Upload limits use two definitions
Depending on the site, “1 MB or less” can mean 1,000,000 bytes or 1,048,576 bytes (1,024 × 1,024). The difference is about 48 KB, so a file sized for the larger definition can be rejected by a site that uses the smaller one.
Free CVT's target-size compression meets the smaller definition so the file passes both: 100 KB means 100,000 bytes and 1 MB means 1,000,000 bytes. The KB figures in the result list use 1,024 units, so a result may show slightly under the target, such as “96 KB”.
Quality first, resolution second
JPG and WEBP get smaller as the quality value goes down. The tool halves the search range step by step to find the highest quality that fits the target. If quality alone is enough, the resolution stays the same as the original.
If the file is still too big at quality 40, the tool does not go lower; it reduces the width and height and searches the quality again. Below 40, blocky artifacts become obvious, so at the same file size a slightly smaller photo looks cleaner. When files were downscaled, the completion message says how many.
PNG and already-small files are handled differently
PNG is lossless, so the quality value does not apply. With the output format set to “Same as original”, a PNG reaches the target by reducing its resolution only. If you do not need a transparent background, switching the output to JPG or WEBP keeps more detail.
If the original is already under the target and in the same format, it is kept as is instead of being saved again, so it does not lose quality for nothing.
What to expect from each target
100 KB suits small images such as ID or application photos. A phone original of several megabytes will usually be downscaled, so if the site specifies a pixel size, resize to it first with the image resizer and then compress for a sharper result.
500 KB suits photos for the web and messengers, and 1 MB suits receipts and documents where small text must stay readable. A 5.8 MB noise test image (3000×2000) became 962 KB at the 1 MB target and 94 KB at the 100 KB target. Noise is the hardest case to compress, so ordinary photos usually lose less resolution.
What to check in the result
Open the saved file and check that faces, text and edges are not smeared. For document photos compressed to 1 MB, the test is whether the smallest text is still readable.
Files that could not reach the target are listed separately in the completion message. In that case, crop the photo to the part you need, or reduce its pixels with the image resizer, and compress again. Your original file is not changed, so keep it.
Keep the original and check the content, size and format of the result.