Image Resizer
Resize images to your desired dimensions
Input Information
Result
Image Resizer is a browser-based image processing tool that accurately resizes your uploaded image to the desired width and height. Using the HTML5 Canvas API to re-render the image in the browser means no data is sent to any server, fully protecting your privacy.
It can be used for many purposes: resizing images for blog posts, adjusting SNS post dimensions, generating website thumbnails, reducing email attachment sizes, and tuning print image resolution. It handles all browser-supported image formats such as JPG, PNG, GIF, WebP, and BMP, and the result downloads as a PNG (with transparency support).
The maintain-aspect-ratio option lets you resize safely without distorting the original proportions, while you can also manually adjust the ratio to intentionally create distorted images. The original and result previews let you instantly compare before and after.
It runs directly in any web browser with no software installation or sign-up, and is very handy when you want to quickly resize images without professional editors like Photoshop or GIMP.
Term Glossary
- HTML5 Canvas API
- An API that lets scripts draw images and graphics in the browser. This tool uses Canvas drawImage to resize images in memory.
- Aspect Ratio
- The ratio of an image’s width to its height. Resizing while preserving it keeps the image from being squashed and preserves its natural shape.
- Lossless Compression
- A compression method that loses no image quality. PNG is a prime example, guaranteeing output identical to the original.
Image Resize Algorithm:
This tool resizes images on the client side using the HTML5 Canvas API drawImage() method. The browser’s Canvas 2D context internally applies an interpolation algorithm to produce the new-sized image.
Step 1 - Load Image: The uploaded image file is read via the FileReader API and converted to a data URL, then loaded into an Image object. During this step, metadata such as the original width and height is extracted.
Step 2 - Calculate Target Size: The user sets the target width and height. When the maintain-aspect-ratio option is enabled, the other axis is auto-calculated from one axis based on the original aspect ratio.
Step 3 - Canvas Rendering: A new Canvas element is created, its size is set to the target width/height, and the original image is re-rendered to the target size via the drawImage() method.
Step 4 - Output: The resized image is converted to a PNG data URL using the Canvas toDataURL() method.
Supported Input Formats: All browser-supported image formats such as JPG, PNG, GIF, WebP, BMP | Output Format: PNG (transparency supported, lossless compression)
Effective Usage Tips:
1. Use the maintain-aspect-ratio option by default
Enabling the "Maintain Aspect Ratio" checkbox preserves the original image proportions so it resizes naturally without distortion.
2. Prefer shrinking over enlarging
Enlarging beyond the original size adds pixels, which can cause blur or stair-step artifacts (aliasing). Shrink to a smaller size than the original whenever possible.
3. Calculate the right size for your purpose beforehand
- Web blog body image: 800~1200px width
- SNS post (Instagram): 1080x1080px
- SNS post (Facebook): 1200x630px
- Website thumbnail: 300~400px width
4. Understand PNG characteristics
The result is saved as PNG. PNG uses lossless compression so quality is preserved, but the file size can be larger than JPG.
5. Use professional tools for bulk resizing
When resizing many images at once, dedicated tools like ImageMagick, GIMP, or Photoshop are more efficient.
6. Back up the original image
Save the original separately before resizing. Since a resized image cannot be undone, keep the original in case you need it.
7. Your privacy is protected
All image processing happens entirely within the browser, and your uploaded image is never sent to any external server.