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How to batch resize images to exact sizes, in your browser

2026-10-01 · 6 min read

To batch resize images, drop the folder into a bulk image resizer, choose a target size (a preset such as Instagram post 1080 by 1080, or your own width and height) and run it once. Every file is resized in the same pass and downloaded as a zip. In AstralBatch this happens inside your browser tab, so nothing is uploaded.

TL;DR: pick a preset or custom dimensions, leave Keep aspect ratio ticked unless you want images stretched, and choose PNG, JPG or WebP as the output. With Keep aspect ratio on, each image is fitted inside the target box without cropping, so a photo with a different shape ends up smaller than the preset in one direction.

What does batch resizing actually do to each image?

Each image is decoded, resampled to the new pixel dimensions with the pica resizing library, and encoded again in the output format. Reducing an image discards pixels, so it cannot be undone: always resize from the original files, never from copies you already shrank. The resampling is not a crop. It changes how many pixels the whole picture is made of.

Which preset sizes can you resize to?

Platforms change their recommended sizes from time to time, so treat the presets as starting points and check the platform's own help page before a big upload. If your target is not in the list, choose Custom dimensions and type any width and height.

Why is my resized photo not exactly 1080 x 1080?

Because Keep aspect ratio is on by default and it means fit within the target, not fill it. The image is scaled by the largest factor that still fits inside the box, so a 4:3 landscape photo sent to a 1080 x 1080 preset comes out 1080 pixels wide and 810 tall. Nothing is cropped and nothing is padded. If you need every file at exactly the preset size, untick Keep aspect ratio, which stretches the picture to the exact numbers and distorts anything that does not already have that shape. For true square or banner crops, crop in an editor first and resize second.

The same rule works in the other direction: an image smaller than the target is enlarged to fit. Enlarging invents pixels by interpolation and adds no real detail, so avoid sending small thumbnails to a large preset.

What image upscaling can and cannot do: what enlarging can and cannot recover.

Which output format should you choose when resizing?

• PNG is the default. It is lossless and keeps transparency, but photos come out large.
• JPG is the smallest choice for photographs and opens everywhere. It has no transparency, so transparent areas are filled with white.
• WebP is usually smaller than JPG at similar quality and supports transparency, but some older software cannot open it.

JPG and WebP outputs are written at a fixed quality of 0.92 on the browser's 0 to 1 scale. If file size is the real goal, resize first and then run the results through the compressor.

What is the checklist for resizing a whole folder safely?

• Work on copies, or keep the originals in a separate folder.
• Resize one or two files first and open them to check the shape and sharpness.
• Decide whether stretching is acceptable before unticking Keep aspect ratio.
• Choose JPG or WebP for photos and PNG for graphics with transparency.
• Expect output file names like photo_1080x1080.jpg: the size in the name is the target you chose, not necessarily the exact pixels of that file.
• More than one file downloads as a single zip named astral_batch_resized.zip.

• MDN: Responsive images, how browsers pick which image size to load
• web.dev: Learn Images, sizing and optimizing images for the web

Batch resize images in your browser: presets or custom sizes, PNG, JPG or WebP, no account.

How to batch compress images and see what each file saved: after resizing, shrink the files further without touching the dimensions.

Frequently asked questions