Reviewed and updated September 2026 - the core information here remains accurate for JPEG, which remains the most common format for sharing and delivery. We have a separate article on AVIF in the works, which is increasingly relevant as more browsers and software adopt it.
JPEG image file sizes are highly variable - even when generated with the same settings, no two images are likely to generate files of the same size (though they could be close). Two different images with the same dimensions will generate files of different sizes - often drastically different.
JPEG files have two components that we care about: the image data and the EXIF meta data.
The size of the image data is driven by three factors - the image pixel dimensions, the Q ("quality") setting and the image contents. Smaller image dimensions will produce smaller files and lower Q settings compress more and generate smaller files, but the impact of the image contents is highly variable.
Too low a Q setting will result in visible image artefacts - most obviously banding/ posterization in areas with smooth gradients or blocking when the setting is really low.
Too high a Q setting will just generate large, bloated files with no visible benefit (it's very common advice to maximise the quality setting, but it's not really the best advice).
On top of the image data, the EXIF segment of JPEG files can also be bloated with XMP metadata (especially from LrC which can save all the parametric edit information to the file).
PPI doesn't matter
The output PPI setting is largely irrelevant - its setting doesn't make a file high or low resolution. At this stage, it's an arbitrary number (unless you're generating your JPEGs by setting physical dimensions in which case the dimensions and the PPI setting determine the pixel sizes). It's the pixel dimensions that count, not the PPI setting - 1000px at 72ppi is the same 1000px set at 300ppi) and they look the same on screen and in print.
An example
As an example (I use Lightroom Classic (LrC)), a 21MP image might generate a 10.4MB JPEG at a quality setting of 100%, 6.1MB at 90% and 3.5MB at 80% Taking it a little bit further, I get 2.3MB at 70% and 1.6MB at 65%. Unless the image has smooth tonal gradients exhibiting banding/ posterization, the 2.33MB/70% JPEG is probably good for a 20” print (if I’m printing bigger than 20” the source image gets a lot more scrutiny before I commit ink to paper).
Limiting that same image to a maximum dimension of 1900 pixels and setting JPEG quality to 60 generates a 220kb file - that's big enough to fill most screens at 1:1 and the quality is generally fine for on-screen viewing (the artefacts are obvious if it isn't - look for banding in any smooth gradients in tone).
Combatting banding
Banding happens in part because we're using 8-bit files (we're stuck with that in traditional JPEGs), but also because lower Q settings don't cope well with smooth tonal gradients (skies, studio backgrounds, etc.).
To help avoid banding artefacts in your JPEGs, try adding a little noise before exporting JPEGs. This may sound counter-intuitive to image quality, but it's a tried and tested technique.
I have a standard setting as a preset in Lightroom Classic - Amount: 5, Size: 25 & Roughness:50. That's not enough to be noticeable, even when pixel peeping. These values aren't overly scientific - I played with them once and have stuck with them since.
I'm pretty sure that the concept of adding noise probably freaks some pixel-peepers - just remember, stop looking at pixels and admire the image.
My own settings
Unfortunately for me, many of my images have smooth tonal gradients - while I’d love to upload with a lower Q setting, I tend to use 80% across the board for web and social media. Most online services will resize and recompress as part of their content delivery.
For lab prints, I export JPEGs from LrC at 90% quality (and don’t resize up or down). The output file sizes vary dramatically - I could be ordering a 10” print from a 5MB file or a 40” print from a 3MB file - the file size simply isn’t something to worry about.
I also use a 90% quality setting to export from LrC for competition images. That doesn't appear to affect my results.
And, of course, as mentioned above, I always add a little noise in Lightroom.
Questions? Get in touch.
Barrie, Fiona, Elsa 🐾 & Mac 🐾