Photo Resizer & Compressor

How resizing and compression work in the browser

This tool changes an image's dimensions and file size using the HTML5 Canvas API — the same technology browsers use to draw graphics. When you process an image, it is loaded into an off-screen canvas sized to the new width and height, the original is redrawn onto that smaller canvas (which is the actual resampling step), and the canvas is then re-encoded to your chosen format. Resizing and compression are two different levers: shrinking the pixel dimensions removes data outright, while compression quality controls how aggressively the encoder discards detail at whatever dimensions you keep. The result is a brand-new file you download; the original on your disk is untouched.

The three resize modes

Two behaviours are worth knowing. In Exact Dimensions mode, the Lock Aspect Ratio toggle uses your width as the reference and recomputes the height from the original proportions, so the image is never stretched. In Max Dimension mode the tool will not upscale: if your limit is larger than the image already is, it keeps the original size rather than blowing it up into a blurry result.

A worked example

Drop a 4000×3000 pixel, 4.2 MB JPEG straight off a phone into the queue and choose Max Dimension with a limit of 1200. Because the width (4000) is the longer side, the tool sets the new width to 1200 and scales the height proportionally to round(1200 × 3000 ÷ 4000) = 900, giving a 1200×900 image. With JPEG output at quality 85, the re-encoded file typically lands around 200–300 KB — roughly a 95 percent reduction — which the row reports as a green "smaller" badge. The downloaded file is renamed automatically with a _resized suffix (e.g. beach_resized.jpg) so it never overwrites your original.

Choosing the output format

• JPEG — lossy, ideal for photographs. It exposes the quality slider, which maps directly to the encoder: quality 85 is usually visually indistinguishable from 100 at a fraction of the size.
• WebP — a modern format that typically beats JPEG by 25–35 percent at the same visual quality, also with a quality slider. Supported by all current browsers.
• PNG — lossless, best for screenshots, logos, line art, and anything with sharp edges or text. The quality slider does not apply to PNG; it always encodes at full fidelity, so it will be larger than JPEG for photos.

The quality slider runs 1–100 and is divided by 100 to feed the canvas encoder, so a slider value of 85 becomes an encoder quality of 0.85.

Batch processing and ZIP download

You can queue many images at once, each shown with its original dimensions and file size alongside the predicted new size. Click Process All to run them through the canvas one after another, then download each individually, or use Download All as ZIP to bundle every result into a single archive. The ZIP is built with the JSZip library packaged entirely in memory in your browser — no server assembles it.

Practical tips and common mistakes

• Match format to content. Saving a screenshot of text as JPEG produces blurry, ringing artifacts around the letters; use PNG. Saving a photo as PNG produces a needlessly huge file; use JPEG or WebP.
• Resize before you compress. A 4000-pixel image squeezed to low quality still carries far more data than the same image scaled to 1200 pixels at high quality. Cutting dimensions is the single biggest size win.
• Quality 80–85 is the sweet spot for JPEG and WebP. Below about 70 you start to see blocking; above 90 the file grows with little visible gain.
• Don't expect to recover quality. Upscaling a small image only invents pixels — it never adds real detail, which is why Max Dimension mode refuses to enlarge.
• Animated GIFs lose their animation. The canvas captures only the first frame, so a multi-frame GIF becomes a still image.

Privacy

Every step — reading the file, drawing it to the canvas, re-encoding, and zipping — happens locally in your browser. Your photos are never uploaded to BoxTool's servers. The only external request is fetching the JSZip library from a public CDN, and that is loaded only if you choose to download a ZIP; resizing and individual downloads work with no network at all.