How to resize an image without losing quality

Pixel size and file size are not the same

A picture has two sizes, and they are easy to mix up:

• Pixel size, such as 1200 × 900 px: how many dots make up the picture. This decides how much detail it can show.
• File size, such as 180 KB: how much space the saved file takes. This depends on the pixel size, the format and the quality setting.

“Resizing” changes the pixel size. “Compressing” changes the file size without changing the pixels. Both can lose quality if done badly, in different ways: too few pixels look soft, too much compression looks blocky.

Rule 1: making a picture smaller is safe

Shrinking a picture throws away pixels you do not need. A 4000 × 3000 phone photo shown at 1000 × 750 still looks sharp, because no screen or form shows the extra detail anyway. Any decent tool (Paint, Preview, your phone, a browser tool) does this well.

Rule 2: making a picture bigger always loses something

Enlarging has to invent pixels that were never there, so the result looks soft or blurry. “AI upscalers” guess at missing details, which can look good on landscapes but changes faces and text. For forms and documents, never enlarge: start from the largest original you have.

Rule 3: resize once, from the original

Every time a JPG is saved, a little detail is lost. Resizing an already resized copy, or saving the same file again and again, adds up. Keep your original and make each new size from it.

Rule 4: keep the shape

If you change the width and height by different amounts, the picture stretches: faces look wide, signatures look squashed. Keep maintain aspect ratio (or scale proportionally) on. When a form needs a different shape, such as 3.5 × 4.5 cm for a photo, crop to that shape first and then resize.

Rule 5: choose the right format

Saving a screenshot as JPG gives fuzzy halos around the letters; saving a photo as PNG makes a huge file for no visible gain.

Rule 6: compress last, and not too hard

A JPG has a quality setting. At a high setting (about 80 percent and up) most photos are hard to tell from the original, at a fraction of the file size. Below about 50 percent, blocks and smudges appear, first in the sky and on skin.

When the KB limit is tiny, use fewer pixels instead of lower quality

Forms often allow only 20 KB or 50 KB. You can get there in two ways:

1. Keep the full pixel size and push the JPG quality very low. The file fits, but the picture is blocky and smudged.
2. Make the picture smaller in pixels and keep a clean quality. The file fits, and the picture looks sharp at its size.

The second way almost always looks better, and it is what our image compressor does. These are our sample phone photos at each limit:

If the form fixes the pixel size or the size in cm, the tool keeps that size and lowers only the quality, as little as possible: try resize image in cm or resize image to 20 KB.

Quick checklist

• Start from the original, largest file.
• Crop to the shape you need.
• Shrink, never stretch, with the aspect ratio locked.
• JPG for photos, PNG for screenshots and text.
• Compress last; if the KB limit is small, reduce the pixels rather than the quality.