JPG to PNG Converter

About JPG to PNG Converter

Why Convert JPG to PNG?

Most people convert JPG to PNG to stop quality loss from further edits, to get a format with alpha transparency, or to meet a platform requirement (e.g. favicons, app icons, some print workflows). PNG's DEFLATE compression is bit-for-bit reversible, so editing and re-saving does not degrade the image the way repeatedly saving a JPEG does.

Reason	Explanation
Lossless editing	PNG preserves every pixel exactly. Each time you edit and re-save a JPEG, quality degrades - PNG avoids this
Transparency support	PNG supports an alpha channel for transparent backgrounds. JPEG does not
Text and sharp edges	JPEG compression creates artefacts around sharp edges and text. PNG keeps them crisp
Further processing	If the image will be edited, composited, or layered, PNG is the better working format
Platform requirements	Some platforms, tools, or APIs specifically require PNG input

How the Conversion Works

The browser decodes the JPEG into raw RGBA pixels, draws them onto an HTML5 Canvas at original dimensions, then re-encodes that pixel buffer as PNG via canvas.toBlob(). No server processes the file and no quality slider is involved, because PNG encoding is lossless by design - the output captures the decoded JPEG pixels exactly.

Worked example. A 1920×1080 JPEG photo of 380 KB decodes to 1920 × 1080 × 4 bytes = 8,294,400 bytes of raw RGBA in memory. Canvas draws this untouched. PNG then applies a per-row filter (None, Sub, Up, Average, or Paeth - chosen per line by the encoder) and pipes the result through DEFLATE with a 32 KB sliding window. Because photographic data has high entropy, DEFLATE cannot compress it much, and the PNG typically ends up around 2.8-3.5 MB. Every compression artefact already baked into the JPEG is preserved - the PNG is larger but not visibly better than the JPEG it came from.

Step	What Happens
1. Upload JPEG	File is read into memory using the FileReader API
2. Decode pixels	Browser decodes the JPEG and creates an Image object
3. Draw to Canvas	Image is drawn at original dimensions onto an offscreen Canvas
4. Export as PNG	Canvas.toBlob() encodes the pixel data as lossless PNG
5. Preview and download	Side-by-side comparison with file sizes shown

JPEG vs PNG: When to Use Each

Factor	JPEG	PNG
Compression	Lossy - quality decreases with more compression	Lossless - no quality loss regardless of file size
Transparency	Not supported	Full alpha channel (256 levels of transparency)
File size (photos)	Much smaller (typically 5-10x)	Larger - every pixel stored exactly
File size (graphics)	Larger due to artefacts around sharp edges	Often smaller for simple graphics with few colours
Best for	Photographs, natural images, web delivery	Screenshots, graphics, logos, images with text, transparency
Colour depth	8-bit (16.7 million colours)	Up to 16-bit (trillions of colours)
Re-editing	Quality degrades with each save	No quality loss on re-save

File Size Expectations

Converting from JPEG to PNG will almost always increase the file size, because PNG uses lossless compression while JPEG is lossy. The increase depends on the image content:

Image Type	Typical JPEG Size	Typical PNG Size	Increase
Photograph (1920x1080)	200-500 KB	2-5 MB	5-10x larger
Screenshot with text	100-300 KB	200-600 KB	1.5-2x larger
Simple graphic / logo	50-150 KB	20-80 KB	Actually smaller in many cases
Illustration with flat colours	100-250 KB	80-200 KB	Similar or smaller

Batch Conversion

Select multiple files at once or drop a batch into the drop zone. Each file is converted independently with its own before/after preview, size readout and download button. The browser processes files in parallel via Promise.all, so a 10-image batch completes in roughly the time of the slowest single image, not the sum. A 100 MB per-file cap prevents the tab from running out of memory on very large originals.

What Does PNG Actually Do Differently?

PNG stores RGBA pixels in rows, applies one of five filters per row (None, Sub, Up, Average, Paeth - defined in the W3C PNG Third Edition), then runs the filtered bytes through DEFLATE. JPEG instead splits the image into 8×8 blocks, runs a type-II Discrete Cosine Transform on each, quantises the resulting frequency coefficients (this is the lossy step), then encodes them with run-length and Huffman coding. That quantisation is why JPEG degrades on every save, and why converting to PNG locks in whatever quality the JPEG currently has.

Aspect	JPEG (ISO/IEC 10918)	PNG (ISO/IEC 15948)
Compression algorithm	DCT + quantisation + Huffman	Row filter + DEFLATE (LZ77 + Huffman)
Lossy step	Quantisation of DCT coefficients	None
Transparency	No alpha channel	Full 8-bit alpha channel
Max colour depth	8 bits per channel	Up to 16 bits per channel
Re-save behaviour	Quality decays per save	Bit-identical on every save
Typical photo size	200-500 KB at 1080p	2-5 MB at 1080p

When to Convert and When Not To

Convert JPG to PNG when the image will be edited again, composited, needs transparency added, or is a screenshot or logo where sharp edges matter. Do not convert photographs that will only be viewed or uploaded to the web - the PNG will be several times larger with no visible improvement, and some platforms (Instagram, WhatsApp, most CMS uploaders) re-encode everything back to JPEG on upload anyway.

Scenario	Convert?	Reason
Screenshot with text for docs	Yes	PNG keeps text crisp; JPEG blurs edges
Logo going into a design file	Yes	Needs transparency and clean edges
Photo staying as a photo on the web	No	Bigger file, no visible benefit
Image about to be re-edited multiple times	Yes	Stops generational JPEG quality loss
Favicon or app icon source	Yes	Most icon toolchains expect PNG input
Print photo at final stage	No	Printers typically prefer high-quality JPEG or TIFF

Important Note About Quality

Converting JPEG to PNG does not recover quality lost during the original JPEG compression. If the source JPEG was saved at low quality and already has visible blocking around edges, haloing around text or colour banding in skies, those artefacts are baked into the pixel data and will be preserved exactly in the PNG. Think of the conversion as wrapping the existing pixels in a different container, not as a restoration. If detail is missing, only re-shooting the image or running it through an AI upscaler can genuinely add it back.

File Size: What to Expect After Conversion

Expect a PNG to be 3-10× the size of the source JPEG for photographs, roughly 1.5-2× for screenshots with text, and potentially smaller than the JPEG for simple flat-colour graphics. The reason is that DEFLATE compresses repeated byte patterns well (flat colours and sharp edges) but struggles with the high-entropy, noise-like data produced by JPEG decompression of a photo. PNG optimisers (pngcrush, OptiPNG, zopflipng) can shave another 10-30% off typical PNG output by trying every filter combination - browser canvas.toBlob picks defaults instead of exhaustively searching, so browser-produced PNGs are never the smallest possible.

Browser Support and Privacy

Canvas-based image conversion works in every current browser (Chrome, Edge, Firefox, Safari, Opera) on both desktop and mobile. Because the entire pipeline runs locally - FileReader, Image decode, Canvas draw, toBlob encode, URL.createObjectURL preview - nothing ever leaves the device. There is no upload, no server-side processing, and no analytics call tied to the image contents. The only network traffic is the initial page load. This matters for screenshots that contain private data, ID cards, medical images, or anything covered by GDPR or HIPAA where uploading to a third-party converter would be a breach risk.

Common Mistakes

• Using PNG for every photo. A 6 MB PNG of a holiday snap loads five times slower than the 800 KB JPEG it was made from, with no visible difference. Keep photos as JPEG unless there is a specific reason not to.
• Expecting transparency to appear automatically. JPEGs do not have an alpha channel. The converted PNG will have a solid background until you remove it in an editor.
• Relying on PNG to "improve" a low-quality JPEG. PNG preserves the current pixels - including the current artefacts. Quality lost in JPEG quantisation cannot be restored by a format change.
• Running hundreds of multi-megapixel files at once on a phone browser. Each decoded image sits in memory as raw RGBA. Ten 24 MP images can exceed 700 MB of RAM. On mobile, batch in groups of 5-10.

If you need to go the other direction, the PNG to JPG converter handles that. You can also compress your images before or after conversion, and resize images to a sensible pixel dimension first so the PNG does not become unnecessarily huge. Everything stays on your device - the tool works offline once the page is loaded.

Sources

• W3C - Portable Network Graphics (PNG) Specification, Third Edition
• IETF RFC 1951 - DEFLATE Compressed Data Format
• ISO/IEC 15948 - PNG International Standard
• ISO/IEC 10918-1 - JPEG International Standard
• MDN - HTMLCanvasElement.toBlob()
• Wikipedia - JPEG compression overview