Understanding image quality vs file size
Every digital image is a grid of pixels. When you resize or compress an image, you're changing how those pixels are stored. The trick is finding the sweet spot where the file is small enough for your needs but still looks sharp to the human eye.
There are two main ways to reduce file size: reducing dimensions (fewer pixels) and increasing compression (storing pixels more efficiently). The best approach uses both strategically.
Lossy vs lossless compression
- Lossy (JPG): Discards some data the eye barely notices. Great for photos. At quality 80–85%, most people can't tell the difference from the original.
- Lossless (PNG): Keeps every pixel intact but files are larger. Best for screenshots, logos, and images with text or transparency.
- Modern formats (WebP, AVIF): Offer better compression than both JPG and PNG at similar quality. WebP gives 25–35% smaller files than equivalent JPGs.
Practical tips for quality preservation
- Start from the highest quality source. Never resize an already-compressed image multiple times — each round of compression degrades quality.
- Resize to the actual display size. A 4000×3000 photo displayed at 800×600 wastes bandwidth. Resize to the exact dimensions you need.
- Use quality 80–85% for JPG. This is the sweet spot where file size drops significantly but quality loss is invisible to most viewers.
- Choose the right format. Photos → JPG or WebP. Screenshots → PNG. Illustrations → SVG if possible, otherwise PNG.
- Keep aspect ratio locked. Changing width and height independently stretches images unnaturally.
When to resize by dimensions vs by file size
Sometimes you need a specific pixel size (like 600×600 for a passport photo), and sometimes you need a specific file size (like 50KB for an exam portal). These are different goals:
- By dimensions: Use when you know the exact pixel width and height needed. The tool adjusts quality to make it fit.
- By file size: Use when a portal says "maximum 50KB" or "under 200KB." The tool adjusts both dimensions and quality to hit your target.
- By percentage: Use when you want to scale proportionally — like making an image 50% of its original size.
Common mistakes to avoid
- Enlarging a small image makes it blurry — you can't create detail that doesn't exist.
- Saving as PNG when you don't need transparency wastes space — use JPG for photos.
- Compressing the same image repeatedly (re-saving as JPG multiple times) gradually destroys quality.
- Using screenshots instead of actual image exports — screenshots have compression artifacts and wrong color profiles.