Free Online Image Resizer: Resize Images to Exact Pixels

Pixels, File Size and Display Size Are Three Different Numbers

Almost every confusion about image sizing comes from treating these as one thing.

The third is the one people expect the file to control and it does not. An image displays at whatever size the layout gives it, and the browser scales the pixels to fit. Making the file bigger does not make the picture appear larger; it only means more data is downloaded and then discarded.

Enlarging Cannot Add Detail

Reducing an image discards pixels, which is straightforward. Enlarging it has to invent pixels that were never captured, and there is no way to invent the right ones.

Interpolation estimates each new pixel from its neighbours, which produces a larger image that is softer than the original rather than a more detailed one. A 400-pixel image scaled to 1600 pixels contains exactly as much real information as it did before, spread across sixteen times the area.

The practical rule is to always start from the largest version you have. If the only file available is small, enlarging it is a compromise rather than a fix, and modest enlargement — up to perhaps 150% — survives better than dramatic scaling.

Aspect Ratio, and Why Images Get Squashed

Aspect ratio is the relationship between width and height. Change one without the other and the image distorts — faces widen, circles become ovals, and text stretches.

New height = Original height × (New width ÷ Original width)

A 1600 × 1200 image resized to 800 wide needs a height of 1200 × (800 ÷ 1600) = 600. Keeping the ratio locked handles this automatically, and it should be the default.

When an image genuinely has to fit a different shape — a square avatar from a landscape photo — crop rather than stretch. Cropping removes part of the picture and keeps the rest correct. Stretching keeps everything and makes all of it wrong, which is far more noticeable.

What Size Does an Image Actually Need to Be?

The useful rule for screens is roughly twice the display width, which covers high-density displays without wasting data.

Anything substantially above these is downloaded and thrown away. A phone photograph at 4000 pixels used as a 600-pixel thumbnail wastes most of its file size before a single pixel is drawn.

DPI Does Not Affect Screen Images

This causes more wasted effort than any other sizing misconception. DPI — or PPI — describes how densely pixels are placed when an image is printed. It has no effect on how an image appears on a screen.

A 1200-pixel-wide image is 1200 pixels wide whether its metadata says 72 DPI or 300 DPI. Changing that number alters nothing visible on a website. Being asked for a "300 DPI image" for print is really a request for enough pixels: at 300 DPI, a photo printed 6 inches wide needs 1800 pixels. The pixel count is the real requirement and DPI is just the unit it was expressed in.

Working Order

• Start from the largest original you have, and keep it. Resizing is destructive, so work on a copy.
• Resize to about twice the intended display width, with the aspect ratio locked.
• Choose the format next — photographic or graphic decides it, and the image converter covers the trade-offs.
• Compress last, if the file is still larger than you want, using the image compressor.
• Check the result at its real display size rather than zoomed in.

Resizing here happens in your browser, so images are never uploaded and nothing leaves your device.