPS coordinates normally takes a deliberate clean-up — and bad news if you were relying on the capture date to sort your files. Make a note of it beforehand, or keep the original.

On the colour side, a PNG carrying a wide-gamut profile such as Display P3 or Adobe RGB can come out looking slightly duller: the canvas works in sRGB. The shift stays subtle except on heavily saturated reds and greens, but if colour accuracy is critical — print work, brand guidelines — you are better off going through software that handles profile conversion explicitly.

When it is better not to convert

Four cases where JPG is the wrong instinct:

• A screenshot containing text destined for documentation: quantising the high frequencies smears the characters and leaves a halo around the edges. Stay in PNG — on an interface capture it is already the most compact format, and re-encoding it will not make it any lighter.
• A logo or an icon that has to stay cut out: PNG or WebP, never JPEG.
• A visual that will be retouched later: every export cycle degrades the file a little further.
• A document you need to pass on: if the end goal is something you can email and print, a PDF is usually more useful than a lone JPG, especially across several pages.

A closely related situation comes up all the time: iPhone photos in HEIC, which Windows and plenty of websites refuse to open. That format needs a decoder browsers do not provide, so it goes through the server-side HEIC to JPG tool — one free conversion a day with no sign-up, files being wiped from the servers after an hour; beyond that, the €7-a-year subscription lifts the limit.