How to Batch Convert Images with Command Line Tools

How to Batch Convert Images with Command Line Tools

Converting 500 photos from JPG to WebP one by one would take hours. Batch conversion lets you process entire folders in seconds.

Option 1: Online tool (no installation needed)

For small batches (<50 files, <500 MB total) the fastest approach is uploading files directly:

1. Go to KaijuConverter and select the format pair you need
2. Drag all files at once (or use "Select files")
3. Download the ZIP with the results

Online tools have size limits. For large batches, use the command-line options below.

Option 2: ImageMagick (all static image formats)

ImageMagick is the Swiss Army knife of image manipulation. It converts, resizes, crops, and applies effects in batch.

Install ImageMagick

• Windows: download the installer from imagemagick.org
• macOS: brew install imagemagick
• Linux: sudo apt install imagemagick

Batch convert JPG → WebP

# Convert all JPGs in the current folder to WebP
for f in *.jpg; do convert "$f" "${f%.jpg}.webp"; done

# Windows PowerShell
Get-ChildItem *.jpg | ForEach-Object { magick $_.Name ($_.BaseName + ".webp") }

# With custom quality (0-100)
for f in *.jpg; do convert -quality 85 "$f" "${f%.jpg}.webp"; done

Resize and convert simultaneously

# Resize to 1200px wide (maintaining aspect ratio) and convert to WebP
for f in *.jpg; do convert -resize 1200x "$f" "${f%.jpg}.webp"; done

# Convert and save to an /output/ subdirectory
mkdir output
for f in *.png; do convert "$f" "output/${f%.png}.jpg"; done

Option 3: cwebp (Google's official WebP encoder)

cwebp is faster than ImageMagick for WebP specifically.

# Install
brew install webp          # macOS
sudo apt install webp      # Ubuntu/Debian

# Convert all PNGs to WebP at quality 80
for f in *.png; do cwebp -q 80 "$f" -o "${f%.png}.webp"; done

# Parallel processing (faster on multi-core systems)
ls *.jpg | parallel cwebp -q 85 {} -o {.}.webp

Option 4: FFmpeg (images + video frames)

FFmpeg handles both video and individual images.

# Single PNG → JPG
ffmpeg -i image.png image.jpg

# All PNGs in the current folder
for f in *.png; do ffmpeg -i "$f" "${f%.png}.jpg"; done

# With specific quality (2=best, 31=worst for JPEG)
ffmpeg -i photo.jpg -q:v 2 photo_high.jpg

Option 5: Python + Pillow

For complex workflows (renaming, filtering, metadata handling) Python + Pillow is very powerful:

from PIL import Image
import os

input_folder = "original_photos"
output_folder = "webp_output"
os.makedirs(output_folder, exist_ok=True)

for filename in os.listdir(input_folder):
    if filename.lower().endswith(('.jpg', '.jpeg', '.png')):
        img = Image.open(os.path.join(input_folder, filename))
        new_name = os.path.splitext(filename)[0] + ".webp"
        img.save(os.path.join(output_folder, new_name), "WEBP", quality=85)
        print(f"Converted: {filename} → {new_name}")

print("Done!")

Tool comparison

Tips for reliable batch conversion

• Test with 5–10 files before processing the entire folder
• Keep originals — conversion to lossy formats is irreversible
• Use separate output folders to avoid overwriting source files
• Check output sizes — sometimes WebP can be larger than the source JPG if it was already heavily compressed

With these tools you can fully automate image workflows for your website, blog, or photography projects.

Related conversions

Once you have your batch script ready, these are the most common pair conversions developers run at scale:

• JPG to WebP — typical 25–35% size reduction for the web
• PNG to JPG — flatten PNGs without alpha for thumbnails
• HEIC to JPG — bulk-process iPhone photos for desktop apps
• PNG to WebP — keep transparency, drop bytes
• TIFF to PNG — open up archival scans for web preview
• RAW to JPG — generate JPG previews from camera RAW files