Image dimensions and file size: what is the difference?
“Make this photo smaller” can mean reducing its file size or changing its width and height. Check which one is required before changing the image.
Bytes describe the file; pixels describe its width and height
File size is the amount of data needed to store an image. KB and MB matter for attachments, upload limits and storage. Pixel dimensions count the dots across and down the image. Knowing that a photo is 1600×1066px does not tell you how many KB its file contains.
| Requirement | What to check | Setting |
|---|---|---|
| Attachment at or below 500KB | Saved file size in bytes | Target size |
| Exactly 800×800px | Output width and height | Exact dimensions |
| Longest edge no more than 1200px | The larger dimension | Longest edge |
| 800×800px and no more than 500KB | Dimensions and file size | Dimensions and target size together |
The numbers in this table are examples of how to read a requirement. Use the format, dimensions and size limit specified by the place receiving your image.
A smaller file can keep the same pixel dimensions
JPG and WebP can store the same width and height using less data by changing encoding quality. Equal dimensions do not mean identical detail. Compare small lettering, hair, fabric and other textured areas before keeping a result.
Here is a downloaded PhotoLight sample result, saved as JPG at quality 82 with resizing turned off.
| Property | Original | Result |
|---|---|---|
| Format | JPG | JPG |
| Dimensions | 1600×1066px | 1600×1066px |
| File size | 546,454 bytes | 244,406 bytes |
| Reduction | — | About 55.3% |
These values apply to this example file. Results vary with the browser, photo and settings. Download both files and view them at the same scale to inspect this result yourself.
A 50% resize does not promise a file that is half the size
A 50% resize halves both width and height. A 1600×1066px photo becomes 800×533px. Its pixel count falls from 1,705,600 to 426,400, which is one quarter of the original. That is a calculation of pixels, not a measured file-size reduction.
Encoded bytes also depend on format, quality, colors and texture. Different output settings can produce a larger file even after a resize. Check the resulting px and KB separately. Enlarging an image creates more pixels, but does not recover detail that was missing from the original.
Meet each requirement in order
- Check accepted formats. A small WebP is of no use if the destination only accepts JPG.
- If dimensions are specified, open Resize photos and choose the longest edge, a percentage or exact dimensions.
- If there is a file-size limit, enable a target KB value in Compress photos. Dimension settings are available in the same tool when both conditions apply.
- Check output dimensions, file size, cropping and transparency before saving.
When exact width and height are required, reducing them further to reach a byte limit would break that requirement. Additional downscaling is not used in exact-dimensions mode. If the file misses the target, the tool flags it; a processed result is not automatically a result that meets every condition.
Check the saved file at the size you will use
PhotoLight treats 1KB as 1000 bytes. File managers and upload services may display units differently. For a file close to a limit, check the saved file and whether the destination accepts it rather than relying only on rounded labels.
The comparison preview is scaled to fit your screen. Inspect important text or artwork at the scale at which it will be used. PhotoLight creates a new file and leaves your original unchanged. Selected photos are not uploaded, and original EXIF is not copied into the new file. Names or document content visible within the picture remain visible.