How to Resize Hundreds of Images in Batch with Python

Resize every image in a folder to a target size using Pillow, creating a new subfolder for processed files.

Easy Python 3.9+ Jun 27, 2026 Automation & scripting 1 views 0 copies

image processing batch processing pillow files automation

Requires third-party packages — install first

pip install Pillow

Python code

21 lines

Python 3.9+

import os
from PIL import Image

def resize_images_in_batch(directory, output_size=(800, 600)):
    if not os.path.exists(directory):
        print(f"Directory {directory} does not exist.")
        return
    output_dir = os.path.join(directory, "resized")
    os.makedirs(output_dir, exist_ok=True)
    for filename in os.listdir(directory):
        if filename.lower().endswith(('.png', '.jpg', '.jpeg', '.bmp', '.gif')):
            filepath = os.path.join(directory, filename)
            with Image.open(filepath) as img:
                img_resized = img.resize(output_size, Image.Resampling.LANCZOS)
                output_path = os.path.join(output_dir, filename)
                img_resized.save(output_path)
                print(f"Resized and saved: {output_path}")

if __name__ == "__main__":
    target_dir = "images"  
    resize_images_in_batch(target_dir, (800, 600))

Output

stdout

Resized and saved: images/resized/photo1.jpg
Resized and saved: images/resized/photo2.png
Resized and saved: images/resized/screenshot.bmp

How it works

This script loops through all files in a given directory, filtering by common image extensions. For each image it opens the file with Pillow's Image.open, resizes it using the high-quality LANCZOS resampling filter, and saves the result into a resized subfolder. Using os.makedirs(..., exist_ok=True) avoids errors if the subfolder already exists. The script is designed as a reusable function that takes a directory path and target output size as parameters.

Common mistakes

Forgetting to install Pillow (`pip install Pillow`) before running the script.
Using `os.listdir()` without filtering for image file extensions, which may try to resize directories or non-image files.
Overwriting original images instead of saving resized copies to a separate folder.

Variations

Use `pathlib.Path.rglob('*')` to recursively process images in subdirectories.
Resize proportionally based on max width or height instead of forcing exact dimensions.

Real-world use cases

Preprocessing product photos for an e-commerce site to ensure uniform thumbnail sizes.
Downscaling high-resolution camera images before uploading them to a cloud storage bucket.
Generating compressed image assets for a mobile app to reduce bundle size.

How to Resize Hundreds of Images in Batch with Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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