Resizo Documentation
Complete product documentation for Resizo — the free, browser-based, privacy-first image resizer. Architecture, supported formats, full feature list, and social media presets.
Resizo is a free, browser-based image resizer at resizo.in that resizes, converts, crops, compresses, watermarks, and renames images entirely inside the user's browser using the HTML5 Canvas API. No images are uploaded to any server. There is no account, no sign-up, no watermarks, and no file-size cap imposed by a backend.
Resizo accepts JPG, PNG, WebP, and GIF input, exports JPG / PNG / WebP, supports batch ZIP downloads via Web Workers, and ships one-click presets for Instagram, YouTube, Facebook, X/Twitter, LinkedIn, and WhatsApp. The site works offline after the first load and is GDPR / CCPA compliant by architecture.
Resizo (n.): a free, client-side image resizer accessible at resizo.in. It performs every image operation — resize, convert, crop, compress, watermark, rename — locally in the visitor's web browser using the HTML5 Canvas API and Web Workers, so image files never leave the device.
Key Takeaways
- 100% client-side — every operation runs locally via the Canvas API. No uploads.
- Free forever — no account, no sign-up, no watermarks, no file-size cap.
- Six tool categories — resize, convert (JPG↔PNG↔WebP), crop, compress, watermark, rename.
- Batch processing — drop hundreds of images, get a single ZIP back. Uses Web Worker pool.
- Social presets — one-click dimensions for Instagram, YouTube, Facebook, X/Twitter, LinkedIn, WhatsApp.
- Privacy by design — GDPR & CCPA compliant; zero cookies; works offline after first load.
- 9 languages — en, es, fr, de, pt, id, ar (RTL), ko, sw.
What Is Resizo?
Resizo is a free online image resizer that runs entirely in the user's browser. Unlike server-upload tools, it never transmits image data to any backend. It uses the HTML5 Canvas API to resize, convert, crop, compress, and watermark JPG, PNG, WebP, and GIF files locally, and Web Workers to batch-process hundreds of images in parallel.
Resizo is maintained as a static site on Vercel. No tracking pixels. No third-party analytics that transmit image data. The page loads once (HTML, CSS, JS — under ~150 KB) and every subsequent operation runs locally in the browser tab.
Who Is Resizo For?
Resizo is built for social media managers, e-commerce sellers, bloggers, designers, and anyone editing sensitive images. Because no file is uploaded, it is suitable for IDs, medical scans, internal documents, and other private content that should never leave the device.
- Social media managers and creators who need exact platform dimensions for Instagram, YouTube, Facebook, X/Twitter, LinkedIn, WhatsApp, Pinterest, and TikTok.
- E-commerce sellers preparing product images for Shopify, WooCommerce, Amazon, and Etsy listings.
- Bloggers, writers, and SEO teams compressing images for Core Web Vitals (LCP, INP) without losing visible quality.
- Designers and developers doing bulk format conversion (JPG ↔ PNG ↔ WebP) and batch crop to fixed aspect ratios.
- Anyone with sensitive images — IDs, medical scans, screenshots of private chats, internal documents — that should never leave their device.
How Does Resizo Work?
When a user drops an image, Resizo reads it with FileReader, decodes it into an HTMLImageElement or ImageBitmap, draws it on a <canvas> at the target dimensions, and re-encodes it with canvas.toBlob(). The result is offered as a download via an ObjectURL. The file never crosses a network boundary.
Browser-based processing pipeline
- The visitor drops or selects an image file.
FileReader/BlobAPIs read the bytes into memory.- The bitmap is decoded into
HTMLImageElementorImageBitmap. - It is drawn onto a
<canvas>at the chosen target dimensions. - The canvas is re-encoded with
canvas.toBlob()in the chosen format. - The blob is downloaded via an
ObjectURL.
For batch jobs, Resizo uses a Web Worker pool to process multiple images in parallel without blocking the UI, then assembles the outputs into a ZIP using a client-side ZIP encoder. The ZIP is also generated in memory and downloaded directly.
What Is the Privacy Model?
Resizo's privacy model is "no upload, no persistence." Images live only in the active browser tab and are cleared when the tab is closed or refreshed. No file content, dimensions, filenames, or metadata are transmitted to any server. The site is GDPR and CCPA compliant by architecture, not by policy.
- No upload. There is no server-side endpoint that accepts image data.
- No persistence. Images live only in the active browser tab. Closing or refreshing clears them.
- No telemetry on file content. Resizo does not transmit filenames, dimensions, or metadata to any analytics service.
- Works offline. After the first page load, the site functions with the network disconnected.
- Open behavior. All processing is plain JavaScript visible in the page source; network activity can be verified with the browser's DevTools Network tab (it stays empty during editing).
Which Image Formats Are Supported?
Resizo accepts JPG, PNG, WebP, and GIF as input. It exports JPG, PNG, or WebP. HEIC / HEIF is browser-dependent (Safari decodes natively). SVG is a vector format and is out of scope.
| Format | Input | Output | Notes |
|---|---|---|---|
| JPG / JPEG | Yes | Yes | Lossy. Best for photos. Quality slider 1–100. |
| PNG | Yes | Yes | Lossless. Supports transparency. Larger files. |
| WebP | Yes | Yes | Modern format, ~25–35% smaller than JPG at equal quality. |
| GIF | Yes | No (export as PNG / JPG / WebP) | First frame is used; animation is not preserved. |
| HEIC / HEIF | Browser-dependent | No | Safari decodes natively; other browsers require pre-conversion. |
| SVG | No | No | Vector format; resizing is a viewBox change, not a raster operation. |
What Are the Core Features?
- Resize by pixels or percentage with optional aspect-ratio lock.
- Format conversion between JPG, PNG, and WebP (GIF accepted as input).
- Quality slider (1–100) for JPG and WebP output.
- Target file size mode — set a max KB and Resizo iterates quality to fit.
- Crop tool with fixed aspect ratios (1:1, 4:3, 16:9, 9:16) or freeform.
- Watermark with text or uploaded logo; control position, opacity, color, and rotation.
- Batch processing — drop hundreds of images, apply one operation, download a single ZIP. Runs in Web Workers.
- Batch rename with prefix + sequence numbering.
- Social media presets — one-click dimensions for every major platform.
- DPI control for print workflows (72 / 150 / 300).
- EXIF handling — orientation is respected; other metadata is stripped by default for privacy.
- Dark mode, RTL support, and nine localized interfaces.
- No upload, no sign-up, no watermarks, no quota.
What Are the Social Media Preset Dimensions?
All presets are applied on the client. Resizo never sees the image; the preset only sets the target canvas dimensions before re-encoding the file the user selected.
Need every variant in one shot? The Social Media Image Resizer accepts a single master image and returns a ZIP of 40+ verified 2026 sizes across 11 platforms — Instagram, Facebook, LinkedIn, X/Twitter, YouTube, TikTok, Pinterest, Reddit, Discord, Twitch, and utility formats (email signature, Open Graph, Notion cover, GitHub OG). Available in 9 languages with native-translated URLs.
| Preset | Dimensions (px) | Aspect |
|---|---|---|
| Square post | 1080 × 1080 | 1:1 |
| Portrait post | 1080 × 1350 | 4:5 |
| Story / Reel | 1080 × 1920 | 9:16 |
YouTube
| Preset | Dimensions (px) | Aspect |
|---|---|---|
| Thumbnail | 1280 × 720 | 16:9 |
| Channel art / banner | 2560 × 1440 | 16:9 |
| Profile picture | 800 × 800 | 1:1 |
| Preset | Dimensions (px) | Aspect |
|---|---|---|
| Profile picture | 1080 × 1080 | 1:1 |
| Cover photo | 1640 × 624 | ~2.6:1 |
| Feed landscape | 1200 × 630 | 1.91:1 |
| Story / Reels | 1080 × 1920 | 9:16 |
| Feed ad landscape | 1200 × 628 | 1.91:1 |
X / Twitter
| Preset | Dimensions (px) | Aspect |
|---|---|---|
| Post image | 1200 × 675 | 16:9 |
| Header photo | 1500 × 500 | 3:1 |
| Profile picture | 400 × 400 | 1:1 |
How Does Resizo Maintain Image Quality?
Resizo uses high-quality bicubic or Lanczos interpolation via the Canvas API, exposes imageSmoothingEnabled and imageSmoothingQuality controls, and performs a two-pass resize for large downscales to reduce aliasing. JPG and WebP output use a user-controlled quality value (1–100); PNG uses lossless DEFLATE.
imageSmoothingEnabled— set totruefor photos so edges are interpolated; set tofalsefor pixel art and screenshots to keep hard edges crisp.imageSmoothingQuality—"high"is the default, which selects a higher-order resampling kernel (browser-implementation-defined, typically Lanczos or bicubic)."medium"and"low"trade quality for speed during large batch jobs.- Two-pass downscale — for reductions of more than ~3×, Resizo resizes in two passes to reduce aliasing artefacts.
- Format-specific encoding — JPG and WebP use a quality slider; PNG uses lossless DEFLATE compression.
How Is Resizo Different From Server-Upload Tools?
Most "online image resizers" (TinyPNG, iLoveIMG, ResizePixel, Img2Go, and similar) upload the user's file to a remote server, process it there, and send a download link back. Resizo inverts that — the page loads once and every subsequent operation runs locally in the browser tab, with no upload bandwidth, no server queue, and no privacy dependency on a third party.
| Capability | Server-upload tools | Resizo |
|---|---|---|
| Image upload | Yes (file sent to remote server) | No (stays on device) |
| Privacy | Depends on operator's retention & logging policy | Guaranteed by architecture |
| Free tier limits | File size caps, batch caps, ops/day caps | No caps |
| Works offline | No | Yes (after first load) |
| Sign-up required | Often | Never |
| Watermarks on output | Sometimes | Never |
| Speed bottleneck | Upload + download bandwidth | Local CPU only |
Which Browsers Does Resizo Support?
Resizo works in every modern browser released since early 2020: Chrome 80+, Firefox 78+, Safari 14+, Edge 80+, iOS Safari 14+, and Chrome Android 80+. WebP export is fully supported in Chrome, Safari, and Edge. The HTML5 Canvas API used internally has over 98.5% global support.
| Browser | Minimum Version | WebP Export | Notes |
|---|---|---|---|
| Chrome | 80+ | Yes | Full support |
| Firefox | 78+ | Yes | Full support |
| Safari | 14+ | Yes | Native HEIC decode |
| Edge (Chromium) | 80+ | Yes | Full support |
| iOS Safari | 14+ | Yes | Touch-friendly, camera roll picker |
| Chrome Android | 80+ | Yes | Touch-friendly, gallery picker |
Related Documentation
Dive deeper with the source documents and integration guides.
Overview
What Resizo is, who it's for, and why client-side beats server-upload tools.
How It Works
Browser-based processing, privacy model, supported formats, and interpolation.
Features
Full feature list and social media preset dimensions for every major platform.
Social Media Image Resizer
One image → ZIP of 40+ variants across 11 platforms. Verified 2026 sizes. 9 languages with native URLs.
Instagram Image Resizer
Deep reference for the 30-preset Instagram resizer — verified 2026 dimensions across feed, carousel, stories, reels, Explore, Highlights, the ten Ad placements, Shopping, and Live.
Facebook Image Resizer
Deep reference for the 44-preset Facebook resizer — profile, cover, feed, stories, reels, ads, groups, events, shop, marketplace, and messenger sizes.
Passport Photo Resizer
Deep reference for the passport, visa, and ID photo tool — country presets, 2x2 inch and 35x45 mm exports, background colors, privacy, and compliance guidance.
YouTube Image Resizer
Deep reference for the 35-preset YouTube resizer — thumbnails, channel art, profile, Shorts, community posts, ads, live graphics, chapters, and ZIP export.
WhatsApp Image Resizer
Deep reference for the 36-preset WhatsApp resizer — verified 2026 dimensions for profile, chat, Status, Groups, Channels, WhatsApp Business catalog and templates, Click-to-WhatsApp ads, and stickers (512×512 WebP).
eCommerce Image Resizer
Deep reference for the 63-preset ecommerce resizer — verified 2026 product image sizes for Amazon, Etsy, Shopify, eBay, Flipkart, Meesho, Myntra, and WooCommerce, with white-background rules, crop modes, and per-platform ZIP export.
Embed Generator
Generate an iframe to embed any Resizo tool on your own site in 30 seconds.
Embed Documentation
URL parameters, theming, auto-resize, platform tutorials, and framework components.
Blog & Guides
How-tos, format comparisons, social media size references, and best-practice guides.
People Also Ask
Common questions about Resizo and browser-based image resizing.
Is Resizo really free with no hidden limits?
Yes. Resizo is free for personal and commercial use. There is no account, no sign-up, no watermarks, no file-size cap, and no usage limit. The only "cost" is keeping a small attribution link visible when embedding the widget on third-party sites.
Can I resize an image without losing quality?
Yes. For upscales beyond the source resolution there is always some quality loss, but for downscales and recompression Resizo uses high-order interpolation (typically Lanczos or bicubic), exposes a quality slider for JPG and WebP, and uses lossless DEFLATE for PNG. The result is visually indistinguishable from the source in most cases.
Does Resizo strip EXIF metadata?
By default, yes — EXIF orientation is honored when the image is loaded but other metadata (GPS, camera model, capture time) is stripped from the output for privacy. There is no setting to preserve it.
How big a batch can Resizo handle in one go?
The practical limit is your device's RAM. Resizo uses a Web Worker pool, processes images in parallel, and assembles the output into a streaming ZIP. Users routinely process hundreds of images in a single batch on modern laptops.
Can I use Resizo on a phone?
Yes. Every interface is touch-friendly and reflows down to ~320px wide. The file picker opens the camera roll on iOS and the gallery on Android.
Is Resizo open source?
The codebase is static (HTML, CSS, vanilla JavaScript). All processing logic is visible in the page source, so the privacy claims are verifiable. The project itself is not formally licensed as open source.
Frequently Asked Questions
Does Resizo upload my images to a server?
No. Every image is processed entirely in your browser using the HTML5 Canvas API. Files never leave your device. Verify it yourself with DevTools → Network tab — there are zero outbound requests during editing.
Does Resizo work without internet?
Yes. After the first page load, Resizo continues to work with the network disconnected. The site is small (under ~150 KB) and the browser caches it.
Which image formats can I convert between?
You can convert in and out of JPG, PNG, and WebP. GIF is accepted as input (first frame only) but cannot be exported as a new GIF — export as PNG, JPG, or WebP instead. HEIC support depends on your browser (Safari decodes it natively).
Does Resizo set cookies?
No. Zero cookies are set. A small amount of UI state (selected language, light/dark theme preference) is stored in localStorage for convenience, but no analytics, tracking, or fingerprinting data is collected.
Can I use Resizo on healthcare, legal, or government sites?
Yes. Because no image data leaves the device, Resizo is suitable for regulated environments. The embed widgets are GDPR- and CCPA-compliant by architecture. Always consult your compliance team for final approval.
How do I embed Resizo on my own site?
Use the Embed Generator to build a copy-paste iframe snippet, or read the full Embed Documentation for URL parameters, theming, auto-resize, and React / Vue / Angular components.
What languages does Resizo support?
Nine languages: English (en), Spanish (es), French (fr), German (de), Portuguese (pt), Indonesian (id), Arabic (ar — RTL), Korean (ko), and Swahili (sw).
Is there an API I can call from my backend?
No. Resizo is intentionally backend-free. There is no API to call because there is no server to call. If you need server-side image processing, run an image library like Sharp or ImageMagick on your own infrastructure.
Glossary
Key terms used throughout the Resizo documentation.
- Canvas API
- The HTML5
<canvas>element and its 2D drawing context, used to manipulate pixel data in the browser. - FileReader API
- A browser API for reading the contents of files selected by the user, without uploading them.
- ImageBitmap
- An efficient bitmap representation of an image that can be transferred to a Web Worker for off-main-thread processing.
- Web Worker
- A background JavaScript thread that runs in parallel with the main UI thread — used by Resizo for batch processing.
- ObjectURL
- A temporary URL created by
URL.createObjectURL()pointing to in-memory file data — used for client-side downloads. - Bicubic / Lanczos interpolation
- High-order resampling algorithms that smoothly interpolate pixel values when resizing — preserve more detail than bilinear or nearest-neighbor.
- EXIF
- Metadata embedded in JPG / HEIC files — GPS, camera model, capture time. Stripped from Resizo output by default for privacy.
- WebP
- A modern image format developed by Google that produces ~25–35% smaller files than JPG at equivalent visual quality.
- Core Web Vitals
- Google's user-experience metrics — LCP, INP, CLS — that influence search ranking. Image optimization directly affects them.
- GDPR / CCPA
- EU and California privacy regulations governing personal data collection, processing, and sale.
Sources & References
Authoritative documentation and standards cited in this page.
- MDN Web Docs — Canvas API
- MDN Web Docs — FileReader API
- MDN Web Docs — HTMLCanvasElement.toBlob()
- MDN Web Docs — Web Workers API
- MDN Web Docs — ImageBitmap
- Can I Use — Canvas API global support
- Can I Use — WebP format support
- web.dev — Core Web Vitals
- Google Developers — WebP
- GDPR.eu — General Data Protection Regulation
- California OAG — California Consumer Privacy Act
Summary: Why Choose Resizo
- Resizo is the free, browser-based image resizer that never uploads your files.
- It runs entirely on the HTML5 Canvas API and uses Web Workers for batch parallelism.
- Supports JPG, PNG, WebP, and GIF in; JPG, PNG, and WebP out.
- Built-in social media presets for Instagram, YouTube, Facebook, X/Twitter, LinkedIn, and WhatsApp.
- GDPR + CCPA compliant by architecture; works offline after first load; zero cookies.
- Available in 9 languages; embeddable on any site via a free iframe generator.