How to Reduce Image File Size
Two levers control image file size: dimensions and compression. Most people reach for compression and ignore dimensions, which is backwards — resizing usually saves far more, and costs less visible quality.
Resize first, compress second
This is the whole guide in one line. A photo from a modern phone is often 4000 pixels wide. Displayed in an article at 800 pixels, more than 95 percent of its pixels are discarded by the browser before anyone sees it — you paid to transfer data that was thrown away.
Halving an image's width removes roughly three quarters of its pixels, because area scales with both dimensions. No compression setting achieves that with so little visible cost, for the simple reason that the detail being discarded was never going to be displayed.
Compressing afterwards then squeezes what remains. Doing it in the other order wastes effort compressing pixels you are about to delete.
What width do you actually need?
When unsure, match the largest size the image will ever be displayed at, then add a little for high-density screens. Doubling for retina displays is common advice and is usually more than necessary once compression is applied.
- Full-width website hero: 1600–1920 px
- Image inside an article: 800–1200 px
- Thumbnail or avatar: 200–400 px
- Email attachment for viewing: 1200–1600 px
- Printing at A4: around 2480 px for 300 dpi
Choosing a quality setting
JPEG and WebP quality is a number from 1 to 100, and the relationship to file size is steeply non-linear. Going from 100 to 90 often halves the file with no visible difference at all, because quality 100 spends enormous effort encoding detail the eye cannot resolve.
For photographs, 70 to 80 is the sweet spot and 60 is usually still acceptable. Below 50, artefacts appear first in smooth gradients — skies, skin, shadows — as faint blocky banding.
Images with flat colour and sharp edges behave differently. Screenshots, logos and diagrams show JPEG artefacts as ugly fringing around text at settings that would look perfect on a photograph. Use PNG or WebP for those.
Pick the right format
WebP is supported by every current browser, so for anything going on the web it is close to a free win. Keep JPEG when the file will be opened by older desktop software, where support is less reliable.
- Photographs: JPEG for maximum compatibility, WebP for roughly 25–35 percent smaller files at the same visual quality.
- Screenshots, logos, diagrams: PNG, or WebP which handles both flat colour and photographs well.
- Anything needing transparency: PNG or WebP. JPEG has no alpha channel and will fill transparent areas with white.
Hitting a specific size target
Upload forms often demand something like "under 2 MB" or "maximum 500 KB". There is no formula that maps a quality setting to a file size, because it depends entirely on image content — a photo of a busy street compresses far worse than a photo of a clear sky.
The practical method is to resize to the dimensions you need, compress at quality 75, and check. If it is still too large, drop to 60, then try WebP. Two or three iterations will get you there, and you will almost always land on something that still looks fine.
Doing it with Filembly
Resize by width first, with the height following automatically so nothing is distorted. Then compress the result at the quality you want, optionally converting to WebP for a further saving. Both tools run in memory and store nothing.