この計算機の使い方
Picking the maximum box that fits your need
The resizer works with a maximum box, not a fixed output size: you give it the largest width and height the result may have, and the tool scales the image to fit inside that box while keeping the aspect ratio untouched. A 4,000×3,000 camera photo constrained to 1,600×1,600 comes out as 1,600×1,200 — the long side hits the limit, the other side follows proportionally, and nothing is stretched, cropped, or squashed. The one rule that makes the box model easy to reason about: the output touches at least one side of the box, and never exceeds either. That is what you want for the three most common jobs — a marketplace that caps listing images at 1,600px, an email that refuses multi-megabyte attachments, or a page layout where the image column is 800px wide.
Because the scale is capped at 1, the tool never upscales: an image smaller than the box comes out unchanged, which is deliberate. Enlarging a small photo only spreads its existing pixels thinner — the blur is the file’s resolution, not a fixable artifact — so the honest result for a 400px-wide logo against a 1,600px box is the original 400px image.
What happens during processing
The image is decoded with createImageBitmap and re-encoded in a background Web Worker using OffscreenCanvas, so a 25 MB photo from a modern phone does not freeze the page while it is being resized — the interface stays responsive, and the result panel appears when processing finishes. The output keeps the original format by default: a JPG in, a JPG out; a PNG in, a PNG out. That matters because resizing is a different job from compression — if the goal is a smaller file, the format and quality knobs of the compressor are the right tool, and the two pages are linked.
One side effect is stated plainly on the page: re-encoding produces a clean file, which means embedded metadata — EXIF settings, GPS coordinates, copyright fields — does not survive the process. For photos headed to a public listing that is usually a feature; for an original archive where provenance matters, keep the source file and treat the resized version as a derivative.
Reading the result panel
The panel reports three numbers before you download: the original size, the output size, and the percentage saved, together with the exact output dimensions. That last pair is the one to verify against the platform you are targeting — a listing limit of 1,600px is a hard ceiling, and the reported width and height are the values the platform will actually receive. The percentage saved is the practical number: a 4.8 MB photo that drops below 1 MB is the difference between an upload that succeeds on the first try and one that is bounced back.
If the saving is small or zero, the likely reason is that the input was already efficiently encoded, or that the image was already within the box — in which case the honest output is the original. The tool does not invent savings, and it never degrades quality to hit a percentage.
The local-processing promise
Everything happens on your device: the image is decoded, scaled, and re-encoded inside the page, and the file never crosses the network. That is the point of a resizer that takes photos — a 25 MB original is exactly the kind of file that should not be uploaded to an unknown server just to change its dimensions. The 25 MB input limit exists for the browser’s benefit, not as a network constraint, and the tool says so; for the rare RAW or multi-hundred-megabyte file, a local desktop tool remains the right answer.
よくある質問
画像は引き伸ばされますか？
いいえ。リサイズツールは最大枠に収まるように両方の寸法を同じ係数でスケーリングします。縦横比は常に保たれ、結果は片方の軸で枠より小さくなることがあります。
小さい画像は拡大されますか？
いいえ。scale は最大1に制限されるため、最大寸法より小さい画像はサイズが変わらず出力されます。小さな写真を拡大するとぼやけるだけで、そのままにしておくより悪くなります。
EXIFやGPSのメタデータはどうなりますか？
再エンコードにより、埋め込まれたメタデータは削除されます。ページはその旨をすべての結果で開示します。メタデータが重要な場合（たとえば出所の証明や印刷用）は、元のファイルを保存しておいてください。
Can I resize without losing quality?
The pixel dimensions change, but the resizer preserves the source quality settings — a JPG is re-encoded with quality-oriented defaults rather than a low-quality preset. If you want a smaller file after resizing, the image compressor gives you the explicit quality slider.
What image formats are supported?
JPG, PNG, WebP, and GIF. A GIF input is re-encoded as a single static frame rather than an animation, which is disclosed in the limitations — resize animations with a purpose-built tool if the motion matters.