File optimisation
How to resize an image without losing quality
5 min read
Resizing is the single most effective way to make images smaller, and the one people most often do in the wrong direction. The rules are simple once you understand what a pixel is worth.
Down is safe, up is not
Reducing dimensions discards pixels, and the remaining ones still describe the original detail — a downscaled photo looks sharp. Increasing dimensions has to invent pixels that never existed, which produces softness or artificial-looking edges. Always start from the largest original you have.
Keep the aspect ratio
Set the width and leave the height to be calculated automatically. Forcing both values distorts the picture, which is immediately obvious on faces and logos. If you need an exact frame size, resize first and then crop.
Sensible target widths
- Full-width hero image: 1600–2000 px.
- Blog body image: 1000–1200 px.
- Thumbnail or avatar: 300–400 px.
- Email attachment for reading: 1600 px on the long edge.
Resize before you compress
Compressing a 4000 px image to fit under a size limit destroys detail that resizing would have removed harmlessly. Reduce the dimensions to what will actually be displayed, then apply modest compression on top.
Batch consistently
For product catalogues and galleries, resize every image to the same width in one run. Consistent dimensions make grid layouts behave and keep page weight predictable.
Frequently asked questions
Can I enlarge a small image?
You can, but the result will be soft. No resizer can recover detail the original never captured.
What does DPI have to do with it?
Nothing on screen — DPI only matters for print, where it maps pixels to physical size.
Does resizing reduce quality?
Scaling down re-encodes the file once, which is visually negligible, and usually improves apparent sharpness.
Resize down from the largest original, keep the aspect ratio, and compress afterwards. That order gives the smallest file that still looks right.