Free photo tool
Which image format is smallest?
Stop guessing between JPG, PNG and WebP. Drop an image and this tool encodes it into every format in your browser and compares the real file sizes. It recommends the smallest one that still looks right, with the numbers next to it. Nothing is uploaded.
Your images never leave this device. Every format is encoded in your browser.
Drop an image to compare formats
or
JPG, PNG, WebP, GIF and HEIC, measured on your device
Loading the in-browser HEIC decoder (one time, ~1.5 MB)…
JPG vs PNG vs WebP, and why “best” depends on the image
There is no universally best format
The right format depends on what is in the picture. Photographs, with many colors, smooth gradients and grain, compress beautifully with JPEG or WebP, which throw away detail your eye won’t miss. Screenshots, receipts, diagrams and logos, with few colors, hard edges and text, look worse and often weigh MORE as JPEG, because lossy compression smears sharp edges. For those, a lossless PNG is both cleaner and smaller.
Why measure instead of guess
Rules of thumb get you close, but the only way to know is to encode the actual image and weigh the results. This tool does exactly that. It re-encodes your image into JPEG (at two quality levels), a normal PNG, an indexed PNG, and WebP, then reads the real byte sizes and shows them to you. The recommendation is whichever is smallest while still preserving what matters: transparency, or the sharpness of text and edges.
Done in your browser
Every one of those encodes happens on your device with the browser’s own image pipeline. No server sees your picture. Most “convert” sites upload it. This one measures on your device, so you can compare formats for a screenshot of something private without it ever leaving your phone or laptop.
Now actually shrink it
Once you know the format, resize and compress the image to the size you need.
Resize & compress images →See what’s inside first
Curious what a photo is carrying, such as GPS, device and timestamps, before you convert it?
Photo metadata viewer →Questions people ask
Is my image uploaded to measure it?
No. Every candidate format is encoded in your browser and the sizes are measured there. Nothing is sent to a server. You can run it with your Wi-Fi off once the page has loaded.
PNG or JPG: which is smaller?
It depends entirely on the image, which is why this tool measures instead of guessing. For photographs JPG is usually far smaller. For screenshots, logos and text-heavy images a palette PNG is often both smaller AND sharper, because JPG blurs hard edges. Drop your image in to see the real numbers.
Should I use WebP?
WebP is usually the smallest of all at a given quality, so it’s great for your own websites and storage. The catch is compatibility: a few older apps, some email clients and older software still handle WebP poorly. If you’re sending a file to someone, JPG or PNG is the safer bet.
What’s an “indexed” PNG?
A PNG that stores an exact palette of the image’s colors and one small index per pixel, instead of a full color per pixel. For images with 256 or fewer distinct colors, which covers most screenshots and logos, it is completely lossless and dramatically smaller than an ordinary PNG. This tool includes its own encoder for it.
Why isn’t JPEG offered for some images?
JPEG can’t store transparency. When your image has see-through areas, offering JPEG would silently flatten them onto a solid background. So the tool leaves it out and compares the formats that keep the transparency.
Does WebP here include lossless WebP?
No. A browser’s canvas can only produce lossy WebP, so that is what is measured. For a lossless result the tool compares PNG and the indexed PNG instead, which are genuinely lossless.
Honest notes
The numbers are real measurements, not estimates: each size is the actual byte length of encoding your image into that format, in your browser, right now. The recommendation is whichever measured smallest while preserving transparency and edge sharpness.
The WebP figure is lossy only. Browser canvases can’t produce lossless WebP, so this tool doesn’t pretend to. When lossless matters, the PNG and indexed-PNG figures are the ones to compare.
Very large images are scaled to a sensible maximum before measuring, to keep memory and time reasonable. The downloaded file is that same processed image. Keep your originals if you need full resolution.
The picture you’re optimising is still on your device.
Whatever format wins, the original is still in your library. Sealby keeps the ones that matter encrypted on your device, readable by no one but you.
iPhone & iPad · iOS 17+ · Free