How to Compress an Image to Under 100 KB (and Keep It Sharp)
The short answer
A phone photo is usually a few megabytes, and a form that says “under 100 KB” wants something roughly thirty times smaller. Quality alone can’t get you there without making the photo look smeared. The reliable order is:
- Shrink the pixels to what the form will actually show, for example 1024 or 1280 pixels on the longest side.
- Then lower the JPG quality until the file fits.
- Save as JPG (or WebP if the form accepts it), not PNG.
Compress image does steps 2 and 3 for you: choose “Fit under a file size”, pick 100 KB and press Compress. Set “Also resize” first and the result will usually be sharper. Everything happens in your browser, so the photo isn’t uploaded.
Why the file is big in the first place
File size follows two things: how many pixels there are, and how hard each one is compressed.
A 12-megapixel phone photo is 4032 × 3024 pixels. Most forms display your upload in a box a few hundred pixels wide, so almost all of those pixels are wasted. Here’s what resizing alone does to the pixel count:
| Longest side | Pixel size (4:3 photo) | Megapixels | Share of the original |
|---|---|---|---|
| 4032 (as shot) | 4032 × 3024 | 12.2 | 100% |
| 1920 | 1920 × 1440 | 2.8 | about 23% |
| 1280 | 1280 × 960 | 1.2 | about 10% |
| 1024 | 1024 × 768 | 0.8 | about 6% |
| 800 | 800 × 600 | 0.5 | about 4% |
JPG file size roughly follows the pixel count, so dropping from 12.2 to 1.2 megapixels removes around nine tenths of the work before you touch quality. Then a moderate quality setting finishes the job, instead of a harsh one.
“100 KB” can mean two numbers
NIST distinguishes the SI prefix kilo (1,000) from the binary kibi (1,024). Software is inconsistent about which it means by “KB”:
- 100 KB (decimal) = 100,000 bytes
- 100 KiB (binary) = 102,400 bytes
A file of 101,000 bytes passes one portal and fails another. The safe target is under 100,000 bytes, which passes either way. Our compressor counts 1 KB as 1,000 bytes for exactly this reason. Windows File Explorer divides by 1,024, so a file of 99,000 bytes shows there as about 96.7 KB.
JPG, PNG or WebP?
| Format | Compression | Transparency | Best for under-100 KB photos? |
|---|---|---|---|
| JPG | Lossy, adjustable quality | No | Yes. Opens everywhere and every form accepts it. |
| WebP | Lossy or lossless | Yes | Smaller still, if the form accepts .webp |
| PNG | Lossless | Yes | No. Big for photos; good for screenshots and logos |
MDN describes JPEG as “particularly useful for photographs” and notes it has no transparency. PNG is lossless, which MDN says makes it preferable “for more precise reproduction of source images, or when transparency is needed”, and that precision is exactly why a photo saved as PNG is heavy. Google reports lossy WebP files are 25 to 34% smaller than comparable JPEGs at equivalent quality, which can be the difference between 120 KB and 85 KB. But many upload forms still accept only JPG and PNG, so check before choosing WebP.
If your photo is a PNG and the form takes JPG, converting is the single biggest saving. If it’s a screenshot of text or a logo with flat colours, PNG can stay PNG; it compresses those well.
Worked approach: a profile photo for an application form
Say the form wants a JPG under 100 KB and your photo is a 4032 × 3024 JPG of about 3 MB.
- Open Compress image and add the photo.
- Choose “Fit under a file size” and pick 100 KB.
- Set “Also resize” to “Longest side 1280 px” (or 1024 px). The tool never enlarges, so a smaller photo is left alone.
- Keep “Same format” for a JPG, or choose JPG if you’re starting from PNG.
- Press Compress and read the line under the file. It tells you the new size, the pixel size, the quality used, and whether pixels were reduced further to fit.
- Compare before and after with the slider. Zoom in on hair, text and sky, where compression shows first.
- Download the file, or all files as a ZIP for a batch.
What the tool is doing
In “Fit under a file size” mode, the tool tries quality 92 first. If that’s too big, it checks quality 35, the lowest it will go, then narrows the gap in halving steps to find the highest quality that fits. If even quality 35 is too large, it scales the picture down using the fact that size roughly follows pixel count:
next scale ≈ current scale × √(target ÷ current size) × 0.95
For example, if a photo is still 180 KB at the lowest quality and the target is 100 KB: √(100 ÷ 180) × 0.95 ≈ 0.71, so it tries about 71% of the width and height and searches again.
That’s why resizing yourself first helps: you choose the pixel size on purpose, and the quality search lands higher.
When you need exact pixel dimensions
Some forms specify both a size and dimensions, like “600 × 600 pixels, under 100 KB”. Do it in two steps: Resize image to the exact size first (pixel mode, or the 600 × 600 square preset, which trims the overflow from the centre), then compress the result to 100 KB. Resize keeps proportions by default; fill in only the width and the height follows the photo’s shape.
What else affects the result
- Detail. A photo of a busy street compresses worse than a portrait against a plain wall. Two photos of the same pixel size can land at very different file sizes.
- EXIF data. Phone photos carry camera details and often GPS location. “Remove EXIF data” is ticked by default, so that information doesn’t take up part of your 100 KB and your location isn’t shared.
- Format support. The compressor takes JPG, PNG and WebP. Convert iPhone HEIC photos with HEIC to JPG first.
Common mistakes
- Dragging quality down to 20 on a full-size photo. You get blocky artifacts and the file may still be too big. Resize first.
- Saving a photo as PNG to “keep quality”. It makes the file larger, not smaller.
- Compressing the same file repeatedly. Each lossy save loses a bit more detail. Start again from the original with stronger settings.
- Cropping out too little. If the form shows a head-and-shoulders photo, crop to that before compressing; you’re paying bytes for background.
- Ignoring the KB definition. Aim under 100,000 bytes, not 102,400.
Quick decision guide
| Your situation | What to do |
|---|---|
| Phone photo, form wants under 100 KB | Resize to 1024 to 1280 px, then fit under 100 KB as JPG |
| Exact dimensions and a KB limit | Resize to the dimensions, then compress |
| Screenshot or logo | Keep PNG, or try JPG if it has photos in it |
| Form accepts WebP | WebP gives the best quality at the same size |
| iPhone HEIC photo | Convert to JPG first |
When you’re ready, Compress image handles one photo or a whole batch on your own device.