Pixels, aspect ratio and fit
Every digital image is a grid of pixels. A photo from a modern phone is typically 4032 × 3024 pixels (12 megapixels) or larger, much more than most uses need. A full HD screen is 1920 × 1080, most websites display images under 1600 pixels wide, and email or chat apps often downscale anything larger anyway.
The aspect ratio is the ratio between width and height. 4032 × 3024 is 4:3; 1920 × 1080 is 16:9. When you change the size without keeping the ratio, you have to decide what happens to the difference, and that is what the fit options are for:
- Crop to fill keeps the subject large and trims from the edges evenly. Best for thumbnails, avatars and social posts.
- Fit and pad keeps the whole picture and adds bars. Best for product photos and anything where cropping would cut off important content.
- Stretch forces the image into the box and distorts it. Only useful for textures or deliberate effects.
How resizing works here
Images are decoded in a background Web Worker, rotated according to their EXIF orientation, and then resampled with the Lanczos3 filter from the Squoosh project, compiled to WebAssembly. Lanczos looks at a wider neighbourhood of pixels than the simple bilinear scaling used by most browsers and phone apps, which preserves fine detail and avoids the jagged or blurry look of cheaper methods. Transparent images are resized with premultiplied alpha, so semi-transparent edges do not pick up dark fringes.
Resizing and file size
Resizing is the most effective way to make an image file smaller. Pixel count grows with the square of the dimensions, so a 4000 × 3000 photo has four times as many pixels as a 2000 × 1500 version, and the file is usually around three to four times larger. If an upload form rejects your photo for being too big, reducing the long side to 2000 pixels often solves the problem on its own, and the result still looks sharp on a laptop or phone screen. For a strict size limit, follow up with the image compressor.
Common sizes
Pre-set pages exist for frequently needed dimensions, including 1080 × 1080 for square posts, 1920 × 1080 for full HD, 1280 × 720 for thumbnails and 1200 × 630 for link previews. For platform-specific sizes in one place, use the social media image resizer.