PNG to JPG Converter — Lossless Source to Lossy DCT
PNG (W3C 2nd Edition, 10 November 2003 / ISO/IEC 15948:2004) is the lossless web standard for raster graphics with optional 8-bit alpha; JPEG (ITU-T T.81 approved 18 September 1992 / ISO/IEC 10918-1:1994) is the lossy DCT-based standard optimised for photographs. Most JPG files in the wild are JFIF-wrapped (JPEG File Interchange Format Version 1.02, Eric Hamilton at C-Cube Microsystems, 1 September 1992) — the de facto file container around the bare T.81 entropy-coded bitstream. The conversion handles two technical details bare format-swapping tools often miss: (1) PNG's alpha channel is composited against a configurable background colour (default white) using the WHATWG Canvas 2D Context, since JPEG has no alpha; (2) ICC colour profile handling defaults to sRGB (IEC 61966-2-1:1999, the foundational web RGB standard). Quality control (1–100) maps to JPEG quantisation table scaling per ITU-T T.81 Annex K — 85 is the standard web sweet spot for visually lossless photo encoding. The conversion runs locally in the browser via Canvas toBlob('image/jpeg', quality).
How to convert PNG to JPG
- Drop a .png file onto the tool or click to browse — single file or batch.
- Pick a quality level (1–100). 85–90 is the safe sweet spot for photos; 75 is fine for most web use; below 70 introduces visible compression artefacts.
- Pick a background colour for transparent regions (default white) — alpha pixels are composited against this colour since JPEG has no alpha channel.
- Review before/after sizes (typical 5–10× reduction for photos), then download the JFIF-wrapped JPG. The original PNG is not modified.
Common use cases
- Shrinking heavy PNG screenshots (lossless 1–5 MB) before emailing or uploading where 100–500 KB JPG suffices for the photographic regions.
- Converting PNG-exported camera photos to smaller JPG for messaging apps, social platforms, or web galleries that prioritise download speed.
- Producing JPG fallbacks from PNG sources for platforms that strictly require JPG (some print services, legacy CMSes, photo prints).
- Cutting page weight by replacing photographic PNG hero images with quality-tuned JPGs (85 quality typical) — improves Largest Contentful Paint without visible quality loss.
Frequently asked questions
How does PNG → JPG handle transparency?
PNG (W3C 2nd Ed, 2003) supports tRNS palette transparency + 8-bit alpha (RGBA); JPEG (ITU-T T.81, 1992) has no alpha channel. The tool composites transparent pixels against a configurable background (default white) via Canvas 2D, then encodes opaque RGB to JFIF JPEG. Keep PNG or convert to WebP/AVIF if transparency matters.
What does the JPG quality slider actually do?
Quality 1–100 maps to JPEG quantisation tables per ITU-T T.81 Annex K. Higher = smaller quantisation steps = more DCT coefficients preserved = larger file. Quality 90+ visually indistinguishable from lossless; 75–85 is the web sweet spot; below 70 produces visible block boundaries and ringing.
Why are sizes 5–10× smaller for photos but only 2× for graphics?
JPEG's DCT + chroma subsampling exploits smooth gradients and lower colour acuity in natural photos. Flat-colour graphics with sharp text edges violate both assumptions: edges become high-frequency content JPEG degrades, saturated colours smear under subsampling. Use PNG for graphics, JPG for photos.
What is JFIF and why is it in JPG file headers?
JFIF v1.02 (Eric Hamilton, C-Cube Microsystems, 1 September 1992) is the de facto file container around the bare ITU-T T.81 bitstream. APP0 marker carries density (DPI) + thumbnail metadata. RFC 2046 (1996) specified JFIF as the canonical image/jpeg form for internet transport.
Does the conversion preserve the colour profile?
Browsers default to sRGB (IEC 61966-2-1:1999, the foundational web RGB standard). PNG iCCP / JPEG APP2 ICC profiles are stripped through the standard Canvas 2D path. Modern browsers (Chrome 94+, September 2021; Safari 15.2+, December 2021) honour wide-gamut profiles via canvas.getContext('2d', { colorSpace: 'display-p3' }).
When PNG → JPG saves space — and when it costs quality
JPEG's compression efficiency depends on content type. Natural photographs benefit substantially: smooth tonal gradients compress efficiently when high-frequency DCT coefficients are quantised to zero, and chroma subsampling (typically 4:2:0 — half horizontal, half vertical resolution for chroma channels) exploits the human eye's lower acuity for colour vs luminance. A photographic PNG typically shrinks 5–10× when converted to JPEG quality 85, with no visible loss to most viewers. Flat-colour graphics, logos with sharp edges, and screenshots of text are different: their content is dominated by sharp edges (high-frequency content that JPEG quantisation degrades into visible ringing) and saturated brand colours (which chroma subsampling smears into nearby pixels). For these, PNG's lossless DEFLATE (RFC 1951 Deutsch, May 1996) often wins on size and clearly wins on quality. The transparency story is one-way: PNG → JPG always discards the alpha channel, compositing transparent pixels against the chosen background. If transparency matters, the right destination is WebP or AVIF (both support alpha and out-perform PNG on size for photographic content). The Canvas 2D Context conversion path keeps everything in sRGB by default; wide-gamut profiles (Display P3, Rec.2020) require explicit colorSpace: 'display-p3' on the canvas context to avoid being silently downsampled to sRGB.
- Adjustable JPEG quality 1–100 mapped to ITU-T T.81 Annex K quantisation tables
- Configurable background colour for alpha compositing (PNG transparency → opaque JPEG)
- Output JFIF v1.02 container (Eric Hamilton, 1992) compatible with all browsers and editors
- sRGB IEC 61966-2-1:1999 colour space handling (default web RGB)
- Real-time before/after size comparison (typical 5–10× reduction for photos at quality 85)
- Browser-side via WHATWG Canvas toBlob('image/jpeg', quality) — no upload, no server
Free. No signup. No file uploads. Ads via AdSense (consent required).
Sources (6)
- W3C (PNG Working Group) (2003). Portable Network Graphics (PNG) Specification (Second Edition). W3C Recommendation 10 November 2003 / ISO/IEC 15948:2004 — source format with alpha channel + DEFLATE compression.
- ITU-T (CCITT) Study Group VIII & ISO/IEC JTC 1/SC 29/WG 10 (JPEG) (1992). Information technology — Digital compression and coding of continuous-tone still images: Requirements and guidelines. ITU-T Recommendation T.81 (18 September 1992) / ISO/IEC 10918-1:1994 — target JPEG baseline DCT bitstream.
- Hamilton, E. (C-Cube Microsystems) (1992). JPEG File Interchange Format (JFIF) Version 1.02. 1 September 1992 — de facto JPEG container; specifies APP0 marker, density units, thumbnail handling.
- Freed, N., & Borenstein, N. (1996). Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types. RFC 2046, IETF (November 1996) — registers image/jpeg media type referencing JFIF as the canonical interchange form for internet transport.
- International Electrotechnical Commission (IEC) (1999). Multimedia systems and equipment — Colour measurement and management — Part 2-1: Default RGB colour space — sRGB. IEC 61966-2-1:1999 — default RGB color space for the web; flatten target when discarding PNG alpha channel against background.
- WHATWG (live). HTML Living Standard — Canvas 2D Context + HTMLCanvasElement.toBlob(). html.spec.whatwg.org/#2dcontext (browser conversion mechanism: decode PNG → composite alpha against background → toBlob('image/jpeg', quality)).
These are the W3C, ISO/IEC, ITU-T, and IETF specifications the tool implements or builds on. Locate them on w3.org, iso.org, itu.int, or datatracker.ietf.org.