Converting Many Images at Once: Batches, ZIP Files and Names

A batch from start to finish

1. Select all the files in your file picker (hold Shift or Ctrl/Cmd to select several) or drag them onto the page. A single batch can hold up to 100 images.
2. Choose the output format first. The default is JPEG.
3. Set the remaining options once: quality, background color for transparent areas and maximum size. They apply to every image in the list.
4. Read the results list. Each row shows the size before and after, and the pixel size when it changed.
5. Download single files from their rows, or use the ZIP button to get all of them together as converted-images.zip.

The important detail is step 3. Changing any setting afterward converts the entire list again, which for 100 large photos takes a while. Settle the format, size and quality on two or three files before you add the rest.

How the output files are named

The naming rule is mechanical: the original file name is kept and only the extension changes, written in lowercase. If the new extension would be the same as the old one, "-converted" is added to the name so the output never looks identical to the source. The examples assume the output format shown in the second column.

Input file	Output format	Output name
IMG_0412.HEIC	JPEG	IMG_0412.jpg
banner.png	WebP	banner.webp
logo.png	PNG	logo-converted.png
scan 07.bmp	PNG	scan 07.png

Because the rule only swaps the extension, two inputs that share a base name but differ in extension (for example banner.png and banner.webp, both converted to JPEG) produce the same output name. The ZIP then adds -2 to the later file (banner.jpg and banner-2.jpg), which keeps both but does not tell you which source each came from. If your folder holds such pairs, rename one of them before adding it. The same goes for identically named files that come from different folders.

Keeping the files in the right order

Names decide the order you see in most file managers once the files are extracted, and plain alphabetical sorting puts IMG_10 before IMG_9. If the sequence matters, such as pages of a scan or frames of a time-lapse, rename with leading zeros before adding the files: IMG_009 and IMG_010 sort correctly everywhere. Since the converter only swaps the extension, the order and numbering you set up beforehand survive the conversion unchanged, and nothing needs to be renamed afterward.

What the ZIP does and does not do to file size

A ZIP is a convenience, not a size reduction. This converter's ZIP stores the files without compressing them, so it comes out slightly larger than the files it holds. Even a compressing ZIP barely helps: I generated 20 synthetic 1920 by 1080 pictures (smooth gradients, soft shapes and a little noise, not real photographs), saved each as JPEG at quality 90 and packed them with the standard Deflate method in Python. The 20 JPEG files came to 9,845,704 bytes in total; that archive was 9,744,022 bytes, about 1 percent smaller.

The reason is that JPEG data is already compressed, so the general-purpose compression inside a ZIP finds almost nothing left to remove. Real photos behave much the same way; flat graphics saved as BMP or uncompressed formats are the exception and shrink a lot. In practice, if the batch is too large to email, reduce the pixel dimensions or lower the quality, since archiving alone will not get you there. Zipping 100 photos of 4 MB each still gives roughly 400 MB.

Settings that behave differently across a mixed batch

A folder is rarely uniform, and the one-settings-for-all model has consequences.

Maximum size. The limit applies to the longest side of each image. Rasters are never enlarged, so a 900 pixel wide picture stays at 900 even if you choose 1920. A set of 4000 by 3000 photos limited to 1600 comes out at 1600 by 1200, which is 1.92 megapixels instead of 12, or 16 percent of the original pixels.

Transparency. When some inputs are PNGs with transparent areas and the output is JPEG, they all receive the same background color. If they are meant for different backgrounds, split them into separate batches; the reasoning is in the background color article.

Quality. The slider only affects JPEG and WebP outputs. A batch converted to PNG ignores it, as the quality guide describes in detail.

Unreadable files. Some inputs depend on what your browser can decode, HEIC above all. Those files are reported individually, so the rest of the list still converts. If a whole folder is HEIC, check the HEIC conversion guide before you start. Animated GIFs in the list contribute only their first frame.

When a large batch slows down

Everything happens in your browser, so a big batch uses your device's memory and processor. A 4032 by 3024 photo expands to about 48.8 MB once decoded (12,192,768 pixels at 4 bytes each), regardless of how small the file is on disk. Phones and older laptops can struggle if many images of that size are processed in one go.

If the tab becomes unresponsive or reloads, split the folder into groups of 25 to 50 and run them one after another; each group produces its own ZIP. This is a practical guideline, not a documented limit: the ceiling of 100 images is fixed, but how comfortably your device handles it depends on image size and available memory. Closing other tabs before a large run also helps.

A worked scenario: 80 product photos for a shop page

A seller has 80 photos from a camera, 6000 by 4000 pixels (24 megapixels each), and needs web copies. The list loads in one batch. Because each image is above roughly 16.8 megapixels, the converter's Original size option would scale each down automatically, so choosing an explicit maximum is the more predictable path. The seller picks JPEG, 1600 pixels and the default quality, checks the first few rows for the before and after sizes, and downloads the ZIP.

The pixel count falls from 24,000,000 to 1600 by 1067, about 1.7 megapixels, roughly 7 percent of the original pixels. File sizes shrink by a similar order of magnitude, though not exactly in proportion, so the seller reads the sizes on the page rather than assuming. For choosing the target dimensions and format of the pictures themselves, the guide to choosing a format for a website covers the trade-offs.

Checking the results list before you download

The results list is the quality control for the whole batch, so scan it instead of downloading blind. Look for rows where the new size is larger than the old one, which happens when a JPEG becomes a PNG or when quality is set high, and for rows whose pixel size is smaller than you expected, which means the maximum size setting is capping them. A single odd row, say one file that grew while the rest shrank, usually means that file was already heavily compressed or was a different kind of picture, and it is easy to convert separately with other settings.