Bulk Image Format Converter
Convert multiple images between PNG, JPEG, and WebP formats in bulk with quality control, optional resizing, and ZIP download — all in your browser.
このツールについて
Convert batches of images between formats without installing any software or uploading files to a server. All processing happens locally in the browser using the Canvas API. Drop multiple images onto the upload area — PNG, JPG, WebP, GIF, or BMP — and they appear in the conversion queue showing the original filename, dimensions, and file size. Select the target format from PNG, JPEG, or WebP. For JPEG and WebP, a quality slider (1–100) lets you balance file size against visual fidelity; a value of 85 is typically indistinguishable from 100 at a significantly smaller size. The optional resize step constrains the output to a maximum width or height while preserving the original aspect ratio — handy for preparing web-ready thumbnails. Each result row shows the original format, the new format, the before and after file sizes, and a percentage reduction. Download individual converted files with a single click. The Download All as ZIP button uses JSZip to bundle all outputs into one archive without any server round-trip. Process hundreds of images in one go — the tool queues them sequentially using promises to avoid blocking the browser UI.
使い方
- 1 Drag and drop image files onto the upload area, or click 'Choose Files' to select multiple images.
- 2 Select the target format: PNG, JPEG, or WebP.
- 3 For JPEG or WebP, adjust the quality slider (1–100); 85 is a good default.
- 4 Optionally enable resizing and enter a maximum width or height in pixels.
- 5 Click 'Convert All' to process the queue — a progress bar shows completion status.
- 6 Click the download icon next to an individual image, or click 'Download All as ZIP' to save everything at once.
Why convert image formats at all
PNG, JPEG, and WebP each make a different trade-off, and the right choice depends on the picture. PNG is lossless and supports transparency, which makes it ideal for logos, screenshots, and graphics with sharp edges — but it is heavy for photographs. JPEG is lossy: it throws away detail the eye barely notices to shrink photos dramatically, at the cost of no transparency and visible artifacts at low quality. WebP is the modern compromise — it does both lossy and lossless compression and typically produces files 25–35% smaller than the equivalent JPEG or PNG at the same visual quality, with transparency support too. Converting a folder of PNG photos to WebP, or JPEGs to a smaller modern format, can cut a page's image payload substantially. This tool does that in bulk, for many files at once, without ever uploading them.
How the conversion happens — entirely in your browser
There is no server step. Each image is read with the browser's File API, drawn onto an off-screen HTML canvas at the chosen dimensions, and exported in the target format using the canvas toBlob() method. Because the decoding and re-encoding use the browser's own image engine, the input can be anything your browser can already display — PNG, JPEG, WebP, GIF (its first frame), or BMP — and the output is whatever you select. Files never leave your device, which is what makes the tool safe for screenshots, client work, or anything confidential. Images are processed one after another using promises, so the browser stays responsive and a progress bar tracks the queue even when you drop in hundreds of files.
The quality slider, concretely
For JPEG and WebP, the quality slider (1–100) maps directly to the encoder's quality parameter. It is hidden for PNG because PNG is lossless — there is no quality dial to turn. The relationship between quality and file size is steeply non-linear: the last few percentage points cost a lot of bytes for detail almost nobody can see. A setting of 85 is the widely accepted sweet spot, usually visually indistinguishable from 100 while being roughly 30–50% smaller. Drop toward 60 and you will start to see blocky artifacts in smooth gradients and skies; push to 100 and you mostly pay for invisible precision. Each converted row reports the before and after sizes and the percentage change, so you can dial the slider with real feedback rather than guesswork.
A worked example
Say you drop in a 2.4 MB PNG screenshot, choose WebP, and leave quality at 85. The tool draws the PNG to a canvas and re-encodes it; the result might land around 280 KB — a reduction the row shows as roughly -88%. Now switch the target to JPEG at the same quality: the file may be larger than the WebP and, critically, any transparent areas in the original turn white. That is not a bug.
The transparency trap you must know about
JPEG has no concept of transparency. When you convert a PNG or WebP that contains transparent pixels to JPEG, those pixels have to become some solid colour — and this tool fills them with white before encoding. So a transparent logo converted to JPEG comes out on a white rectangle. If your image relies on transparency, convert to WebP or keep it as PNG; reserve JPEG for full-frame photos that have no transparent regions. This single rule prevents the most common surprise people hit with format conversion.
Optional resizing for web-ready output
The Max Dimension field constrains the output so the longest side does not exceed the pixel limit you set, scaling the canvas down while preserving the aspect ratio automatically. Enter 800 and a 4000×3000 photo becomes 800×600; a portrait 3000×4000 becomes 600×800. This is the fast way to turn camera-sized originals into thumbnails or email-friendly images. Leave the field blank to keep original dimensions. Note that the tool only shrinks images that exceed the limit — it never upscales a smaller image, which would only add blur and bytes.
Downloading: one file or a ZIP
After conversion, each row gets a download button for that single file. To grab everything at once, Download All as ZIP loads the JSZip library from a CDN the first time you click it, bundles every converted image into a single archive in browser memory, and triggers one download — still with no server round trip. The only external request the tool ever makes is fetching that ZIP library on demand; the images themselves stay local.
Practical tips and common mistakes
- Don't re-encode JPEGs repeatedly. Each lossy save discards more detail. Convert from the highest-quality source you have, not from an already-compressed copy.
- Animated GIFs lose their animation. The canvas captures only the first frame, so a GIF becomes a still image. Use a dedicated video/GIF tool if you need motion preserved.
- PNG won't shrink a photo. Converting a noisy photograph to PNG often makes it bigger, because lossless compression can't exploit the eye's tolerance for detail loss. Use JPEG or WebP for photos.
- Check the savings column. If a conversion shows a size increase (a
+%), you probably picked the wrong target format for that image — switch and reconvert. - Very large batches use memory. Thousands of high-resolution images are held in memory until you clear the queue; process in reasonable groups on low-RAM machines.
よくある質問
Resize and compress images in the browser using Canvas API — by percentage, exact dimensions, or max dimension — with batch processing and ZIP download.
ブラウザで JPG・PNG・WebP 画像を圧縮 — アップロード不要。
プリセットのアスペクト比でブラウザ内の画像をインタラクティブにトリミング、回転、反転できます。
任意の画像ファイルをBase64エンコードされたデータURI文字列に変換します。