Image Resizer

How Browser Image Resizing Works

ConvertUnlimited uses the HTML5 Canvas API to resize your images directly in your browser. When you add a file, your browser decodes the image data into RAM. When you specify new dimensions, the browser's graphics engine draws that image onto a canvas of the new size, applying interpolation algorithms to maintain visual clarity.

Because all calculations happen on your device's CPU and GPU, your images are not intentionally uploaded by this tool. This local-first approach ensures local browser-based processing and supports batch processing of large photo sets.

Resizing vs. Compression

While both techniques reduce file size, they work differently:

• Resizing changes the physical dimensions (pixel width and height) of the image. This is ideal for meeting specific requirement for websites, social media, or email attachments.
• Compression reduces the data size while keeping the same dimensions by optimizing how the color data is stored.

Our tool allows you to do both simultaneously by adjusting the dimensions and the output quality slider.

Understanding Aspect Ratio

The "Lock Aspect Ratio" feature ensures your image doesn't look stretched or squashed. When enabled, changing the width will automatically calculate the correct height based on the original image's proportions (and vice versa). If you need exact specific dimensions that don't match the original shape, uncheck this box, but be aware it may distort the image.

Best Practices for Image Sizing

Choosing the right size is crucial for web performance. Here are some common standards:

• Website Heroes: 1920px to 2560px width.
• Blog Posts: 800px to 1200px width.
• Social Media Posts: Usually 1080px to 1200px square or portrait.
• Thumbnails: 150px to 300px width.

Always try to resize your images to the exact size they will be displayed on screen to achieve the fastest loading times and best SEO scores (Core Web Vitals).

Local processing model

ConvertUnlimited provides the resizing logic as a static script that runs in this tab, rather than uploading files to a server for processing, as some "online" tools do. Selected file contents are processed locally in your browser for this workflow.

Resampling and DPI math

Resizing here changes only pixel dimensions; it does not read, write, or preserve a DPI/PPI metadata tag. A resized file's on-screen behavior depends purely on its width and height in pixels — the browser's canvas encoder does not embed a print-resolution value at all. To hit a specific print size at a specific DPI, calculate the target pixel dimensions yourself (inches × DPI) and enter those as the width and height:

The tool uses the Canvas 2D API's built-in image smoothing set to its highest quality level (imageSmoothingQuality: "high") when drawing the resized image. The browser chooses the actual resampling filter; this is not user-selectable, and browsers are not required to use an identical algorithm, so the same source image can look very slightly different when resized in, for example, Chrome versus Safari.

Transparency and JPEG output

Unlike some other tools on this site, the resizer does not explicitly fill transparent pixels with a background color before encoding to JPEG. If you resize a transparent PNG or WebP and choose JPG as the output format, the result depends on how the browser's JPEG encoder handles the dropped alpha channel — in practice this commonly renders as black rather than white, since the canvas starts fully transparent and JPEG has no alpha to preserve. Preview the result before relying on it, or export to PNG or WebP instead if transparency needs to be visible or absent-safe.

Batch Resizing Tips

• Memory Management: If you are resizing 100+ high-resolution photos, your browser may temporarily slow down. For the smoothest experience, process images in batches of 20-50.
• ZIP Downloads: After processing a batch, use the "Download All (ZIP)" button to bundle all your resized images into one organized file.
• Format Choice: WebP often gives a good size-to-quality balance for modern websites; JPG remains the safer choice for compatibility with older software.
• Mixed batches: Entering one target width with "Lock Aspect Ratio" checked applies that width to every image in the batch, and each image's own height is computed from its own original aspect ratio — images are not forced to match the first file's proportions.

Failure modes and limits

• Very large source files. Extremely large images (50 MB+ RAW-derived exports, very high pixel counts) can exceed a browser tab's available memory during decode; if a tab crashes, process fewer files at once.
• Upscaling. Enlarging an image beyond its native resolution cannot invent detail that was not captured; the browser's smoothing filter interpolates between existing pixels, so upscaled results look softer than a genuinely higher-resolution source.
• Transparency to JPEG. As above, converting a transparent image to JPG discards the alpha channel with an unpredictable resulting background color rather than an explicit fill.
• Mobile devices. Phones and tablets have less available memory; keep batches smaller than on desktop.

Frequently Asked Questions

Is there a file size limit?

We don't impose a software limit, but very large images (e.g., 50MB+ RAW exports) may exceed your browser's allocated memory. If a tab crashes, try processing fewer files at once.

Will my images lose quality?

Downscaling (making an image smaller) always involves some data loss, but modern browser interpolation makes this nearly imperceptible at typical viewing sizes. Upscaling (making an image larger) cannot add detail that was not there and will look softer or blurrier.

Can I convert formats while resizing?

Yes. You can upload a PNG and export it as a resized WebP or JPG directly using the format selector.

Does resizing set a specific print DPI?

No. Only pixel width and height change; no DPI or PPI tag is written to the output file. Calculate the pixel dimensions you need for a target print size and DPI yourself (inches × DPI) and enter them directly — see the table above for common examples.

What resampling algorithm is used?

The browser's built-in Canvas 2D image smoothing, set to its highest quality option. The specific filter is chosen by the browser and is not user-selectable, so results can differ very slightly between browsers on the same source image.

What happens to transparency when I resize a PNG to JPG?

The alpha channel is dropped without an explicit background fill, so the result depends on the browser's encoder; transparent areas commonly turn black rather than white. Preview the output, or keep the format as PNG or WebP if transparency needs to remain visible.

Can I batch-resize images that have different original aspect ratios?

Yes. A single target width applies to every image in the batch, but each image's height is computed from that image's own aspect ratio, not from the first file added.

What if I enter both a width and a height with "Lock Aspect Ratio" checked?

Width takes priority: the height is recalculated from the original aspect ratio using the entered width. Uncheck "Lock Aspect Ratio" if you need an exact, independently chosen width and height, keeping in mind this can stretch or squash the image.