To resize a JPG, you change its pixel dimensions, its width and height measured in pixels. The trick to keeping it sharp is simple: shrink it evenly (never stretch one side more than the other) and don't try to blow a small image up into a large one. Every device you own can do this with a built-in app, and this covers the good ones.
First, let's clear up the thing most people mix up.
Resizing vs compressing: they're not the same
These two get used interchangeably, but they do different jobs.
- Resizing changes the dimensions. A 4000 x 3000 photo becomes, say, 1200 x 900. Fewer pixels, physically smaller image.
- Compressing keeps the dimensions the same but lowers the file size by trimming data the eye barely notices. A 1200 x 900 photo at 2 MB might drop to 300 KB.
You often want both. A photo straight off a phone is huge in both dimensions and file size, so resizing the pixels down and then compressing gets you a lean image that still looks good. If your real goal is a smaller file, our guide to reducing JPG file size covers that side in detail.
How to resize a JPG on Windows (Photos app)
The Photos app is the easiest built-in option.
- Right-click your JPG and choose Open with > Photos.
- Click the three-dot menu in the top corner and pick Resize.
- Choose one of the presets, or select Define custom dimensions.
- Enter your new width. Leave the aspect ratio locked so the height follows automatically.
- Click Save resized copy.
How to resize a JPG on Windows (Paint)
Paint gives you a little more control and it's on every Windows PC.
- Open your image in Paint.
- Click Resize in the toolbar.
- Switch from Percentage to Pixels.
- Make sure "Maintain aspect ratio" is checked. This is the important box.
- Type your new width. The height fills in on its own.
- Go to File > Save As > JPEG picture.
Keep that aspect ratio box checked and your image scales evenly. Uncheck it and set mismatched numbers, and you'll squash or stretch the picture out of shape.
How to resize a JPG on Mac (Preview)
Preview is built into every Mac and handles this cleanly.
- Double-click your JPG to open it in Preview.
- Go to Tools > Adjust Size.
- Make sure "Scale proportionally" is checked.
- Set the width and height dropdown to pixels.
- Enter a new width. The height updates by itself.
- Press Command + S to save.
Why stretching wrecks an image
Every JPG has an aspect ratio, the relationship between its width and height. A 1600 x 900 image is 16:9. As long as you scale both sides by the same amount, that ratio holds and the picture stays true to itself.
The moment you change width and height by different amounts, the ratio breaks. Faces go wide, circles turn into ovals, text leans. That's the stretched look. Keeping "aspect ratio" or "scale proportionally" checked is what prevents it, and it's why every method above tells you to leave that box on.
One more limit worth knowing: you cannot enlarge your way to more detail. A 400 x 300 image blown up to 1600 x 1200 just stretches the pixels it already has, so it comes out soft and blocky. Resizing removes detail cleanly; it can't create detail that was never there. Always start from the biggest original you've got.
The faster route: resize and shrink in one step
If you want to resize the dimensions and cut the file size at the same time, our JPG compressor does both in your browser. Set the dimensions you need, and it scales the image while squeezing the file down, so you don't have to run two separate tools.
It's handy when you have a target in mind, like fitting an upload limit. If you need to hit a specific size, our guide to getting a JPG under 100KB shows exactly how far to push the dimensions and the quality together.
Resize first to the dimensions you actually need, then compress. Do it in that order and you'll get the smallest, sharpest file for the job.