この計算機の使い方
How compression actually shrinks a photo
A JPEG is already compressed; what the compressor does is re-encode it at a quality level you choose. The trade-off is the fundamental one of lossy formats: lower quality throws away more of the detail the encoder judged unimportant — and in return the file gets smaller. The slider makes the trade visible: at 90% quality a photo may be barely smaller than the original, at 60–80% it is often a fraction of the size with no visible difference on a normal screen, and below 50% artifacts start showing up, especially along edges and in gradients. The page reports the exact output size at the current setting before you download, so finding the smallest acceptable file is a matter of moving the slider, not guessing.
The sweet spot depends on the image, not on a magic number: a landscape with smooth gradients compresses far better than a photo of fine texture like foliage, and a screenshot with flat colors compresses better still. That is why the tool shows the real output size at every quality setting instead of applying a fixed preset.
The PNG problem, and what actually works
PNG is a lossless format: re-encoding it at a lower quality changes almost nothing, because lossless compression has no quality knob to turn — the data must survive intact. If a PNG is large, the effective move is to change the format: converting to WebP or JPEG typically cuts the file by 60–80% because those formats are allowed to discard information. The compressor supports converting PNG input to WebP or JPG output alongside keeping the format, and the sibling converter covers the general case. If the image is a diagram, logo, or anything with sharp edges and flat colors, WebP usually beats both: the same visual at a fraction of the size.
The exception that keeps PNG alive: transparency. If the image needs an alpha channel and it must survive as-is, PNG is the format that keeps it losslessly, and the compressor says so honestly instead of pretending quality reduction will fix it.
Reading the savings numbers
The result panel shows the output size and the percentage saved — the two numbers that decide whether the file will pass an email limit, an upload cap, or a page-weight budget. A 6.2 MB JPEG re-encoded at 70% quality typically lands around 800 KB, an 87% saving, and the panel makes it visible in one glance. The percentage is relative to the original, so it is honest about what the re-encode achieved: an already-compressed image will show a small saving, and the tool reports the real number rather than a target.
Before downloading, glance at the result. The size number can look perfect while a detail — a product label, a face, a line of text — has visibly degraded; the visual check is the final gate, and it is the one the tool cannot do for you.
Metadata, privacy, and the local promise
Re-encoding produces a clean file: EXIF, GPS, and copyright metadata are not carried over, and the page discloses this on every result. For photos you are about to publish that is usually what you want; for files where provenance matters, keep the original as the archive copy.
Everything runs on your device — the image is decoded and re-encoded in a background Web Worker, and the file never leaves the browser. That is the meaningful difference from a "compress image online" service that asks for a 6 MB upload: the compressor has no upload, so the photo, its location data, and its contents stay local by construction, and the 25 MB input limit exists only to keep the page responsive.
よくある質問
どの品質を使えばよいですか？
写真の場合、60〜80%がほとんどの画面でサイズと品質のバランスがとれます。50%を下回ると画質劣化が目立ち始めます。スクリーンショットや平坦な色のグラフィックは、低い品質でもよく圧縮されます。スライダーは各設定での正確な出力サイズを表示するため、許容できる最小のファイルを見つけられます。
PNGが小さくならないのはなぜですか？
PNGはロスレス形式のため、品質を下げて再エンコードしてもほとんど変わりません。PNGファイルを実際に小さくするのはWebPやJPEGへの変換（この圧縮ツールの姉妹ツールである変換ツールで利用可能）で、多くの場合60〜80%小さくなります。
圧縮するとメタデータは削除されますか？
はい。再エンコードによりEXIF・GPS・著作権メタデータのないきれいなファイルが生成され、その旨がすべての結果に表示されます。メタデータが重要な場合は、元のファイルを保存しておいてください。
Why does the same quality setting produce different sizes for different images?
The quality slider sets how much detail the encoder may discard, but the resulting size depends on the image: smooth, low-detail photos compress far more than busy ones. That is why the panel shows the exact output size at every setting instead of promising a fixed reduction.
Is the compressed image good enough to print?
Probably not — printing needs the full resolution and quality of the original. Compression is for screens, email, and the web; keep an uncompressed master for anything that will be printed or edited later.