What does the JPEG quality number actually control?
A JPEG encoder splits the image into 8 × 8 blocks, converts each block into 64 frequency coefficients with a discrete cosine transform, then divides every coefficient by the matching entry of a quantization table and rounds it. Large divisors turn more fine-detail coefficients into zero, and zeros cost almost nothing to store. The quality number only chooses how big those divisors are. In the widely used libjpeg scale, quality 50 uses the example tables printed in the JPEG standard; above 50 the tables shrink linearly (quality 75 uses half the baseline values, 90 uses 20%), and at 100 almost every divisor is 1. Below 50 they grow quickly (quality 25 doubles them). That is why the curve is so uneven: going from 90 to 100 typically adds more bytes than going from 10 all the way to 80.
Why is quality 80 in one app not quality 80 in another?
Because there is no standard scale. Each program decides how its slider maps to quantization tables and color subsampling, and some use their own tables altogether, so the same number can give files that differ by 30% or more. Compare results by file size and by eye, never by the number alone.
| Where you export | Scale | Worth knowing |
|---|---|---|
| Photoshop: File > Save a Copy > JPEG | 0–12 | 7 and above keep full color resolution (no 4:2:0 subsampling) |
| Photoshop: Save for Web (Legacy) | 0–100 | 0–50 use 4:2:0 subsampling, 51–100 do not |
| Photoshop: Export As | 1–7 | Default 6; 7 is close to Save a Copy at 12 |
| Lightroom Classic: Export | 0–100 | Only 13 real steps: 77–84, for example, all give the same file |
| GIMP 3: File > Export As > JPEG | 0–100 | Default 85; can reuse the original file's quality settings |
| Browsers (canvas toBlob, toDataURL) | 0–1 | Each browser picks its own default when none is given |
| macOS Preview: File > Export | Least to Best slider | No number shown |
| Fast WebP Converter compressor | 1–100 | Or type a target size in KB instead |
What is chroma subsampling (4:2:0)?
Human vision resolves brightness detail much better than color detail, and JPEG exploits it. After converting to one brightness and two color channels, most encoders keep the color at half width and half height, written 4:2:0: one color sample for every 2 × 2 block of pixels. That removes three quarters of the color data and is invisible in nearly all photos. It shows on small, saturated details, such as thin red text on a blue background, where colors smear across the edges. Turning it off (4:4:4) typically makes the file 30–40% larger at the same quality number, so keep 4:2:0 for photos and switch to 4:4:4, or to PNG, for graphics with colored text. Lossy WebP always uses 4:2:0.
How big is a JPEG at each quality setting?
Typical sizes for a detailed outdoor photo saved with a standard libjpeg-based encoder and 4:2:0 color. Your own files can be 30–50% smaller or larger depending on detail and noise.
| Quality | 12 MP photo (4032 × 3024) | 1600 × 1200 photo | What you see at normal size |
|---|---|---|---|
| 100 | 7–10 MB | 1.3–1.6 MB | Identical to the source; most bytes wasted |
| 95 | 4–5.5 MB | 700–900 KB | Identical |
| 90 | 2.6–3.5 MB | 450–580 KB | Identical |
| 85 | 2–2.8 MB | 350–450 KB | Identical |
| 80 | 1.6–2.2 MB | 280–360 KB | Identical for photos (the usual sweet spot) |
| 75 | 1.4–1.9 MB | 240–300 KB | Very hard to tell; slight softening when zoomed |
| 70 | 1.2–1.6 MB | 210–270 KB | Fine for most web photos and thumbnails |
| 60 | 1–1.4 MB | 170–220 KB | Softer texture; possible banding in skies |
| 50 | 0.9–1.2 MB | 150–190 KB | Blocks visible in smooth areas when zoomed |
| 30 | 0.6–0.85 MB | 100–140 KB | Visible blocks and halos |
Two things stand out. Between 100 and 85 the file shrinks by roughly 70% with no visible change, because the encoder is mostly discarding noise. Below 70, each step saves fewer bytes while artifacts start to appear. That is why 70–85 is the sweet spot for web photos.
What JPEG quality should you use?
- Photos for a website or blog: 75–85, with 80 as the default.
- Large hero images behind text: 65–75; overlays and size hide the softness.
- Product photos with zoom: 80–88, so texture survives the magnifier.
- Thumbnails: 65–75; small display sizes hide artifacts.
- Screenshots, text, line art: 90 and above with 4:4:4, or better, PNG or lossless WebP.
- Prints and archives: 90–95, or keep a lossless master.
- Upload forms with a size limit: use a target size rather than guessing a number; see compress an image to a specific size.
Should you use progressive JPEG?
A progressive JPEG stores the image in several passes, so the browser can show a blurry full-size version first and sharpen it as the rest arrives, instead of drawing it from top to bottom. File size is usually within a few percent of a baseline JPEG either way (encoders such as mozjpeg make progressive files slightly smaller). It is a good choice for large images on the web. Some very old devices, such as certain TVs and photo frames, may not open progressive files, so keep baseline for those.
Why you should never resave a JPEG over and over
Every save runs quantization again on an image that already has rounding errors. Re-saving an untouched file at the same quality changes little after the first rounds, but any edit, crop or resize shifts the content against the 8 × 8 grid and produces fresh losses each time. After many generations, edges get blocky and colors drift. Keep the original or a lossless master, make every JPEG from it in one step, and if you only have a JPEG, recompress it once at 80–88 rather than lower. In GIMP, the option to reuse the original's quality settings limits the damage when you made only small edits. More in does compressing images reduce quality.
How do you set JPEG quality without installing anything?
- Open the JPG compressor in Chrome, Edge, Firefox or Safari, on a computer or a phone.
- Drop your JPG files. With Same as original, the output stays JPG.
- Set Quality to 80 (75 for thumbnails, 85–88 for product zoom), or choose Target size and type the maximum in KB.
- Download. EXIF and GPS metadata are removed, and if a file cannot get smaller, the original is kept instead of a bigger copy.
For web pages, the same photo saved as WebP at quality 80 is typically another 25–35% smaller than the JPEG; see the JPG compression guide for the web for when to switch.