How to Compress a PNG: A Developer's Guide to Smaller Files

Lossy vs Lossless Compression

Understanding the difference between lossy and lossless compression helps you choose the right approach for your PNG files. Each method has trade-offs between file size and image fidelity.

Lossless Compression

Lossless compression reduces file size without discarding any image data. The compressed file can be decompressed to produce an exact pixel-for-pixel copy of the original. This is ideal when you need perfect quality preservation.

• Typical Reduction — 10-30% smaller files
• Quality — 100% preserved—no visual degradation
• Best For — Logos, text, screenshots, technical diagrams

Tools like OptiPNG and Zopfli use advanced algorithms to find more efficient ways to encode the same data. They try different filter strategies and compression parameters to minimize file size without losing information.

Lossy Compression (Quantization)

Lossy compression achieves dramatically smaller files by reducing the number of colors in the image. A technique called quantization maps millions of potential colors down to 256 or fewer, stored in an optimized palette.

• Typical Reduction — 50-80% smaller files
• Quality — Minor degradation, often imperceptible
• Best For — Photos, illustrations, web graphics

Comparison Table

For a deeper dive into these concepts, see our guide on lossless vs lossy compression.

Both lossy and lossless PNG compression preserve transparency (alpha channel). You never need to worry about losing transparent backgrounds when compressing PNG files.

Command-Line PNG Compression

For developers and power users, command-line tools offer precise control and automation capabilities. These tools integrate into build scripts, CI/CD pipelines, and batch processing workflows.

pngquant (Lossy)

pngquant is the go-to tool for lossy PNG compression. It uses a sophisticated algorithm to reduce 24-bit PNGs to 8-bit palette images with alpha channel support.

pngquant --quality=65-80 --speed=1 --ext=.png --force image.png

• Typical Reduction — 60-80%
• Speed — Fast (under 1 second for most images)
• Best For — Web graphics, photos, illustrations

The quality parameter sets minimum and maximum quality thresholds. If the result would fall below minimum quality, pngquant exits without creating output—useful for automated pipelines where quality standards must be met.

OptiPNG (Lossless)

OptiPNG optimizes PNG files without any quality loss. It tries multiple compression strategies to find the smallest possible representation of the same image data.

optipng -o7 image.png

• Typical Reduction — 5-20%
• Speed — Moderate (higher -o levels take longer)
• Best For — Final optimization pass, archival images

The optimization level (-o0 to -o7) controls how many compression strategies are tested. Higher levels produce smaller files but take exponentially longer.

Zopflipng (Maximum Lossless)

Zopflipng uses Google's Zopfli algorithm to achieve the absolute smallest lossless PNG files. It's significantly slower than OptiPNG but typically achieves 3-8% additional reduction.

zopflipng -y image.png image-optimized.png

• Typical Reduction — 3-8% beyond OptiPNG
• Speed — Slow (minutes for large images)
• Best For — High-traffic images where every KB matters

Batch Processing Script

Automate compression across entire directories with a simple shell script:

# Compress all PNGs in current directory with pngquant
for file in *.png; do
    pngquant --quality=65-80 --ext=.png --force "$file"
done

# Follow up with lossless optimization
optipng -o5 *.png

Learn more about our compression pipeline to understand how we chain these tools for optimal results.

When to Use Other Formats

PNG is excellent for images with transparency, text, and sharp edges. However, newer formats like WebP and AVIF often achieve smaller file sizes while maintaining quality. Understanding when to use each format helps you make the right choice.

PNG Strengths

• Universal Support — Works in every browser, email client, and image editor
• Lossless Quality — Perfect for screenshots, logos, and text-heavy images
• Transparency — Full alpha channel support
• No Artifacts — Clean edges without JPEG-style compression artifacts

When WebP or AVIF Might Be Better

WebP and AVIF are modern formats that often achieve 30-50% smaller files than PNG for the same visual quality. They also support transparency and lossy/lossless modes.

For web projects, consider serving WebP with PNG fallback using the HTML picture element. This gives you the best of both worlds—smaller files for modern browsers and full compatibility for everyone else.

PNG Compression Questions Answered

Here are direct answers to common questions about how to compress a PNG.