Does resizing an image reduce its file size?

Yes, reducing the dimensions of an image typically reduces its file size because fewer pixels need to be stored. However, the output format also matters. This tool outputs PNG which is lossless. For maximum file size reduction, resize first then use an image compressor.

Can I resize images without losing quality?

Downscaling (making smaller) generally preserves quality well. Upscaling (making larger) may cause pixelation since new pixels must be interpolated. For best results, avoid enlarging images beyond their original dimensions.

What image formats are supported by this resizer?

This tool supports all image formats that your browser can decode, including JPEG, PNG, WebP, GIF, BMP, and SVG. The output is always in PNG format which supports transparency and provides lossless quality.

Image Resizer

ブラウザ内で画像をリサイズ。幅・高さを指定してアスペクト比維持も可能。

How to Use the Image Resizer

This free online image resizer lets you change image dimensions directly in your browser using the HTML5 Canvas API. Upload any image file (JPEG, PNG, WebP, GIF), set your desired width and height, and download the resized result as a PNG file. All processing happens client-side, so your images are never uploaded to any server, ensuring complete privacy and instant results.

Aspect Ratio Lock

By default, the aspect ratio lock is enabled. When you change either the width or height, the other dimension is automatically calculated to maintain the original proportions. This prevents your image from being stretched or distorted. Uncheck the lock if you need to set custom dimensions independently, for example when creating images for specific social media requirements or banner sizes that differ from the original aspect ratio.

Common Use Cases

Image resizing is essential for web development, social media posting, email attachments, and document preparation. Different platforms require specific image dimensions: Instagram posts work best at 1080x1080 pixels, Twitter headers at 1500x500, and Facebook covers at 820x312. Resizing images before uploading them reduces page load times and bandwidth usage. For e-commerce sites, consistent product image dimensions create a professional, uniform appearance across your catalog.

Quality Considerations

Upscaling (making images larger) may result in pixelation since new pixel data must be interpolated. For best quality, resize images to smaller dimensions or stay close to the original size. The tool outputs PNG format which is lossless, preserving quality during the resize operation. For photographs where file size matters more than pixel-perfect quality, consider using the Image Compressor tool after resizing.

Image Resizer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リサイズツールです。画像をアップロードし、幅と高さをピクセル単位で指定してリサイズできます。アスペクト比ロック機能により、画像の縦横比を維持したままサイズ変更が可能です。処理はすべてクライアントサイドで行われ、PNG形式でダウンロードできます。

よくある落とし穴と対処 / Common Pitfalls

落とし穴 / Pitfall	問題 / Problem	対処 / Fix
縦横比が崩れて歪む Distorted proportions	幅と高さを独立に指定すると被写体が伸びる Setting both dimensions freely stretches the subject	アスペクト比ロックを使う。比率の計算はAspect Ratio Calculatorで Keep the ratio locked
拡大でぼやける Upscaling looks blurry	ピクセル情報は増やせないため、拡大は常に画質劣化を伴う You cannot invent missing pixels	原則「縮小のみ」。大きい元画像を確保する Resize down only; source larger originals
Retinaでぼやける Blurry on Retina displays	表示サイズちょうどにすると高密度ディスプレイで不足 1x assets look soft on 2x screens	表示サイズの2倍で書き出し、CSSで半分に表示する Export at 2x the display size
リサイズしたのに重い Still heavy after resizing	PNG出力は写真だとサイズが大きくなりがち PNG is large for photographic content	リサイズ後にImage CompressorでJPEG/品質調整 Compress to JPEG afterwards
SNSで端が切れる Edges cropped on social media	プラットフォームごとに推奨比率が異なる(OGは1.91:1等) Each platform crops to its own ratio	目的の比率にトリミングしてからリサイズ Crop to the target ratio first

用途別サイズ早見表 / Size Cheat Sheet

OGP画像:        1200 x 630
X(Twitter)投稿: 1600 x 900 (16:9)
Instagram:      1080 x 1080 / 1080 x 1350
YouTubeサムネ:  1280 x 720
ブログ本文画像:  幅 800-1200 (表示幅の2倍目安)
ファビコン元:    512 x 512

Why browser-only?

Resizing happens with the Canvas API entirely in your browser — your photos are never uploaded, which matters when they contain faces, documents, or location-revealing scenes. It also means instant results with no queue and full offline support. Confirm in the DevTools Network tab: zero requests while resizing.

リサイズの全てがブラウザ内で完結し、写真がアップロードされることはありません。人物・書類が写った画像も安心して処理できます。