ホーム記事 ツール会社概要 サポート購読

// 無料オンラインツール

画像サイズ変更オンライン

画像の幅と高さをピクセル単位で任意に変更。アスペクト比を固定して歪みを防止し、画質を調整して即座にダウンロード。100％無料 - 画像はブラウザを離れません。

When to resize vs. crop vs. compress

Resize when you need specific pixel dimensions - e.g., a thumbnail slot that requires 400×300px. Crop when you want to change the aspect ratio or remove unwanted edges. Compress when the dimensions are correct but the file size is too large. For web publishing, combine all three: resize to display dimensions, crop to the correct ratio, then compress for fast loading.

How resolution affects file size

File size scales roughly with the number of pixels (width × height). Halving both dimensions reduces the file size by approximately 75% before any compression. A 3000×2000 photo resized to 1500×1000 will be about one-quarter the file size at the same quality setting. This is why resizing to display dimensions is the most impactful optimization step - compression alone can only do so much.

Common dimensions cheat sheet

  • Email header: 600×200px
  • Blog featured image: 1200×630px
  • Product image (e-commerce): 1000×1000px
  • HD wallpaper: 1920×1080px
  • Passport photo (US): 600×600px (2×2 inches at 300dpi)

Frequently asked questions

Enlarging an image (upscaling) always loses quality because the tool must invent pixel data that wasn’t there - this creates blurring. Downscaling (making the image smaller) does not lose quality in a perceptible sense; it actually improves sharpness by averaging multiple source pixels into each output pixel. Our tool uses the Canvas API’s bilinear interpolation for smooth downscaling.
Resizing changes the canvas dimensions while keeping all pixels (potentially squishing or stretching, or with aspect ratio lock, letterboxing). Cropping removes the outer parts of the image to change its shape. If you need to change the aspect ratio without distortion, crop first, then resize.
Common sizes: Instagram square 1080×1080px, Instagram landscape 1080×566px, Twitter/X post 1200×675px, Facebook cover 820×312px, LinkedIn banner 1584×396px, YouTube thumbnail 1280×720px. For quick presets, try our platform-specific converter pages.
Yes. When you resize a PNG (or any format with an alpha channel), the Canvas API preserves transparency. The output will be saved as PNG to retain the transparent background. If you download as WebP, transparency is also preserved - WebP supports alpha channels.