How does browser-based image compression work?

This tool uses the HTML5 Canvas API to decode the image, render it to an off-screen canvas, and re-encode it as JPEG at your chosen quality level. All processing happens in your browser, so your images are never uploaded to any server.

What quality setting should I use for image compression?

For most use cases, 70-80% quality provides a good balance between file size and visual quality. Photographs compress well at 60-80%. At quality settings above 80%, the visual difference is typically imperceptible while still achieving significant size reduction.

Does image compression preserve transparency?

This tool outputs JPEG format which does not support transparency. If you need to preserve transparency, use PNG or WebP format instead. The JPEG output is ideal for photographs and images where transparency is not needed.

Image Compressor

ブラウザ内で画像を圧縮します。品質を調整してダウンロード可能。

品質: 70%

ファイルサイズ優先品質優先

How to Use the Image Compressor

This free online image compressor reduces file sizes directly in your browser using the HTML5 Canvas API. Select an image file, adjust the quality slider to balance file size and visual quality, and the tool compresses your image instantly. You can then download the compressed version as a JPEG file. Since all processing happens client-side, your images are never uploaded to any server, ensuring complete privacy and fast results regardless of your internet connection speed.

Why Compress Images?

Image compression is essential for website performance, reducing storage costs, and faster file sharing. Large images slow down web pages, increase bounce rates, and hurt SEO rankings. Google recommends keeping page load times under three seconds, and images are typically the largest assets on a web page. Compressing images before uploading them to your website can dramatically improve page speed, user experience, and search engine performance. Email attachments also benefit from smaller file sizes.

How It Works

This tool uses the browser Canvas API to decode the image, render it to an off-screen canvas, and re-encode it as a JPEG at the specified quality level. JPEG compression is lossy, meaning some visual information is discarded to achieve smaller file sizes. At higher quality settings (80-95%), the visual difference is typically imperceptible to the human eye while still achieving significant size reduction. Lower quality settings produce smaller files but may show compression artifacts like blurring or banding.

Tips for Best Results

Start with the quality slider around 70-80% for a good balance between quality and size. Photographs typically compress well to 60-80% quality. Graphics with sharp edges or text may benefit from higher quality settings. For web use, aim for images under 200KB. If you need to preserve transparency, note that this tool outputs JPEG format which does not support transparency. For transparent images, consider using PNG or WebP format instead.

Image Compressorの使い方

このツールはブラウザ上で動作する無料の画像圧縮ツールです。HTML5 Canvas APIを使用して画像を読み込み、品質スライダーで圧縮率を調整し、JPEG形式でダウンロードできます。すべての処理はクライアントサイドで行われるため、画像がサーバーにアップロードされることはなく、プライバシーが完全に保護されます。

よくある落とし穴と対処 / Common Pitfalls

落とし穴 / Pitfall	問題 / Problem	対処 / Fix
JPEGの再圧縮を繰り返す Re-compressing JPEG repeatedly	非可逆圧縮のため、保存のたびにブロックノイズが蓄積する Lossy artifacts accumulate with each save	元画像(PNG/RAW)を保管し、配布用だけ圧縮する Keep a lossless master; compress only exports
透過PNGが黒/白背景になる Transparent PNG gets a solid background	JPEGはアルファチャンネル非対応 JPEG has no alpha channel	透過が必要ならWebPへ変換(透過対応で軽量) Use WebP for transparency
文字・図形がにじむ Text and line art look blurry	JPEGは写真向け。エッジの鋭い画像はノイズが目立つ JPEG is tuned for photos, not sharp edges	スクリーンショットや図はPNGのまま運用する Keep screenshots/diagrams as PNG
EXIF(位置情報)の扱い EXIF metadata handling	スマホ写真にはGPS座標が埋まっていることがある Phone photos may embed GPS coordinates	Canvas経由の再エンコードでEXIFは除去される(本ツールは除去側)。残したい場合は専用ツールを Canvas re-encoding strips EXIF (this tool does)
品質を下げてもサイズが減らない Size barely shrinks at lower quality	既に強く圧縮済み、または解像度自体が大きい Already compressed, or resolution dominates	先にリサイズで表示サイズまで縮小してから圧縮 Resize to display dimensions first

品質設定の目安 / Quality Guide

品質 90-100: ほぼ劣化なし(印刷・保存用)
品質 75-85:  Web標準。肉眼では差がわかりにくい ← 推奨
品質 50-70:  サムネイル・一覧用
品質 50未満: ブロックノイズが目立ち始める

例: 4000x3000 のスマホ写真 (3.2MB)
→ 幅1200にリサイズ + 品質80 → 約180KB (94%削減)

Why browser-only?

Photos are among the most personal files you have — family pictures, ID documents, screenshots of private conversations. Server-side compressors upload all of that before returning a smaller file. This tool compresses with the Canvas API entirely in your browser: the image never leaves your device, EXIF data (including GPS) is stripped by re-encoding, and it works offline. Watch the DevTools Network tab — zero upload requests.

写真は最も個人的なファイルです。本ツールは圧縮の全てがブラウザ内で完結し、画像がアップロードされることは一切ありません。再エンコードによりGPS等のEXIF情報も除去されます。