When a portal says your file must be under 100KB, you might assume that a 99.9KB file will pass. Usually it does. But upload systems don't always count bytes the same way your operating system does. Some servers measure the raw file. Others measure the encoded form after the browser wraps it in a multipart HTTP request, which adds a small overhead. A file that reads as 99KB on your desktop has failed at the server end often enough that targeting 96KB has become the safer habit. It costs you nothing in visible quality and eliminates a whole category of frustrating retries.
There's also the question of what "under" actually means to the system. Most portals use a strict less-than check: exactly 100KB fails if their threshold is 100,000 bytes and your file is 102,400 bytes (which is 100 kibibytes, a common source of confusion). The difference between 100KB in decimal and 100KiB in binary is about 2.4KB. If you're cutting it close, that gap can be the difference between a submission that goes through and one that doesn't. Staying comfortably under removes the ambiguity. If your portal is stricter than 100KB, the compress under 50KB tool handles that with the same approach.
The format matters as much as the file size
A lot of upload failures aren't about file size at all. They're about format. iPhone users shooting in HEIC, Android users with PNG screenshots, designers exporting WebP from Figma — these files hit portals that were built expecting a plain JPEG and get rejected before the size check even runs. The error message usually says something vague like "invalid file type" or just the generic size error again, which sends people off compressing a file that wasn't the problem in the first place.
JPEG is the one format every institutional portal accepts without question. It's been the standard for two decades and no upload system in government or HR has ever deprecated it. This tool outputs JPEG by default. If you're starting from a HEIC file from your iPhone, it converts and compresses in one step. If you'd rather handle the conversion separately first, the HEIC to JPG converter does that on its own, and then you can compress the result here. Either way, the output going into the portal is a standard JPEG that nothing will reject on format grounds.
What the photo actually looks like at 96KB
This is the part people are most anxious about, and usually without reason. A professional headshot or passport-style photo displayed at 200 by 200 pixels on a recruiter's dashboard needs very little data to look sharp. At 96KB, the compression applied is typically light enough that you'd struggle to spot a difference between the original and the output even at 400 by 400 pixels. The binary search algorithm finds the highest encoder quality that still fits under the target, so you're never getting more compression than necessary.
The exception is very large images with high detail, like a photo taken in RAW or a high-megapixel camera image. In those cases the tool scales the dimensions down proportionally as well as compressing, which keeps the output looking clean rather than introducing heavy JPEG blocking. If you need the image to stay at its original dimensions and the quality result still isn't acceptable, the portal limit itself is the constraint, not the tool. For situations where the portal accepts files up to 200KB, the compress under 200KB tool gives you more headroom to work with.