Upload limits turn up on job applications, exam registrations, visa and government portals, and email. The message rarely explains itself — “file size greater than 100 KB”, “max 2 MB”, “image too large” — and it is easy to confuse file size with image size. This page explains the units, the one ambiguity that causes rejections, and how to get under a limit without wrecking the file.
The units, from smallest to largest
| Unit | Equals | Roughly the size of |
|---|---|---|
| 1 byte (B) | 8 bits | One letter of plain text |
| 1 kilobyte (KB) | 1,000 bytes | A short email with no attachments |
| 100 KB | 100,000 bytes | A passport-sized photo, or a one-page text PDF |
| 1 megabyte (MB) | 1,000 KB | A minute of MP3 audio, or a small web photo |
| 1 gigabyte (GB) | 1,000 MB | About an hour of standard-definition video |
| 1 terabyte (TB) | 1,000 GB | A laptop’s whole drive |
Each step is a thousand times the last. So 0.5 MB is 500 KB, 2 MB is 2,000 KB, and a 3 MB phone photo is thirty times over a 100 KB limit.
Note the capital B. KB is kilobytes; Kb or kbit is kilobits, one-eighth as much. File sizes are almost always given in bytes, and internet speeds in bits — a 100 Mbps connection moves about 12.5 MB per second.
What “greater than 100 KB” is telling you
It means your file is bigger than the form allows and you need a smaller one. It says nothing about the picture’s dimensions in pixels, though the two are linked: a 4000 × 3000 photo has twelve million pixels to store, a 600 × 600 one has 360,000, and fewer pixels is the single biggest reason a file gets smaller.
Some forms also set a minimum, such as “between 20 KB and 50 KB”. That is usually there to stop people uploading a tiny, unreadable thumbnail. Aim for the upper half of the range rather than the lowest size you can reach.
1,000 or 1,024? The ambiguity behind “it says 98 KB but the form rejects it”
There are two definitions of a kilobyte in use, and they disagree by 2.4%:
- 1 KB = 1,000 bytes — the international standard, and what macOS, iOS and most storage makers use.
- 1 KB = 1,024 bytes — the older computing convention, and what Windows File Explorer still shows. Strictly this unit is a kibibyte (KiB).
The gap grows with each step: a “megabyte” is either 1,000,000 or 1,048,576 bytes, a 4.6% difference. In practice it means a file Windows shows as 98 KB is 100,352 bytes — which is over a limit enforced as 100,000 bytes. You cannot tell from the form which one it uses.
The safe move is to aim a few percent under the stated number. For a 100 KB limit, anything up to 97 KB as Windows shows it (99,328 bytes) passes either way. The compress-to-size tool handles this for you: when you ask for 100 KB it aims at 100,000 bytes, the smaller reading, so the result passes whichever definition the upload form uses.
Typical sizes, and typical limits
| File | Usual size |
|---|---|
| Phone photo (JPEG, 12 megapixels) | 2–5 MB |
| Phone photo (HEIC, iPhone default) | 1–3 MB |
| Screenshot (PNG) | 200 KB – 2 MB |
| Passport-style photo, 600 × 600 JPEG | 40–150 KB |
| Scanned signature, cropped | 10–40 KB |
| One-page PDF typed in Word | 30–150 KB |
| One scanned page as PDF | 300 KB – 2 MB |
Exam, government and job-application portals commonly ask for a photo somewhere between 20 KB and 200 KB and a signature under 50 KB, and cap documents at 1 to 5 MB. Gmail caps a message’s attachments at 25 MB.
How to get a photo under the limit
- Reduce the dimensions first. Halving the width and height leaves a quarter of the pixels and roughly a quarter of the file. A form that wants a small photo almost never needs more than 600 to 1,200 pixels across. The image resizer does this.
- Use JPEG for photos. PNG is lossless and is the wrong format for a photograph; the same picture as a PNG can be five to ten times larger. iPhone HEIC files are rejected by many portals outright — convert HEIC to JPG first.
- Then lower the quality. JPEG quality between 60 and 80 is hard to tell from the original at normal viewing size, and much smaller.
- Drop the metadata when the budget is tight. Camera details, location and an embedded preview image can add tens of kilobytes — a real share of a 50 KB limit. The EXIF remover strips them without re-encoding the picture.
Or skip the guesswork: the compress-to-size tool takes the limit you were given and searches for the best quality that fits under it, and tells you plainly if a file cannot get there at its current dimensions.
How to get a PDF under the limit
A PDF of typed text is already small; if it is large, it is almost always because of the images inside it — scans, photos, or a document saved from a phone camera. Those images are what has to shrink.
That can be done two ways. A server-side compressor recompresses each image inside the PDF and leaves the text alone. A browser cannot do that reliably, so the PDF compress-to-size tool takes the other route: it re-renders each page as an image at the quality needed to hit your target. That works very well on scans — the pages were images already — but it turns selectable text into a picture of text, so for a typed document, try the lossless option in the PDF compressor first.
A quick checklist
- Is the limit a file size (KB/MB) or a pixel size (for example 600 × 600)? Many forms set both.
- Is there a minimum as well as a maximum?
- Which formats are allowed? JPEG and PDF are the safe defaults; HEIC and WebP often are not.
- Aim a few percent under the maximum, to survive the 1,000-versus-1,024 difference.